Larger Props For Efiency And Engine Life When Running Big Engines Slow

[Brian Degulis]

Our boat is a 1985 61' MY powered by 2 12V71TI 650HP @2300 RPM Originally it had a top speed of 18kts. To do that speed it required every bit of HP to get it up on some level of plane and you couldn't back off much at all before it fell back into the hole. Running the boat that way for me made no sense at all burning 70GPH and operating at about 95% of Max just isn't for me. There really wasn't a happy middle ground either up to 11kts you could run pretty efficiently but between that and 18kts it is just horribly inefficient throwing a huge wake and not really going anywhere. It would be interesting to here from some others with 61s and see if you feel the same.

So I decided pretty early on that we where going to run this boat somewhere near hull speed. After experimenting a little I decided that 10.5 kts was about as high as we could go efficiently there is some lift at that speed but not very much fairly small wake yet fast enough for the stabilizers to work. With the original OEM prop it took 1500 RPM to get 10.5 kts and the fuel burn was just over 19GPH with one Gen running. The common wisdom with boats is that the engine should be run at about 80% of max for good life or in my case around 1850 RPM any less and your at risk of low combustion temperature and shortened life. On boats that's basically true but it's not because of low RPM it's because of light load at lower rpm caused by the difference between the way the engine produces power and the way the prop absorbs it. Think about a semi truck with an engine rated 500 HP @2300 RPM. If that truck is running with an empty trailer or no trailer at all and the driver wants to go 55 MPH he shifts up until the engine is turning as slow as possible without over loading or lugging he doesn't stop shifting at 1850 RPM @ 55 mph. He does this because he wants to load the engine properly for efficiency and life. Now if our boats had controllable pitch props we could do the same thing but they don't we are stuck with a fixed ratio that's generally set up to allow the engine to turn full RPM and produce Max HP.

The next thing I did was to gauge the engines out and see just what's going on @ 1500 RPM so I put gauges on to read turbo boost, air box pressure and exhaust temp. At 1500 RPM there was no readable turbo boost or air box pressure and the exhaust temp was 375 degrees. So basically the engine was running like a lightly loaded natural. Since the compression in a turbo is about 95 lbs less than a natural it was really running like a natural that's completely worn out running cold and inefficiently. So I decided if I want to run this boat at this speed I needed to lower the RPM and increase the load on the engine. If you look at the power curve for these engines it's pretty easy to why it's running so poorly. @ 1500 RPM the engine is capable of producing 475 HP but the OEM prop is only taking around 150 HP so the engine is only producing 34% of what it's rated for at 1500 RPM.

The first thing I did was to de rate the engines from 650 HP @2300 RPM to 525 HP @ 1600 RPM. I chose this rating based on some prop calculations the OEM prop was 32 X 31 with the engine de rated the prop called for would be 35 X 34. I would have liked to go bigger on the dia with less pitch and I had the hull clearance to do it. I went with 33 X 35 five blade only because they where available used from a member of this forum at a fair price. So I hauled the boat and put them on here are the results.

The RPM required to go 10.5 kts is now 1150 RPM OEM was 1500 RPM. The turbo boost is now 6Hg OEM was 0 the air box pressure is now 8Hg OEM was 0 and the exhaust temp is now 575 degrees OEM was 375. Fuel consumption is now just under 15 GPH OEM was just over 19 GPH. I would have liked to see a little more turbo boost and air box pressure but I didn't want to de rate any more than I did. I was very pleased with a 4 GPH drop in fuel consumption for the same speed. The improved efficiency comes from a much more complete burn and a larger dia prop is more efficient at near hull speed. There is also a gain in turning the engine slower. If you take a look at the fuel consumption curves on these engines they run about 18 HP per gallon per hour at WOT fully loaded When you reduce the RPM to 1200 they run about 19.5 HP per gallon per hour fully loaded. I'm guessing that because it simply takes more fuel to drive the engine at a higher RPM even with no load at all and I'm guessing that your percentage of waste heat goes up also.

The engines will now turn up to 1620 RPM within acceptable parameters after that they begin to overload so my new top speed is around 14kts. The one down side to doing this is that I now have a little to much thrust at idle before the change it was just about right maybe a little on the high side now I'm idling at 5.6 kts. It's manageable but I have to be careful when maneuvering or coming up on a spring line. Aside from that I think it's great at the lower RPM I'm smoother and quieter and definitely running cleaner the exhaust noise is a little louder and deeper but you really don't hear unless your on the stern.

In the interest of accuracy I should let you know how the fuel usage was determined. It was done the same way for the OEM boat and after the changes the GPH is total for both engines with one generator running and AC on. I filed up one 600 Gal tank to the point where it was gurgling out the vent then we went from my home in Tampa FL to Key West FL around 235 NM the trip involved only around 40 mins of running time below 10.5 kts over water. When we arrived in Key WEST we re filled the tank again until it was gurgling out the vent. For both trips the stabilizers were on and the fuel was treated with a cetane booster. I'm thinking it should be very accurate over a 230 NM run.

In my opinion any turbo engine should not be run continuously under lightly loaded conditions I believe you need to have some turbo boost and air box pressure and at least 500 degrees of exhaust temp measured before the turbo. Any less than that and I think you lose efficiency and shorten life and now that we are approaching $4 a gallon for fuel using less doesn't hurt either. I also think that a large engine properly loaded and turning slow should have great life between rebuilds. My background is all commercial I've been building tugs for cover 20 years. I listen to the folks on this forum and it seems that around 3000 hrs is the norm between rebuilds. In commercial world 3000 hours would be viewed as a complete failure 15000 would be more the norm. There are a few different reasons for this but the biggest one is that HP per CI is much much lower in commercial applications as is the RPM. With de rating and over proping you may not get to 15000 hrs but I bet you can get to 10000 hrs and that's a whole lot better than 3000.

Brian

[krush]

Nice write up.

To prevent from overloading, do you set a stop on the throttles?

[MikeP]

This article may have been posted before. If so, I apologize. It covers the D/E in a new Norhavn but also deals a bit with engine efficiency issues that Brian was dealing with. Pretty interesting and looks like, as has been stated by others here before, the way of the future (short of Hydrogen power and fuel cells).

http://www.nordhavn.com/constr_con/diesel_electric.php4

[Trojan]

Lots of nice work Brian. I think your sure heading in the correct direction for your boat. But like you said, I think you should have tried to find the larger wheels. I think you would have shown better results. That heavy pitch is not helping you. But I could be wrong. I'm in the process of changing wheels on my 43dc also. I have 26 X 27 4 blade now. When I go back in the water. I will be 29 X 28 3 blade. It should prove interesting. Nice to here someone is trying something different. I don't know where you draw the line from a planing hull to a displacement hull once a boat gets past a certain length and tonnage. You almost need to talk to the designer on what he had in mind. Keep us posted if you try any further changes. Just remember with a turbo you have less cylinder compression to compensate for the air boost. So running to slow of rpm will defeat the usefulness of the turbo. Maybe a little less pitch and a little more rpm would balance out and increase your speed with no fuel increase. Just a thought. Fun isn't it. Good luck. To bad it cost so much money to play.:D

BILL

[Boss Lady]

If going sailboat speed fits your use profile this might help you get max efficiency, but some sailboats are faster.

I like the idea of a motorsailer with large twin engines when you want to go fast, and sails for making long range travel affordable. Free is better than cheap!

Hatteras made 3 motorsailers, maybe one of them will be available when I retire.

A wide beam boat with a retractable keel, that is the ticket.

[Brian Degulis]

Nice write up.

To prevent from overloading, do you set a stop on the throttles?


Thanks I limit the RPM by re setting the governers with the shims and spacers from Detroit. It's the same procedure used to set no load RPM and it's just trial and error to hit the RPM you want.

Brian

[krush]

Thanks I limit the RPM by re setting the governers with the shims and spacers from Detroit. It's the same procedure used to set no load RPM and it's just trial and error to hit the RPM you want.

Brian

But does that prevent from creating an overload condition? You could be over fueling and overloading the engine at that "max rpm" if the prop is too big.


I'm not going to get into the turbo argument again, but I'm sure you know spool has almost nothing to to with RPM.

Chris, sail boats are long and narrow for a reason :-)

[Boss Lady]

You are going in the right direction. All fixed propellers are most efficient at one rpm and a compromise at all others. Once you pick the rpm you want to operate, you can size the propeller to give maximum thrust and load the engine for the HP it is capable of providing at the lower rpm. It will go into an overload situation as soon as you try to run the rpm higher. I agree with you that your engines will like the increased load and will come up to proper operating temps. You have already proven that point. I will be tweeking my props for optimum cruise, instead of top speed, which is where most of our boats are set.

[REBrueckner]

Fascinating post, a story well told. Bravo on the improved fuel consuption at cruise speed!!

"The first thing I did was to de rate the engines from 650 HP @2300 RPM to 525 HP @ 1600 RPM.."

What's involved with this/how do you derate? Smaller injectors, I guess, what else???

Having taken this step, what would have been the results of just operating with the derated HP and OEM props? Seems like you would have had to run at a higher RPM to achieve the required HP for 10.5 knot cruise relative to the original setup. Maybe the loading would still be too low??? Are the 33 x 35 props you're using more efficient than the older.

Finally, I am wondering about air box pressure. When one uses Walker Airseps or RACOR CCV's, I thought air box pressure became slightly negative. Would this affect the work you have done in any way?

Thanks for a great post.

[MikeP]

I'm certainly in agreement that their is a "sweet spot" in engine operation and that optimizing for that will give the best economy at that speed range.

Of course, there is a large difference between best engine power efficiency and best overall fuel efficiency. Lower power production (lower RPM, all else being equal) is always better for fuel consumption. For example, on our 53MY, 640RPM will yield 6.3 knots and 3NMPG (WOW!). 1100 RPM will yield 9 knots and 1 NMPG, 1400 will yield 10K and .75NMPG. At 640 RPM, eng temps will not get much above 140, at least they won't in the 20-30 minutes I have operated the boat at those speeds to maneuver in the harbor area. I suspect they never will with everything set up as per oem.

So regardless of how well you set up the engine to operate at IT'S best efficiency, it will use more fuel than it would if the RPM (load) is reduced. Obviously, this only matters if you are concerned about the absolute best fuel economy.

As has been often mentioned, boats are set up as a compromise. But it is handy to have the full range of speed available should I want to use it. So it seems to me there is another option available if fuel economy is a driving issue. But the eng temp has to be dealt with.

IF one is happy with 6 knots/3NMPG for extended periods, one has to deal with the eng temp issue. Other than that, there is no bad consequence for operating at low RPM. The low temps are the issue at low RPM as DD points out in the service and operating manual. SO how do you raise the temp?

..To achieve max range it seems that you could obtain proper eng temps at very low RPM by simply restricting the RW flow through the system. If this was done with valves of some sort - or simply partially closing the RW seacocks, it should be possible to operate the engine at very low speeds while maintaining correct temps. Obviously you would have to monitor a MECHANICAL temp gauge to determine the correct valve opening to achieve the temps you want at low RPM. When you want "normal" speed, open the RW valve completely and you have normal water flow. Alternatively might be a "Popoff" diverter valve on the water pump that would, when the valve was in the divert position, dump water from the pump directly into the exhaust header. When the valve was "off" you would have normal flow, when "on" it would divert and therefore provide less cooling water flow to the heat ex. Keep in mind that the Tstats would work normally - opening at the appropriate temp.

I have no idea if this would actually work but if achieving 1500NM on a (700 gal) tank of fuel, as opposed to 500NM - at 6 knots instead of 9 - is useful, it might be interesting to try. If you already have mech temp gauges in the engine room (if you don't, PUT THEM IN), it's as simple as fooling with the seacock and monitoring the temp gauges.

6 knots is pretty darn slow though... ;)

[chris]

Very interesting thread to me. I am currently trying to figure out what props to use on my 48MY. Most of my running is done at 10kts. I have to turn 1500rpm to acheive 10kts. I would like to acheive 10kts at 1000 rpm and have the eng. temps at 180. I am now running 26x21x4bl props with 1.5:1 Allisons. 6v92's rated at 425hp.
Chris

[Brian Degulis]

Re setting the governers prevents overloading because the governer hold the RPM below the point where overload can Happen. In my case overload starts a 1620 RPM so the governers are set at 1600 RPM so when everthing is clean the governer will dictate top RPM not the prop load.

When I spoke of running cold I should have been more specific. The water temp should always be within range no matter what speed your running. The problem with running very lightly loaded is the combustion temp is to low this happens even if the water temp is right on. The only way to increase the combustion temp is to increase load. To increase load for the same speed you must reduce RPM with larger props this will load the engine more at a lower RPM. Every engine has a rating for the HP that can be produced at any given RPM the closer you get to that rating the better.

To de rate I simply re set the governers for a max Rpm of 1600. The only benifit of going to smaller injectors would be a slightly better atomization but the gain would be very very small. In doing this I know that the posability exists that I may have to change it back to OEM when it comes time to sell right now it's just props and re setting the governers to get back to original.

[Brian Degulis]

[QUOTE=krush;79750]But does that prevent from creating an overload condition? You could be over fueling and overloading the engine at that "max rpm" if the prop is too big.


Now I understand your question 1600 RPM was used as the MAX RPM for the prop calculation. After I did the change I used exhaust temp to verify that the calculation was correct. The engines ran within parameters up to 1620RPM beyond that the EX temp goes up above the range as does turbo boost and air box pressure. After doing the check I set the RPM to 1600.

Brian

[akintosyali]

A motor sailer by Hatteras???

[SKYCHENEY]

Here's the motorsailer:

http://www.hatterasowners.com/Brochures/65SY/65SYArchives.htm

[Jackman]

Doesn't overloading occur across the performance curve no matter what rpm you limit yourself to with a prop that is too big or pitched too high for factory spec WOT?

[REBrueckner]

"Doesn't overloading occur across the performance curve..."

Seems like Brian's test data reports a resounding: not all!!

In fact his test data shows most of our engines are UNDERLOADED at lower RPM as evidenced by lower exhaust temps and less than ideal efficiency, and are only close to proper loading (and exhaust temps) in the 1800 to 2000 RPM range. "Overloading" would occur in Brian's case over 1600 RPM where he began testing at higher than normal exhaust temperatures....hence his limiting RPM to 1600 RPM.

It appears that Brian would be unable to use them at higher RPM. Sounds good for extended hull speed cruising so far.

Had he attempted to use even larger props, then he likely would have over loaded his engines and this would have been reflected in higher than rated exhaust temperatures.

I got wrapped up in all that data a bit too much: All Brian actually did was to put on bigger props. Anybody can do that and run at lower RPM and be more efficient while losing top end speed. His insight was that after the change he tested for proper operating exhaust temps and mechanically limited his engines (to 1600 RPM) so that his engines can't be accidently overload.
Again I say: BRAVO!!!

[krush]

Now I understand your question 1600 RPM was used as the MAX RPM for the prop calculation. After I did the change I used exhaust temp to verify that the calculation was correct. The engines ran within parameters up to 1620RPM beyond that the EX temp goes up above the range as does turbo boost and air box pressure. After doing the check I set the RPM to 1600.

Brian


Yeah, I just wanted to verify you "shifted the curves" so that prop HP and engine HP meet at 1600rpm.

[MikeP]

One issue that troubles me a bit re the data - Conventional wisdom for long boat engine life is to deprop slightly, thereby INCREASING the eng RPM for the same power output and reducing the load on the engine. It has been posted here as well as on sites such as Boatdiesel and numerous other places.

The data presented and conventional wisdom seem to recommend the opposite for best boat engine life. They can't both be right.

Life is so confusing...

[Brian Degulis]

One issue that troubles me a bit re the data - Conventional wisdom for long boat engine life is to deprop slightly, thereby INCREASING the eng RPM for the same power output and reducing the load on the engine. It has been posted here as well as on sites such as Boatdiesel and numerous other places.

The data presented and conventional wisdom seem to recommend the opposite for best boat engine life. They can't both be right.

Life is so confusing...

They are both right but for diffrent purposes. You might consider reducing pitch if you run at or near WOT because in that range your are loading the engine at or near 100% of it's rated load.

In my case I have a boat with 1300 HP and at my normal operating speed it only requires 300 HP to push it. So I was operating an extremly lightly loaded engine. The larger props go a long way to load the engines better at a lower RPM while producing the same thrust that the smaller OEM props produced at a higher RPM. But I can no longer produce 1300 HP I'm down to about 900 at the new lower RPM.

The whole concept becomes very clear if you take a look at the power/ prop curves. Think of it as shifting gears in your car or truck you simply change the ratio of RPM and thrust produced at the prop. In a sense your changing the boats operating parameters to better suit the way you plan to use it.

Rob is exactly correct you could just go to bigger props and change nothing on the engines but then it would be possible to overload if you push the throttles up to much. It's basically just good engineering to prevent that from being possible.

Brian

[Boss Lady]

Variable pitch props would allow us to have the best of both worlds. you could add more pitch for more thrust at lower rpms and adjust loading of the engines at any rpm you wanted to operate.

We do this in airplanes all the time. We use less pitch to let the engine make maximum rpm for take off, and climb to altitude. Once at altitude we reduce rpm, add pitch to get more speed and then lean the engine to get max cruise without melting the heads or exhaust.

Boats in europe have had variable pitch props for years. We have been lazy because we have historically had cheap fuel. I bet you will begin to see VP on new boats here real soon for the same reasons they have them in europe (maximize fuel efficiency across all operating rpms)

The most cost effective way to do it now is to have two sets of props, one for normal operation and the other for max economy.

[krush]

Airplanes mostly have variable pitch because the density of air changes so much with altitude. Even at the same power setting, a good climb prop sucks up high at cruise (won't get into nerd details).


Water's density remains constant so the advantage of a variable pitch prop isn't as great as in an airplane. A continuously variable transmission may be another alternative. "Our" problem has to do with the linear production of power from an engine vs the exponential (J curve) power absorption of the prop--not the changing density of air and great variance in forward velocity.

[Dan Mapes]

Are the exhaust gas temps in the manuals? This is all quite interesting. We have Covington 6-71TIs and are going to be cruising at reduced RPMs. We can run wide open at 21K on glass at around 2200 but we are thinking that 12-1300 rpm is where we will run, perhaps lower.
I like the idea of increasing prop size/pitch but we already idle at 5.8 knots.

[Brian Degulis]

I haven't ever been able to find anything from DD regarding stack temp. The common wisdom on commercial aplications is for continous running above 500 but not over 700 for long life.

Brian

[Dan Mapes]

We don't have pyrometers installed so I am wondering which ones are available. Presumably since we both have 71's the exhaust manifolds are the same, and they would be mounted prior to the coupling that goes up to the turbo?

[mikfin]

Hi,

Nice engineering! Which prop calculator did you use? Is it available online?

Thanks,

Mike

[akintosyali]

I think the props on my gas boat are original. So, the question is, has the prop technology changed so much in the last 40 years that my props are obsolete? Or can the existing props still be shaped and trimmed into something a lot more efficients??

[Brian Degulis]

We don't have pyrometers installed so I am wondering which ones are available. Presumably since we both have 71's the exhaust manifolds are the same, and they would be mounted prior to the coupling that goes up to the turbo?

I like Etna for pyrometers the same folks that make the digital tachs (I like them also)

The prop calculations where done by General Propeller In Bradenton FL they have some very good programs that not only take into acount the normal HP reduction lengh weight etc But can also factor in the prop your using now and how it loads the engine and what speeds it provides. By factoring some real world data you can get much closer on a calculation.

Brian

[Brian Degulis]

I made a mistake Hewit is the pyrometers not Aetna they are only tachs

Brian

[Boss Lady]

Airplanes mostly have variable pitch because the density of air changes so much with altitude. Even at the same power setting, a good climb prop sucks up high at cruise (won't get into nerd details).


Water's density remains constant so the advantage of a variable pitch prop isn't as great as in an airplane. A continuously variable transmission may be another alternative. "Our" problem has to do with the linear production of power from an engine vs the exponential (J curve) power absorption of the prop--not the changing density of air and great variance in forward velocity.

Hi Krush, I like it when you shoot holes in your own argument. Chris

[krush]

Hi Krush, I like it when you shoot holes in your own argument. Chris

That's a cop-out if I ever read one. At least back it up....

[Boss Lady]

Airplanes mostly have variable pitch because the density of air changes so much with altitude. Even at the same power setting, a good climb prop sucks up high at cruise (won't get into nerd details).


Water's density remains constant so the advantage of a variable pitch prop isn't as great as in an airplane. A continuously variable transmission may be another alternative. "Our" problem has to do with the linear production of power from an engine vs the exponential (J curve) power absorption of the prop--not the changing density of air and great variance in forward velocity.

The reason the prop that gives good climb performance sucks at cruise is exactly the same problem we have with our fixed propellers. You have a compromise in pitch and size chosen to give better performance at a given performance parameter. A variable pitch control prop gives you a much wider tuning for any given operating condition and the corresponding improvement in transferring available engine power to work (locomotion)

Controllable pitch propellers for airplanes are called "constant speed props", and are designed to give maximum loading without overload of the engine.

I have yet to see an internal combustion engine produce linear power, whether you are talking HP or torque. Quite the opposite. I raced cars and motorcylces for many years and have attended many dynometer pulls. These are called power "curves" not power "lines" as stated above.

An interesting point in understanding power absorption is that propeller power varies by the cube of the rpm. Consequently, twice the rpm requires 8 times the power to spin it.

Water density is constant, air density does vary with altitude and barometric pressure. Please quantify for the rest of us how much the density changes negate the advantages of using varible pitch in a constant density medium. Please tell us all the advantages of using a continuouly variable transmission instead.

I know you are a young enthusiastic engineer, but please get your facts straight.

[krush]

The reason the prop that gives good climb performance sucks at cruise is exactly the same problem we have with our fixed propellers. You have a compromise in pitch and size chosen to give better performance at a given performance parameter. A variable pitch control prop gives you a much wider tuning for any given operating condition and the corresponding improvement in transferring available engine power to work (locomotion)

Did I disagree? No. The advantage of having a variable pitch prop in a plane (along with it's complexity) is much greater in an airplane for many reason including two I cited: large variation in fluid density and GREAT variation in forward speed


Controllable pitch propellers for airplanes are called "constant speed props", and are designed to give maximum loading without overload of the engine.

They are only called that if they have a governor that keeps the RPM constant. Thanks for the lesson though...I fly a plane with one.



I have yet to see an internal combustion engine produce linear power, whether you are talking HP or torque. Quite the opposite. I raced cars and motorcylces for many years and have attended many dynometer pulls. These are called power "curves" not power "lines" as stated above.

Well, here are a few. Seeing that you are blatantly wrong in this respect reduces your credibility for your other comments:

All these "curves" look pretty damn linear to me throughout the nomral operating range.

http://vf-engineering.com/images/kit/bmw/e36_m3/dyno1.gif

http://www.btinternet.com/~madmole/images/chart2.JPG
http://www.frontierpower.com/images/makingsense-engine-b.jpg




An interesting point in understanding power absorption is that propeller power varies by the cube of the rpm. Consequently, twice the rpm requires 8 times the power to spin it.

Thanks captain obvious for that revelation...even though this point has been addressed many times on this forum, I'll be sure to credit you with letting us know.


Water density is constant, air density does vary with altitude and barometric pressure. Please quantify for the rest of us how much the density changes negate the advantages of using varible pitch in a constant density medium. Please tell us all the advantages of using a continuouly variable transmission instead.

I specifically said I would avoid the nerd talk because that is beyond the scope of this topic.

A CVT would be of benfit because it would allow the loading curve to be shifted left/right by varying the RPM of the prop. This is of use because then the MAX HP output at a given ENGINE RPM could be match with the proper PROP RPM to allow the prop absorb the MAX HP.

Edit (add): This is what Brian did when he put a larger prop on (shifted the prop load curve to the left).



I know you are a young enthusiastic engineer, but please get your facts straight.

I know you are full of yourself and think your internet knowledge is greater than most people's, but some of us fully understand the fundamentals and actually do know what we are talking about--and we didn't learn it via google or wikipedia. Please don't patronize me; if you didn't know my age, you never would've made such a comment.

I don't need to prove anything to anybody, and I believe I have gained the respect of many people on this board in regards to technical topics--at least the respect of people that actually know what they are talking about.

[labrador]

Now Now boys, lets cool down! Boss lady was up early Krush, notice his post was 3:49am. Can't we all just get along! (Rodney King) Although Krush may not have been born when that comment was made!

[Dan Mapes]

I'll start, how big a deal, and dangerous is it to shim the max rpm on a 6-71? I keep hearing that touching these things is to be left to experts on detroits. I sort of think adjusting the rack is along those lines, but max rpm?

[BUSTER]

I'll start, how big a deal, and dangerous is it to shim the max rpm on a 6-71? I keep hearing that touching these things is to be left to experts on detroits. I sort of think adjusting the rack is along those lines, but max rpm?

In what hp config? I have cranked inline 6-71N's up to 3000rpm's"intermittently" with N-80 injectors for thousands of hours,but a pyrometer rules the throttle,if you want piston crowns. Adjusting the rack just synchronizes the injector group to react properly in relation to idle or full throttle,creating even power from each cylinder.

BTW/Considering that the guy with the 12V-71's has naturally aspirated engines.I would expirement with smaller cc injectors and take further advantage of the inertia generated by the large dispalcement of the engines themselves. I would like to know what the EG temp is at present config of his 12V's though.

[Sparky1]

I don't need to prove anything to anybody, and I believe I have gained the respect of many people on this board in regards to technical topics--at least the respect of people that actually know what they are talking about.Im not an engineer but for some body who doesnt have any thing to prove that sure was a lot of proving. I am always interested to learn new things can you give us the web site address where all that information came from?

[Dan Mapes]

In what hp config? I have cranked inline 6-71N's up to 3000rpm's"intermittently" with N-80 injectors for thousands of hours,but a pyrometer rules the throttle,if you want piston crowns. Adjusting the rack just synchronizes the injector group to react properly in relation to idle or full throttle,creating even power from each cylinder.

BTW/Considering that the guy with the 12V-71's has naturally aspirated engines.I would expirement with smaller cc injectors and take further advantage of the inertia generated by the large dispalcement of the engines themselves. I would like to know what the EG temp is at present config of his 12V's though.

I believe they are 380 hp. with 2300 max no load. They have n90 injectors and run through 1.5 capitol gears to 24x22 props. WFO we can hit 21 K on glass and we idle around 6k.
Up around 1800-2000 rpm we get about 800 F degrees on the snails after they have been running at speed a while. When running around 14-1500 rpm I think we get around 500 F.

[bobk]

I'm just catching up with this very interesing thread. As a cruiser (read 8 kt for 3500 miles per year) type, it would be very desireable to gain efficiency and keep temperatures a bit higher. We have a series I 48MY with 425 hp
6v92's and a spare set of props that could be re-pitched. How should I begin?

Bob

[Boss Lady]

You will need the engine power curve for you particular engine, the propeller curve loading for the original propeller, and access to a propeller calculator.

Determine what you can live with for maximum rpm and then select a propeller size and pitch that will load the available HP you have at the new max operating rpm.

If your engine power for a given rpm is say 150 HP, but your propeller is only absorbing 70 HP, then there is 80 left untapped. A larger prop would consume more of the available HP, and would load your engine creating more heat. Proper operating temps for the engine help it combust the fuel adding to efficiency.

There are few tradeoffs to consider. You will have more thrust at idle rpm, this means you will work the transmissions a lot more during slow speed manuevering. You will lose some speed at the top end in exchange for better efficiency at the low end. You should reset your governor to the new max rpm.

There are several members on the board that can help you with this and also recommend several sources for help with the propellers.

[Westfield 11]

I too have been reading and learning from this thread. Now, I have a question: when you re-prop to match your max safe RPM to your hull speed, say 10.5 knots, what happens when the waves and wind are on your nose. Can you still maintain 10.5 without adding some power and thus over temping or stressing the engines. Even worse, what if you have re shimmed the governor and are unable to add power, are you stuck at 4 or 5 knots in 5 foot seas?

How much fudge factor is included to allow for this?

[bobk]

You will need the engine power curve for you particular engine, the propeller curve loading for the original propeller, and access to a propeller calculator.

Determine what you can live with for maximum rpm and then select a propeller size and pitch that will load the available HP you have at the new max operating rpm.

If your engine power for a given rpm is say 150 HP, but your propeller is only absorbing 70 HP, then there is 80 left untapped. A larger prop would consume more of the available HP, and would load your engine creating more heat. Proper operating temps for the engine help it combust the fuel adding to efficiency.

There are few tradeoffs to consider. You will have more thrust at idle rpm, this means you will work the transmissions a lot more during slow speed manuevering. You will lose some speed at the top end in exchange for better efficiency at the low end. You should reset your governor to the new max rpm.

There are several members on the board that can help you with this and also recommend several sources for help with the propellers.


Chris and others: I've probably got less than 5 hours at 10 or more knots. Almost all of the remaining 500 or so hours over the last 18 months is close to eight knots. Speed is of no consequence to our life style, except once in a while I need to bump up a tad to avoid falling off the short chop of the Chesapeake Bay.

So before getting in too deep, the first question I'd like to answer is what sort of efficiency can I attain at eight knots with a prop change, or what speed can I achieve at 6 gph. Any guidance? And how serious would the extra stress on the gears be as was mentioned. The latter sounds like the biggest negative for us.

Bob

[Dan Mapes]

I can't see the gears being negatively impacted since you are not pushing the boat harder than it was meant to go. The transmission is not transmitting more horsepower in fact it is not even transmitting what it is rated for since the boat is traveling slower than max.

[bobk]

I can't see the gears being negatively impacted since you are not pushing the boat harder than it was meant to go. The transmission is not transmitting more horsepower in fact it is not even transmitting what it is rated for since the boat is traveling slower than max.


Maybe the concern is with the clutches?

Bob

[BUSTER]

Maybe the concern is with the clutches?

Bob

Not taking trolling valves into consideration, clutch discs are pressurized and locked up to handle specific torque loads, in both forward or reverse if not,they are loose and in nuetral.Single speed and without torque converters,either direct drive or planetary reduced,common marine transmissions are very rugged and simple.

[Dan Mapes]

Now that would be something worth considering but given the speeds we are supposed to shift at I have to wonder if it means that much. After all, depending on the gears, they offer trolling valves that slip the clutches. When shifting we are quite a ways down the power curve
Perhaps a tad more wear on the clutches but Ill bet it isn't significant.
Depending on the gear design perhaps shifting from forward into reverse at 7/8 knots might mean something.

[Pete]

In reading this very interesting thread, I have noticed that the subject of cooling has not been mentioned. I have never seen a marine DD that had any significant margin on the cooling system. May not be a problem in some areas but us folks that float in 90F degree and above, we think about cooling. The sea water pump is a direct drive/positive displacement pump off the engine rpm, so slowing the rpm means pumping less water. Has anyone done analysis on what happens in August if you try to produce significantly more horsepower at a lower rpm? I would think that marine engine cooling systems are designed off the propeller absorption curve, not the max available horsepower curve.

Pete

[krush]

Pete:

In "theory" the reduction in HP output is linear, and the reduction of cooling water is linear vs RPM as well (postive displacement pump).

I doubt there would be a problem. From what people say here, I would make the deduction that water flow isn't the problem, but rather heat exchanger effectiveness/efficiency (area and fouling).

[Boss Lady]

On the cooling issue.

The engines are currently underloaded and have a problem coming up to proper operating tempertures with the propeller size as provided by the factory when running at less than the suggested cruising rpm. I.E. depending upon water temperatures, my old 8v71s would not even start getting near 180 degrees until the rpm was up near 1700 rpm, especially if the sea water temps were cold.

Margin of cooling is generally for continuous full load duty cycle and is 1900 rpm for most DD engines. Even this stated rpm is erronious, since you can still exceed the safe operating temperatures of the higher HP versions of some engine series at a lower rpm, especially when sea water temperatures are high. Like Krush pointed out fouling also comes into play and it doesn't take much build up to take away a significant amount of cooling capacity. I personally feel that the detroit has ample cooling at the lower rpms, even if you push one harder with a more aggressive prop. If you look at the HP curves you will see that real high HP only comes at the far end of the rpm scale and is usually up at wot (2350 rpm) and HP is normally less than a third down where the cruisers like to run 1200-1400 rpm. So if your engines won't come up to temp at that rpm, then overtemp is almost impossible, if you match propeller power demand with engine output.

I would recommend that you set your new max rpm at least 30 percent above your intended operating rpm and select the prop to match the available power. This will give you some storm margin so that if you can make the speed you want in fair conditions at your new cruising rpm, you will have some power reserve to run against head seas and more control of the boat.

[REBrueckner]

"...So, the question is, has the prop technology changed so much in the last 40 years that my props are obsolete?..."

Great question, I have often wondered also. How about posting that question as a new thread...it's worth it. It's likely complicated enough to warrant a buit of discussion....
For one thing cupping was not around 30 years ago, but I'm unsure what effect that has on efficiency at moderate speeds.

[Boss Lady]

It would be nice if we could entice someone to talk about propeller technology to the forum. However, the successful wheel companies seem to guard secrets like they were the "manhatten project". LOL Most of the time they just tell you the "benefits" you will get out of buying their super duper new computer designed wheels, and not much technical stuff. It is mostly marketing materials. Having said that, there have been significant strides in propellers, some of it has made it from the military to the civilian world. Hatteras' new boats have benefitted from some of this, but for most of us, the re-engineering to put in much larger shafts, deeper gears, and then have the clearance to run larger wheels is not practical. So we end up with slight improvements using some of the new designs in normal sizes. I went to all the trouble of digging into this a few years ago, and found out that you would need prop pockets. Hatteras gets a few percentage points more efficiency because the new boats don't have wasted prop force angled downwards, but is in line with the axis of the hull. That alone is significant, and combined with the slower turning large props gets them some "real" improvements in pushing water efficiently.

For us with older boats, the most practical way to get more is some type of variable pitch retrofit, or as Krush pointed out a variable transmission. So far I don't know of anything reasonable.

[krush]

I wonder if real world gains can be had by fitting nozzles. I know modern tugs use them.

[Boss Lady]

Kort nozzles might be of interest to the hull speed crowd, but have serious drag factors at higher speeds. Maybe there is some engineering that can be focused on this avenue for higher speed boats. Available data is for boats that do heavy stuff like tugs, and cargo.

[Brian Degulis]

I too have been reading and learning from this thread. Now, I have a question: when you re-prop to match your max safe RPM to your hull speed, say 10.5 knots, what happens when the waves and wind are on your nose. Can you still maintain 10.5 without adding some power and thus over temping or stressing the engines. Even worse, what if you have re shimmed the governor and are unable to add power, are you stuck at 4 or 5 knots in 5 foot seas?

How much fudge factor is included to allow for this?

You don't re prop so large that your max speed or RPM is your hull speed for the reason you stated. You want to be able to go a little faster than your cruise in my case cruise is 10.5 kts @ 1150 RPM but max before overload is around 14 kts @ 1600 RPM. So that gives me the ability to deal with head winds and sea conditions or picking up more speed in a following sea for control.

Kort nozzles are great for improving static thrust usually around 40% which is why they are used on tugs. But they are bad at anything above displacement speed Rice makes a speed nozzle mostly for shrimpers and dragers but still not very good above displacement speed.

An 8 kt trawler would gain a lot with a nozzle you don't see it because of the cost. They would also gain a lot with a deeper reduction and larger prop you don't see that because of the cost and the increase in draft.

As far as concerns about enough Raw water capacity at lower RPMs. Its a good observation but really not a concern the cooling capacity required is determined by what the engine is actually putting out in HP not what it's capable of putting out. So in my case I'm turning 1150 RPM and the prop is taking 150 HP the engine is capable of producing around 300HP @ 1150 RPM. So even with the larger prop I'm still not developing near the max HP at that RPM. The cooling capacity comes into play at or near full rated power which on an OEM boat is going to be at or near max RPM.

If your thinking about doing this follow the procedure that I used and explained in the begining of this thread to determine what the max continous eficient speed you want to run at is. Then look at the power curve for your engine and see how much HP is required. Then determine how much higher you want to be able to go above your optimum speed and find the HP required. Then go to the engine power curve and see at what RPM the engine is capable of producing that HP. Then use that RPM and HP to spec out a new prop considering hull clearance also.

It's very hard to know what fuel consumption will be without trying it you can get an idea from a fuel consumption curve but only an idea. The more over powered you are for displacment speedsb with the OEM prop the better the improvement will be.

As far as concerns about the transmission it's very easy to get a rating from the gear manufacturer. It rarely is a problem because even though your increasing the torque at displacment speed the gear is still capable of handling the full rated power of the engine which is an awfull lot more than whats required at displacement speed again it what the engine is producing at that RPM not what it's capable of producing.

You will find that the propeller shaft size and proppler blade loading is more of a concern than the transmission but in almost all cases it's still more than adequte for the job. That's why this aproach is so apealing because un like variable pitch props or multi ratio transmissions increasing prop size is practical aproach without re designing the boat or breaking the bank.

Brian

[Eventide]

Brian - you have done very fine work here. This thread may very well be the difference between some Hatteras owners keeping their cherished craft or, in despair, succumbing to the rising costs of fuel and other boating expenses. There are so few larger boats on the water around here, compared to just a few years ago.

So what you have tried to share with us, and your research and thoughtfulness, could be very important to many of us. I am hoping that you will soon have some 'before and after' fuel consumption figures for us. All the best.

Thank you for your effort.


Eventide
St. Pete

[Pete]

I am going to try the cooling situation one more time and if the heat transfer experts continue to say no problem, I will call it a day. From the prior responses I concluded that the consensus opinion is the design point for detroit's is, cool it enough for WOT and the lower speeds will be OK. I agree with this if you are using props that allow full rpm under load. But, in this thread we are discussing picking a lower rpm and selecting the props to absorb the full horsepower available at that rpm or close to it.

So, here is my hang up. First, I think we all have agreed that the raw water cooling pump volume output is linear and determined by the engine rpm. With this assumption, next take a detroit engine of your choice and obtain the horsepower curves for the engine. All I have seen have prop absorption, and available horsepower, plotted against rpm. Now take a straight edge and draw a line from the lowest rpm point on the available horsepower curve to the wot point on the same curve. While it varies by engine, the available horsepower curve is always higher than the line you just drew. The difference is greatest for turbo engines and less for NA engines but always higher. The line you drew is the available heat absorption of the cooling system, on its best day. Thus if you run at the available horsepower line all the time or just when conditions require the extra power, you will have a heat problem. Now, the flotation water temperature also plays in this so boats in cooler climates have more latitude, but in the 90 degree F areas I think you are asking for problems. You just have to keep below the available cooling line in warm areas. And I will say again, this is the absolute best case for heat transfer, something approached in real life, but seldom achieved. The one thing no body needs is to be in seas that dictate full available power and start seeing the temp rising. Well that is my case for cooling consideration. I think it is real, but will wait for the experts to comment.

Pete

[Boss Lady]

You have two components of the cooling system. The closed circuit fresh water side and the raw water open circuit. The real challenge lies in the amount of transfer through the heat exchanger where these two systems interact with each other. For most applications the amount of over capacity quickly diminishes as HP increases. Take a look at the 8v92 for instance. The 550 HP and the 735 HP have the exact same heat exchanger. With all condtions being the same, e.g. sea water temps, and rpm, which engine do you think is going to get heat saturated first? The ability of the fresh water side to off load heat to the raw water side is a function of heat exchanger surface area, raw water temperatures and flow.

I don't know if anyone on the forum can provide you with a comprehensive study, but the educated guess is that there is ample cooling capacity at the lower HP generated at the lower rpms. The problem experienced currently is not enough heat (load) is being generated at the typical hull speed rpm to even bring the engine up to normal operating temperatures. By increasing the loading lower on the rpm scale, you can get the engine up to temp and operate without the wet stacking and other problems associated with running the engines too cold.

[Brian Degulis]

I don't necassarily agree that raw water output is linear I don't know if doubling the RPM of a rubber impeller pump doubles the output I suspect it does not.

I also don't know if heat rejection or cooling demand is linear again I suspect it is not.

I do know that in my case running WOT 650 HP @ 2300 RPM on the OEM boat my temp ran at 185 degrees. After the change running my de rated WOT 525 HP @ 1600 RPM my temp runs just below 180 degrees. So at the fully loaded de rated lower RPM I'm running cooler.

Maybe one of our engineers can give us some info on how RPM efects Rubber impeller pump capacities and weather heat rejection is linear.

Eventide Check the begining of this thread for before and after fuel consumption it is about 4 GPH less at a 10.5 kt cruise.

Brian

[krush]

Rubber impeller pumps are positive displacement (as opposed to centrifugal pumps) and thus the theoretical output is directly proportional to RPM.

The heat transfer that occurs between the two fluids is dependent on MANY factors. To simplify it though, the amount of heat transfered is determined by the temp difference in the fluids and the heat transfer coefficient.

The concern is that not enough raw water is being pumped to keep the engine cool. The dilemma is that we don't know WHAT is causing our hypothetical engine to over heat. We could not have enough raw water flowing OR we could have bad heat transfer in the heat exchanger.

The test to figure out which one is the problem is simple. Measure the OUTLET temp of the heat exchanger. Not enough Raw water flow, the outlet temp is going to be VERY HOT. Fouled heat exchanger, the outlet temp is not going to be excessively hot.

I spoke in all general form, but calculating the heat transfer is very simple stuff if one can estimate the mass flow and measure the inlet/outlet temps on each side of the exchanger.

To be honest, the fundamentals are not difficult to grasp--which can be a bad thing because people "a little knowledge can be dangerous". Regardless it isn't rocket science and heat flows from hot to cold (2nd law, entropy) and what goes in must come out (conservation, 1st law).

Nerdtalk OUT.

[Trojan]

It's a know fact that Detroit's cooling system are on the short side for cooling reserve. Especially in the warmer areas of the country. If you are running slow rpm wise and running slow speed wise. What is the amount of heat exchanger and strainer trash that's your is engine going to tolerate before you over heat? How slow can you run your engine once it's hot. Once it's hot, idling the motor is not going to cool it back down. Because you have used the reserve. Your running on the edge of the cooling system when you lower the fresh water flow and add heat. Increasing the rpm will only add heat. I think You can kiss that motor good by. You may save some fuel, but will you save a motor. I don't think its a good plan. Very chancy. Now if you change the pump pulleys you might stand a chance by increasing the water flow. You would need to at least double the pump rpm in order to achieve the same flow rate that motor had at the higher rpms.

BILL

[bobk]

Re. cooling capacity, If you reduce the amount of antifreeze or eliminate it (if climatic conditions allow), you will increase the specific heat of the fresh water side and thereby increase the ability to carry away heat.

Also the rubber impellor will have a linear output vs. speed, but if there is much back pressure due to restrictions of the various coolers (inter, after, oil, heat exchanger, return fuel etc) there is the potential for the unit output/revolution to decrease with rpm. Brian's detailed measurements clearly show that in his case he is within allowable parameters. I do wonder if the discharged water temperature changes much.

Bob

[MikeP]

I must admit that I can't understand one issue with the prop change. I have been trying since the post first appeared but I still can't figure it out since it seems to contradict everything I ever learned re dealing with engines in many years of building/modifying/dyno testing engines. I will state that all my experience in this area was with gas engines, not diesel AND that all my experience is with trying to make more power, NOT increase fuel mileage. Admittedly, some items (like blueprinting) will do both, but I never tried to make an engine get "good mileage" as a goal.

But here's the part that confuses me...

The prop change resulted in the engine producing 6PSI of boost at the RPM that pushes the boat at 10 knots as opposed to the oem setup that required 0 PSI of boost. It also runs warmer. Both of these items indicate to me that more HP is now being developed to push the boat at the same speed..

6 Psi of boost on a GAS engine will produce about 40% more HP than 0 boost. This 40% more power takes more fuel. I don't know if it works the same way on a diesel since they are not controlled by restricting the air as gas engines are. However, I still can't get my head around an engine that apparently now produces MORE power to get the same speed out of the same boat while using LESS fuel.

Again, I know gas engines and their operation; I am comfortable tuning/rebuilding my DDs. But I do not have the experience with Diesels that I do with gas and perhaps I don't understand the characteristics of a diesel that make it possible to produce more power while burning less fuel in an engine with no mods other than "gearing."

I want to understand it but, as I said, it seems to violate everything I have ever learned and, to me, seems to violate physical laws! However, I am certainly willing to learn. How is this possible?
http://www.samsmarine.com/forums/images/smilies/confused.gif

oops - can't get the "confused" smilie to show up. :(

[Trojan]

A natural diesel can make its HP any place in the rpm range, a gasser can not. So by increasing the prop load and lowering the rpm you generate your high hp. You then save fuel by lowering the rpm. Fuel requirement on a diesel is constant with every cycle. Fuel does not very like the gasser. To raise hp on a gasser you increase fuel and air. On a diesel you just add air.

BILL

[MikeP]

Well, I always thought I was pretty smart, especially with engines but this sounds like magic to me. It's obvious I need to study diesel operation in order to understand this!

If the boat's speed is the same BUT the boost (and the temp) goes up, doesn't that show that MORE power is now being used to push the boat at the same speed? Is it possible to have BOOST and higher temp and NOT make more power and thereby require more fuel?

IF more power is being produced how is that done with LESS fuel? Or does more boost/higher temps occur with LESS power (and fuel use)? Makes NO sense to me.

Maybe I need to do some serious reading re diesels. Again, the crux for me are 2 questions, Can the engine require more boost but NOT produce more power," followed by, "Can more power be produced but use less fuel?"

This is all somewhat rhetorical - I'm not expecting anyone here to educate me with a detailed explanation on diesel operation. I'll go off and do some research on this question. If I can't understand it, I'll just leave it at that and admit I'm not capable of grasping it, sort of like quantum physics. ;)

[krush]

Maybe I need to do some serious reading re diesels. Again, the crux for me are 2 questions, Can the engine require more boost but NOT produce more power," followed by, "Can more power be produced but use less fuel?"

Yes. The brake specific fuel consumption (lbs fuel per Hour per HP) is not constant throughout the operating envelope of an engine.

Let's use simple numbers: and engine making 100 hp may only need 100lb per hour of fuel (that is 1 lb/hp/hr). Now, lower that to 50 hp at 1600 rpm and it may need 200lb per hour (efficiency drops to 2 lb/hp/hr) but make that 50 hp at 1200 rpm (which may REQUIRE MORE BOOST) and you get your power at 1lb/hp/hr.

The reason behind it all is a bunch of theory and efficiency reasons, but to sum it up simply, on your gas motors you HAD to maintain an Air/Fuel ratio somewhere around 13:1 (and under loading, probably richer, like 10:1). A diesel engine DOES NOT CARE about air fuel ratio.

That's the "simple" reason why you can get better efficiency at lower rpm with boost. The combustion dynamics allow for more energy in the fuel to be converted into propshaft HP instead of lost out the exhaust/coolant.

Better efficiency = better mpg

[bobk]

This is all consistant with the experience a friend had with his 43' Kashing powered with 3208N's. He bought a new set of Michigan wheels from Millers Island Props and they turned out to be way oversized. After two iterations on pitch adjustment, he now has a setup which nets more speed at a given rpm, AND lower fuel burn. He also says he still makes the WOT spec, so maybe there is an inherrent fuel efficiency advantage with his wheels.

Bob

[Brian Degulis]

The cooling capacity issue I think in simply a non issue. Now Krush says the pump is linear and I believe him. I'm still not convinced that doubling the raw water flow or the fresh water flow doubles the systems capacity to cool.

Most DDs and most all diesels have several diffrent ratings 2300 RPM is usually the max or pleasure boat rating 1800 RPM usually the continous rating and some compressors and generators run 1200 RPM. The engine itself is basically the same the injector size may be larger to obtain the max rating but the same turbo or non turbo engine can be set at the factory for many diffrent ratings and if it's the same engine at a diffrent rating THE RAW WATER PUMPS ARE THE SAME SIZE. So Detroit is using the same RAW water pump same heat exchanger and same fresh water pump even though the engine is delivering it's HP at a lower RPM.

A continous duty engine running 1800 RPM is producing more HP at 1800 RPM than a pleasure boat rating is at 1800 RPM Yet Detroit uses the same cooling system for both.

So right now I don't know why but my experience has been that a de rated engine producing more HP at a lower RPM apears to require less cooling capacity even when running at the max RPM and HP. My OEM engines ran hotter at 650 HP @ 2300 RPM than they do at 525HP @ 1600 RPM. Perhaps the cooling capacity is not linear to RPM and HP or perhaps water flow raw or fresh is not proportional to cooling system capacity.

When I get a chance I'm going to talk to my Cummins Aplication Engineer and see if I can get an answer as to why.

Brian

[krush]

Hey Brian, I think the reason there is not an increase in temp at lower rpm is because there is just an "abundance" of raw water to carry away the heat.

If you have more than enough raw water flow to carry away the heat at high RPM, then you should have more than enough at lower RPM...all else being equal.

The fix is very simple if one thinks lower RPM creates a problem....just change the pulley size on the pump as mentioned. However, you have empirical evidence that these concerns are unwarranted.

[MikeP]

OK, after reading responses here and on numerous other sites, I have learned that what's going on here does makes sense. I found many explanations that describe the situation. According to them - the additional air (boost) is providing a more efficient fuel burn BECAUSE the fuel injected at whatever the RPM level was not being burned as efficiently as it could have been with more air. So it was apparently a matter of insufficient air for best combustion. So the combustion was not as efficient as it could have been but it wasn't so bad that black smoke would result. Modern engines with electronic controls have a much more precise fuel/air metering capability and that's why they are more efficient at a wider range of RPM.

THAT makes perfect sense to me! No additional fuel was added, as would have been with more boost on a gas engine. I was assuming that more boost automatically meant more fuel but it is not necessarily the case with a diesel, as was explained by others here and on the various sites I visited.

[Pete]

Many marine DD engines have gear driven raw water pumps mounted in one of the rear accessory drive ports. In these cases someone wanting to increase raw water flow would likely swap their current pump for one with greater capacity.

Pete

[Brian Degulis]

Hey Brian, I think the reason there is not an increase in temp at lower rpm is because there is just an "abundance" of raw water to carry away the heat.

If you have more than enough raw water flow to carry away the heat at high RPM, then you should have more than enough at lower RPM...all else being equal.

The fix is very simple if one thinks lower RPM creates a problem....just change the pulley size on the pump as mentioned. However, you have empirical evidence that these concerns are unwarranted.

The problem with changing the pump speed is that most DDs have a gear driven pump. Take a look at this performance chart the output is not quite linear or proportionete to RPM

[Brian Degulis]

OK, after reading responses here and on numerous other sites, I have learned that what's going on here does makes sense. I found many explanations that describe the situation. According to them - the additional air (boost) is providing a more efficient fuel burn BECAUSE the fuel injected at whatever the RPM level was not being burned as efficiently as it could have been with more air. So it was apparently a matter of insufficient air for best combustion. So the combustion was not as efficient as it could have been but it wasn't so bad that black smoke would result. Modern engines with electronic controls have a much more precise fuel/air metering capability and that's why they are more efficient at a wider range of RPM.

THAT makes perfect sense to me! No additional fuel was added, as would have been with more boost on a gas engine. I was assuming that more boost automatically meant more fuel but it is not necessarily the case with a diesel, as was explained by others here and on the various sites I visited.

It goes even further than that. A turbo engine has around 95lbs less compression than a natural. So if a trubo engine is operating with no turbo boost at all it's running like a very worn out natural. There is not enough compression and heat to get a complete burn. So the same amount of fuel goes in but it gets used much more efiecently your also saving some by turning the engine transmission shaft and prop slower less frictional loss. Then you also gain with a larger dia prop which is more efiecent at displacement speeds

Brian

[Trojan]

My 6-71s have the water pump belt driven off the front of the motor. So you could add an additional pump and have it operate in parallel. The more air you stuff in a diesel the more HP you generate. Providing you have the fuel to burn with it in the proper ratio.
So now were back to the bigger props again. Big 3 blades take less HP than 4 blades. A big pitch requires more HP than a small pitch. And on and on.:D

BILL

[REBrueckner]

Mike P posts:"...So it was apparently a matter of insufficient air for best combustion. So the combustion was not as efficient as it could have been but it wasn't so bad that black smoke would result..."

Yes, that's a big part of the equation. The turbo boost pressure at the reduced RPM (versus none at all previously) makes a difference!! Turbos work!! As noted above, slower RPM turning at cruise also likely means some reduced friction. But torque at the reduced RPM is higher than it was originally at the same RPM, so wear is NOT reduced proportionally. The larger prop at lower RPM should be more efficient than the older smaller prop at higher RPM. Cooling raw water flow at reduced RPM so the raw water pump uses less HP and experiences less wear.

Another way to potentially redesign for slower cruising and extend the above concepts is to put in a larger reduction drive...say 2:1 INSTEAD OF 3:1. Closer to trawler design....Now you can swing an even bigger prop, but few installations permit much of an increase in prop diameter so this approach has limited practical value.

None of the above suggests getting something for nothing: all that's happening is that instead of the engine being tuned for efficiency at say 1800to 2000 RPM and perhaps 400 HP, the changes result in more efficiency at say 1200 RPM and 225 HP (or whatever the actual numbers are). So Brian's changes boost HP to match the new and larger prop at his chosen/designed lower RPM, but prevent him from achieving higher RPMS and HP.

This is analogous to taking a really big cubic inch displacement engine and tuning it to run at low RPM...exactly the design of older, heavy duty naturals before they had the technology/metals/maching to handle high speed RPMs.

[REBrueckner]

Brian,
I have been meaning to ask: How does one measure turbo boost pressure? I'd love to get one reading at my normal reduced RPM cruise speed to see what it is.
Thanks,

[krush]

The problem with changing the pump speed is that most DDs have a gear driven pump. Take a look at this performance chart the output is not quite linear or proportionete to RPM

Brian, that is as about as close to linear as one can get in the real world! It's almost a line (all lines are strait) in the operating range. I stated "therotically" perfectly linear for a reason...nothing in the real world is ever going to be perfect--we know that :)

Compare that to a curve from a centrifugal pump...looks like a prop curve, right? It's because it's basically the same thing LOL

http://www.engineeringtoolbox.com/docs/documents/1114/centrifugal-pump-speed-torque.png
http://www.engineeringtoolbox.com/pumps-speed-torque-d_1114.html

There are also other differences between the pos displacement and centrifugal...a positive displacement doesn't decrease flow with height (or pressure), it just requires more HP.

[Brian Degulis]

Yeah I know it's very close I'm just looking for reasons. I'm thinking that the efect of the rate of flow of the fresh water and raw water is not proportional to cooling capacity. I bet doubling the flow does not equal double cooling capacity if that's true then reducing the flow would have less efect.

Brian

[Brian Degulis]

Brian,
I have been meaning to ask: How does one measure turbo boost pressure? I'd love to get one reading at my normal reduced RPM cruise speed to see what it is.
Thanks,

It's measured at the turbos air outlet in Hg or inches of mercury you can use a simple psi gauge and just convert the reading.

[krush]

Yeah I know it's very close I'm just looking for reasons. I'm thinking that the efect of the rate of flow of the fresh water and raw water is not proportional to cooling capacity. I bet doubling the flow does not equal double cooling capacity if that's true then reducing the flow would have less efect.

Brian

Well you are right. The two governing factors are the conduction across the heat exchanger and is there enough raw water to carry the heat away.

The amount of heat carried away by the raw water is just the massflow*specific heat*change in temp. If you decrease the massflow, the temperature change will increase.

Thus if the exit temperture gets too high, it can "clog up the system" because the rate the heat conducts is driven by the temp difference between the coolent and raw water (think of it as voltage or pressure....you need pressure to move fluid).

So cliff notes: there is usually a limiting factor in the system, but it's hard to say weather it is raw water flow, fouling, or heat exchanger area unless some simple temperature measurements are made.

[Trojan]

I would worry more about the exhaust temp then the actual block temp. I would think that at the lower rpms there would be less water flow and cooling through the shower heads and exhaust manifolds. It seems that it all boils down to the question, is there enough water flow to properly cool the motor at the lower rpms.

BILL

[Brian Degulis]

OK I spoke to a DD Aplication engineer acording to him all DD are designed to run at full rated power throughout the RPM range. That's full rated power not the proppler curve. So the cooling concerns are a non issue.

Brian

[Trojan]

GOOD LUCK!

Bill

[Pete]

I know, I know, I wrote that I would retire from this subject. So, I will do nothing but suggest that a discussion with Covington or J&T would probably be helpful as they did most of the marine application work on the Detroits that Hatteras used. BTW, back in the 80's I attended a Covington school in Greensboro, NC and remember seeing a trailer load of base engines being unloaded. At the time Covington had a small building doing nothing but marine conversions for Hatteras and a few others. They did the cooling systems, along with turbo selection, injectors, exhaust manifolds, etc.. Talk about easy breakin, when an engine was completed, they took it over to the dyno, warmed it up for maybe a couple of minutes and from then on it was WOT. A real show for DD fans.

Pete

[luckydave215]

What we need is one of these in our size.

http://www.autoprop.com/autoprop/international/index.html

I installed one on my last sailboat (38ft, slippery racing hull design, 14,000 pounds, 27 horse Yanmar) and the improvement was dramatic. The boat went from 5.5 knots@3000 rpm to 7 knots@ 1800. Fuel consumption was so small that I went 5 years on 12 gallons, and ran the boat almost every weekend (sailing most of the time of course)
Drag when sailing was reduced 85% because the prop feathers. That would be useful for running on one engine.
The cool thing about this prop design is that its a "constant angle of attack" design, not a "constant speed" design like we're used to on the airplanes we fly. When you apply 10 horsepower to the shaft, the prop pitches to absorb 10 horsepower, and so on. The faster the water is flowing through the prop, the higher the pitch. They work exactly the same in reverse as forward, a real plus for sailboats that don't usually back down worth a crap.
It takes some getting used to approaching the marina, as the boat doesn't slow down when you pull the throttle to idle, the prop pitches up and still pushes the boat with the available power at that engine speed instead of dragging the boat down to "idle speed"
In 10 years of regular use, I had no issues or maintanance different from a fixed pitch prop.
These have been fitted to trawlers with good results, but I don't think they make one in a 1000 horsepower size yet....

[Brian Degulis]

I know, I know, I wrote that I would retire from this subject. So, I will do nothing but suggest that a discussion with Covington or J&T would probably be helpful as they did most of the marine application work on the Detroits that Hatteras used. BTW, back in the 80's I attended a Covington school in Greensboro, NC and remember seeing a trailer load of base engines being unloaded. At the time Covington had a small building doing nothing but marine conversions for Hatteras and a few others. They did the cooling systems, along with turbo selection, injectors, exhaust manifolds, etc.. Talk about easy breakin, when an engine was completed, they took it over to the dyno, warmed it up for maybe a couple of minutes and from then on it was WOT. A real show for DD fans.

Pete

What's being overlooked is that the engine is rated to do what the maufacturer says it can. If you look at the power curves it's telling you that the engine is capable of producing X HP @ X RPM. The propeler curve is showing you what is actually being produced with a typical prop it has nothing to do with the engine and it's cooling systems capacity. So acording to DD if you stay within the HP ratings at various RPMs then you cooling system will handle it. In my case I'm not theorizing I've already done it and can tell you it works at the de rated WOT I'm running cooler than the OEM WOT.

As I said when this came up it's a good observation and it's good to kick things around but it's no very productive to invent problems that don't exist and this is getting to that point.

Brian

Brian

[Boss Lady]

I don't think everyone read your posts thoroughly, or have followed prior threads where this was discussed, before you actually did it.

[krush]

As I said when this came up it's a good observation and it's good to kick things around but it's no very productive to invent problems that don't exist and this is getting to that point.

Brian

Brian, it's an online forum--internet engineers are just part of being in this territory! :rolleyes:

[Brian Degulis]

Brian, it's an online forum--internet engineers are just part of being in this territory! :rolleyes:

I understand that and people throwing around ideas is the purpose of a forum like this I'm not complaining just throwing out my thoughts as well. Did you ever get into a discussion with someone maybe about politics and the person says look (I know I will never convince you and you will never convince me) I walk away from people like that. I can always be convinced and if you manage to do that then I'm the lucky one I gained something from the experience. People that cannot be convinced are usually pretty easy to spot because they present there ideas as facts. They don't start with I think or this is the way I see it they put it up as if it's fact and they never come back and say OK you convinced me they just don't respond when there idea is proven wrong. I often wonder if they change there minds and just don't say that or if they go on believing the wrong thing? These forums are as much a study of human nature as they are about boats.

Brian

[Pete]

OK, this time I will really retire from this thread. In departing, let me just mention a few facts. First, Hatteras used Covington conversions in over 95% of their boats back when 2 cycle engines were installed. DD did not do the cooling system, injector selection, turbo selection, heat exchanger selection, raw water pump selection, etc,etc. Covington did. All that DD is saying is "setup and operate our base engine within our spec. and you will be OK". Covington marinized these engines to drive a boat along the propeller curve. That is why they can crank the horsepower up on engines and have them hold together pretty good. Do what ever makes you sleep at night, but the fact is DD can not speak to the application details of a Covington Marinized engine, only Covington can.

Krush and Brian, I know you are not referring to me with your "internet engineers" comments, as I am a real one complete with the diploma saved with my other important papers. But, it doesn't take a engineering degree to understand what I have written above, just common sense. But then, common sense is often not very common.

Pete

[Trojan]

If you want to run that way that is fine with me and I agree you can run that way. My contention are five. One I think the props are to small and one could even do better with bigger props, less pitch. I know money is a big factor, props are not free. Two, Detroit's cooling systems have always been marginal on cooling on the upper RPM range. Especially in the sun belt. Running on the top end leaves no room for contamination of the system. If you over heat at all, you must reduce RPMs to such a slow rpm to recover that you will have very little forward motion to get you home. But, as long as you are not making any Atlantic crossings. I guess you will be OK.:D Three, you are not going to out run any storms or ruff seas and a following sea could really raise hell. Four, docking or turning around in a crowded marina could be a bear. Five, If I wanted to save fuel money and run blowboat speeds I would have purchase one.:D In a few months of running from now tell us how you like it. Maybe you got something going. As always it's just my opinion. I hope it works for you. In this day and age we need to experiment more. As said before, Good Luck.


BILL

[krush]

Krush and Brian, I know you are not referring to me with your "internet engineers" comments, as I am a real one complete with the diploma saved with my other important papers. But, it doesn't take a engineering degree to understand what I have written above, just common sense. But then, common sense is often not very common.

Pete


Pete, I won't speak for Brian (but I bet he thinks like me), but I agree with you 100% and you most likely already know that--just reassuring.

My comment was just a generalization to ALL internet forums (and real life)...apply it as you see fit :).

Brian, I still find it funny that you catch so much flak from people. I guess they are clueless to the fact that what you did (derating an engine, changing prop) is done everyday in the commercial sector, right?

[Brian Degulis]

Pete If Covington used the prop consumption curve to size the cooling system I would think they would have an obligation to provide you with a power curve that shows that. They are a Detroit Diesel dealer I don't think they would have gotten aproval from Detroit the manufacturer who has to warranty it to do that. If covinton's cooling systems with whatever diffrences they may have is capable of cooling the engine at the max rating it should be able to cool the engine throught the rpm range at the engines rating. If it truly can't I would think they would open themselves up to an awfull lot of liability. Your an engineer with a degree would you design it that way?

Bill I agree more diam and less pitch would work better that decision was based on economics. As far as running at the top of the scale I'm not. At 10.5 kts cruise I'm capable of producing 350 HP and only using 150 HP. My new de rated WOT is 525 HP @ 1600. So I need 300HP to cruise and I've got 1050 HP available hardly running at the top of the scale. In addition when I run @ 1600 RPM I'm running cooler than I did at the OEM 2300 RPM. As far as out running weather sure I can't plane anymore I can Only do 14kts instead of 18kts that's one of the negatives. As far as maneuvering in harbors if your reffering to a little to much power at idle your right another negative. Those are simply decisions you make based on how you use your boat. If you want to go fast this obviously isn't for you. But if your one of the many people that run there boat at or near displacment speed. Doing this will make you run smoother and quieter use less fuel and run your engine in a much more favorable condition that should greatly extend life.

Brian

[bobk]

Very interesting thread to me. I am currently trying to figure out what props to use on my 48MY. Most of my running is done at 10kts. I have to turn 1500rpm to acheive 10kts. I would like to acheive 10kts at 1000 rpm and have the eng. temps at 180. I am now running 26x21x4bl props with 1.5:1 Allisons. 6v92's rated at 425hp.
Chris


Chris, did you ever do anything with your props? I have 26X22 four blade in my 48MY (same engines as yours), Walter has 26X23 four blades in his but probably his 6V92's are rated higher than ours. I'm thinking of increasing pitch for better performance at 8-9 knots. Interestingly the Hatt Owners manual for my 48MY calls for 6X23 four blade. At 1000 rpm I reach 7.8-8 knots. The test report on the boat showed 8.3 knots at 1000 rpm, but the test boat was not stabilized like mine, so I expect some loss.

Bob

[krush]

Thread back from the dead? This is one of the best threads on this site LOL

[REBrueckner]

BobK, Chris
Interesting that your newer 48 MY has 1.5:1 transmissions and approximately 26 x 22 props.

My older 1972 48 YF has 8V71TI's at 435 HP, close to your 425, but I have 2:1, maybe 1.91 to 1, I forget right now, and I am propped with 28 x 32 four bladed props, no cup. I turn up only 2150RPM instead of 2300 WOT.

Increasing pitch will increase your load at lower RPM as will cupping blades. I cruise about 11 knots at 1500 RPM and only about 500 degrees turbo temp, via infrared thermometer test, so I do not like to go lower cruise RPM.


I wonder why Hatteras changed the reduction drive for smaller props in your newer boats: engines,boat, or both...What's the rated RPM of your 6V92's?

[chris]

Chris, did you ever do anything with your props? I have 26X22 four blade in my 48MY (same engines as yours), Walter has 26X23 four blades in his but probably his 6V92's are rated higher than ours. I'm thinking of increasing pitch for better performance at 8-9 knots. Interestingly the Hatt Owners manual for my 48MY calls for 6X23 four blade. At 1000 rpm I reach 7.8-8 knots. The test report on the boat showed 8.3 knots at 1000 rpm, but the test boat was not stabilized like mine, so I expect some loss.

Bob
Bob
Still running my 26x23's. I am looking for a used pair of 29x29 or 30x30's. I want to try a used pair to see how the boat performs before spending $$$ on new.
My speed is the same as yours at 1000rpm. I also have Niads. My water temp runs just under 180. How about yours.
Chris

[chris]

BobK, Chris
Interesting that your newer 48 MY has 1.5:1 transmissions and approximately 26 x 22 props.

My older 1972 48 YF has 8V71TI's at 435 HP, close to your 425, but I have 2:1, maybe 19.1 to 1, I forget right now, and I am propped with 28 x 32 four bladed props, no cup. I turn up only 2150RPM instead of 2300 WOT.

Increasing pitch will increase your load at lower RPM as will cupping blades. I cruise about 11 knots at 1500 RPM and only about 500 degrees turbo temp, via infrared thermometer test, so I do not like to go lower cruise RPM.


I wonder why Hatteras changed the reduction drive for smaller props in your newer boats: engines,boat, or both...What's the rated RPM of your 6V92's?


Rob
I'm not sure why they changed gear ratios. I know the hulls on the 48my's are unique. More of a displacement hull. Maybe that had something to do with it.

Chris

[bobk]

Bob
Still running my 26x23's. I am looking for a used pair of 29x29 or 30x30's. I want to try a used pair to see how the boat performs before spending $$$ on new.
My speed is the same as yours at 1000rpm. I also have Niads. My water temp runs just under 180. How about yours.
Chris


Chris, your original post said you had 26X21 four blades that's why my interest. If indeed they are 26X23, that is the same as my owners manual and Walter's boat. Mine are clearly 26X22 but I believe there is a (small?) cup in the pair that is mounted on the boat. The spares are not cupped.

I run at 160-165F at 1000 rpm. It gets to 180F and holds at 1800 rpm and above regardless of the sea water temperature. That suggests a 180 thermostat, but why would your hold 180 at low rpm???? Any idea what the thermostat is? And does it maintain 180 at high rpm?

Anyway, any guidance guys? I'm going to pull the props because of a vibration and might as well make them more efficient if I can. I run strictly as a trawler for the economy, except to blow out the engine every day or so. And by the way, if I blow it every four hours, there is virtually no smoke, if I wait for 10 hours, I get a puff initially which trails off to nothing after 30-60 seconds, even before the temperature gets up. If for what ever reason twenty or so hours passes before blowing them out, it takes several minutes and the temps will have come up before it clears.

Bob

[chris]

Chris, your original post said you had 26X21 four blades that's why my interest. If indeed they are 26X23, that is the same as my owners manual and Walter's boat. Mine are clearly 26X22 but I believe there is a (small?) cup in the pair that is mounted on the boat. The spares are not cupped.

I run at 160-165F at 1000 rpm. It gets to 180F and holds at 1800 rpm and above regardless of the sea water temperature. That suggests a 180 thermostat, but why would your hold 180 at low rpm???? Any idea what the thermostat is? And does it maintain 180 at high rpm?

Anyway, any guidance guys? I'm going to pull the props because of a vibration and might as well make them more efficient if I can. I run strictly as a trawler for the economy, except to blow out the engine every day or so. And by the way, if I blow it every four hours, there is virtually no smoke, if I wait for 10 hours, I get a puff initially which trails off to nothing after 30-60 seconds, even before the temperature gets up. If for what ever reason twenty or so hours passes before blowing them out, it takes several minutes and the temps will have come up before it clears.

Bob


Bob
I am running 26x21's. My mistake.
I run just under 180 at 1000 rpm. At 1850 - 1900 it runs 185. Those temps are off manual gauges in the er. I very rarely run above 1000 rpm so I am happy with the temp.
I have a vibration in noticeable in my rear st rm. It is at higher rpms. The po of my boat had a vibration test done on the boat and it talks about a few different things that were causing it. One reccomendation they made was to stiffen the hull where the intermediate strut is bolted.
PM me your email address and I will scan and email you the test report. It is several pages .
Chris

[REBrueckner]

Have never heard of a Vibration Test....Is it a formal procedure and or test debvice??? can someone explain what's involved...how it's done....can it isolate different sources???

[chris]

Have never heard of a Vibration Test....Is it a formal procedure and or test debvice??? can someone explain what's involved...how it's done....can it isolate different sources???

Rob
They hook several sensors to different location on the boat. Then ran the boat from 900 to 2300 rpm in and recorded the vibration readings in 100 rpm increments.
If you are interested in seeing the data from mine pm me your email address and I will email it to you.
Chris

[REBrueckner]

Chris,
Thanks for the offer, I'd likely not know what the results mean. Sounds interesting.
Do they have benchmark's for comparison?/ In other words, how helpful are the sensors in isolating an actual problem? Did they make specific interpretations for you....For example, I'd assume the aft most strut adjacent to a prop gets more vibration that an intermediate strut in a drive drain that's ok...so it would be the additional vibration variations beyond those typical variations that would most likely be relevent....