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.

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...