Looking at 56 Hat's 8V92's Problem??

[Danali Star]

We are looking to move aboard fulltime to a larger boat once all of our rental property in Melbourne, FL sells (which has slowed lately, very frustrating). We are currently on a 43ft Gulfstar trawler with twin 100hp Yanmars and burn a whopping 1.5gph total at 8 knots, and although it is great for spending Nov thru June in the Abacos, we feel it is too small to live on full time. We have ruled out many of the 58's Hats prior to 1985 due to either loss of side decks for the size main salon we prefer, or the smallish "feeling" main salon in the yachtfish (vs 18ft beams) to keep side decks. We are leaning heavily toward the Hatteras 56 motoryacht in order to pick up the 18+ft beam with side decks, as well as other features, mainly large sun room/aft deck, larger main salon open to helm, galley down, only the main stateroom is aft of engine rooms, (we like the 58 motoryacht 1985 and later for the same reasons, but they push the price envelope). We would love a cockpit, but that pushes us into 12 cylinders (61cmy etc), and I don't think I am prepared for that. Both the 56 and post 1985 58's seem to be powered with the 8V92's. I have read extensively on several forums, and the frequent comparison to a hand grenade with a loose pin is concerning with the 8V92's. I have zero experience with them. Right now it is my biggest concern.

We do not plan to need speed, we run at hull speed 99 percent of the time in our current trawler, and expect to do the same in whatever we move to next. Fuel consumption is certainly an issue, but perhaps not as much as potential rebuilds on a used boat. One particular 56 on Yachtworld, Party Hatt, claims to do around 20 gph at 12 knots at 1700 rpm. If that example is accurate, I wondered if fuel consumption could be cut and engine life extended with these engines similar to what a friend of mine did with his 8V71's on a 58 Yachtfish. He took off the turbos, downsized the injectors and gets slightly less than 1 gallon per mile, running at 10 knots. OR...the other argument often discussed, decreasing the prop pitch to accomplish similar results. This change won't make or break the decision but if it could be done with the added plus of an extended engine life between rebuilds, I would certainly do it. I would also certainly consider a repower if a more suitable power plant would make more sense, both to engine life and efficiency as I plan to have this boat at least 10+years. Relative to repower, I was concerned about the resale if I repowered for hull speed and efficiency as a potential problem if and when I decided to sell the boat. I thought it might be important to be able to put the turbos back on, original injectors, etc in case the potential buyer wanted the original engine built for more speed. Any and all suggestions and comments are welcome, as we will be making this important decision sooner or later, but we hope to be ready sooner rather than later. It would be great to hear from 56 owners about things they like and don't like about them. Not sure how many were made, but you don't find many for sale. You get a total of only 6 or 7 for sale on yachtworld, versus pages of 58 yachtfish, 58 motoryacht pre 1985, 58 cmy, etc. Is this a sign of people loving their 56's and simply holding onto them.

[Genesis]

Leave the enignes alone and run 1100-1200 RPM.

You'll get 0.7-1.0nmpg on fuel and still have the ability to get out of the way of weather and such if you need to (and are willing to burn the fuel.)

That's how I'd go at it. There's nothing wrong with the 8V92s provided they are not too tightly-wound. Many of them are, but its the idiot with his hand on the throttle that causes the problems, along with a "gotta get 1/2 knot more" nonsense that many owners play.

[Danali Star]

[QUOTE=Genesis;68567]Leave the enignes alone and run 1100-1200 RPM./QUOTE]


Thanks, Genesis......That is excellent news to my ears!!! Thanks for the reply. I had all sorts of dreadful visions going thru my head. Simplified to a T...........

[SKYCHENEY]

I run my 6v92's at about 1400rpm for slow cruise with the occassional shot up to 1950 for 15 minutes or so. They have over 3500 original hours and still seem to be going strong. I would not be affraid of any 92 series engine as long you know that the PO wasn't abusing them.

The problem is that the 92's were not holding up well when they were first introduced. This gave them a very poor reputation and cost GM a bunch of money, not to mention customer loyalty. We owned several of these in trucks and they did not last long (the green ones). Then DD came out with what they called the "silver series" (painted white). These had several improvements and probably every marine version out there has been upgraded to those specs by now.

[Passages]

Agree, the 1978 - 1982, 8-92's had some serious issues but these have been corrected by now.

I don't know about the hand grenade assessment. As long as the HP does not exceed CID, you should be able to expect a reasonable service life.

[chris piazza]

Ive got 8v92's in my 53C. Try and stay in a lower HP rated engines, mine are rated at 550hp, some are even down to 450hp. If you get the 750hp rated 92's as Genisis said these are wound to tight and are short lived. I think the key is a good maintenance program, 1100-1200rpm cruise for 1mpg with the occasional run up to 1950. Also if the boat you chose doesn't already have the Racor CCV'c installed on the 92's this should be your first project

Chris
Superior Nights 53C

[StratPlan61]

The US Army has thousands of 8V92s in their HEMETT 10T truck. It is a heavy duty 4 whl steer, 8 wheel drive workhorse, produced in a dozen configurations. These engines are rated at 450-500hp and enjoy a reliable reputation. As others have noted, prudent operation and a motor configured in a reasonable HP version will outlast most of us. I looked at several 56s before I bought my boat, the 8V92 motors would not be a concern to me.

[Danali Star]

Ive got 8v92's in my 53C. Try and stay in a lower HP rated engines, mine are rated at 550hp, some are even down to 450hp.

Chris
Superior Nights 53C


How do you determine how one is rated. That is, what determines the HP rating. If you are looking at an ad, is there anything that tells you how it is rated other than the "HP" block on Yachtworld where they fill in the blank.

[Brian Degulis]

"Leave the enignes alone and run 1100-1200 RPM"

Yep what Genisis sugested is exactly what I do with 1271TI in a 61MY and get around that kind of burn. I use a cetane booster in the fuel and always run it up a bit for at least a little while each day just to clear it out.

I wouldn't consider removing the turbos I'm pretty sure the TI engine is lower compression than a natural which means that if you needed to run hard your probably be running less eficiently due to incomplete burn.

If you want to consider doing anything with the props you would have to do increase pitch or diameter preferably diameter. That will load the engine closer to rated load at lower RPM but will also cause overload at some point before WOT depending on how much bigger you go. So in efect you will be de rating the engine to a lower HP and RPM. You can get a pretty substantial gain in economy this way (I did it with one of my past boats) but of course you got to balance that against the cost of props and the potential to have to change it back if you want to sell it.

Before I got our 61' lots of people told me that taking a boat that's designed to run well above hull speed and running at hull speed would result in poor preformance and sea keeping ability. I don't see that at all my boat runs very well at hull speed. Most so called trawlers are semi displacment anyway and I think the shear weight of a well built boats like these goes a long way.

Brian

[hattitude]

If you want to consider doing anything with the props you would have to do increase pitch or diameter preferably diameter. That will load the engine closer to rated load at lower RPM but will also cause overload at some point before WOT depending on how much bigger you go. So in efect you will be de rating the engine to a lower HP and RPM.


I'm no expert, but isn't this advice backwards? I thought to lower the load on an engine, you need to make it reach over max rpm (50 to 100) by decreasing pitch or diameter. I thought any engine that doesn't reach max rpm is overloaded throughout it's rpm band, not just at the top.......

Or did I read this wrong...?

[Brian Degulis]

No you read it correctly by going to a larger prop you will not be overloaded thru the entire RPM range. If you take a look at an engine/propeller power curve you will see that the engines power curve is pretty linear. In other words if you have an engine that is capable of producing 200 HP @ 2000 RPM then it's capable of producing about 100 HP @ 1000 RPM. If you look at the propeller curve you will see that's it's not linear if you have a propeller that requires 200 HP to drive at 2000 RPM at 1000 RPM it requires around 50 HP. So in this example when turing the prop at 1000 RPM the engine is only loaded to 50% of it's rated load for that RPM.

Most boats are propped to allow the engine to develop full rated power but at anything less than full rated power the engine is under loaded the lower the RPM the more under loaded it is. Being under loaded is not a realy a bad thing but it's not the best and most efiecient thing either.

What the original poster is considering is how to get the best efiecency at hull speed. By increasing the prop diameter or pitch he will get more thrust at a lower RPM. The engine will be closer to rated load at a lower RPM and he will get the benifit of a larger slower turning prop. These things combined will equal better fuel milage and longer engine life when running at hull speed.

Butt there is no free lunch by doing this he will no longer be able to run at the RPM he could with the original props. He will change the propeller power consumption curve it will now intersect with the engine power curve at a lower RPM and this will be his new and lower WOT. How much lower depends on how much larger the new props are. The governer should be adjusted to this new RPM to prevent the posability of overloading.

So basically you have a boat with engines that are way to big for running at hull speed. Changing to smaller engines isn't practical so look at this as customizing an engine propeller combination that will make running at hull speed more practical and efeicent. But you will lose the ability to use all the power the engine is capable of producing. Usually when this is done it's set up so the engine is now fully loaded at 1800 RPM (the work boat rating) instead of 2300 RPM the pleasure craft rating.

I should qualify my statments I have done this with DD Naturals and I know it works. With TIs I believe you would also have to factor in the turbo boost to insure that at hull speed your not at that point where your loaded up but turning to slow for suficient boost.

Brian

[Genesis]

This cannot be done successfully with turbo engines as they will not spool up until 1600-1800 RPM. Running without boost under heavy load at low RPM will lug the enigne.

The best bet with turbos if you want to "de-rate" the engines is to DOWNPITCH the wheels. This will reduce the demanded output and thus the boat's speed, moving the RPM for a given speed higher. It will not help fuel burn materially however.

If you intend to operate at displacement speeds this is a waste of time and money. It really only applies as a "good idea" if you intend to operate at planing speeds but wish to "de-rate" the engines - it is the cheapest and easiest way to do so, and involves only changing the wheels.

[Danali Star]

[QUOTE=Genesis;68637]This cannot be done successfully with turbo engines as they will not spool up until 1600-1800 RPM. Running without boost under heavy load at low RPM will lug the enigne.QUOTE]

Genesis,

I plan to run pretty much at displacement speeds most of the time. If I am running at 1100 to 1200 rpms is this not considered lugging the engine down? I understood your advice was to run at these lower rpms and leave the engines alone. Is harm done by not allowing the turbos to "spool up" while underway?

Also, in your last paragraph you mentioned something being a waste of time. I believe you were referring to making changes thru the props but wanted to be sure.

Dan

[JLR]

I have the 56 MY with the 892's. Bought the boat with 1000 hours and rebuilt both engines when they hit 2,000 hours. Contrary to the wise advice on this forum, I run mine at cruise speed, (200 off the pins) all the time and think the boat and the motors are great. I burn a total of 47 gallons per hour at cruise and generally run about 16 kts. That type of cruising is what suits me. As far as the boat is concerned, the more people you put on it, the bigger she gets. It is a perfect boat from our perspective. Hope this helps.

[Brian Degulis]

[quote=Genesis;68637]This cannot be done successfully with turbo engines as they will not spool up until 1600-1800 RPM. Running without boost under heavy load at low RPM will lug the enigne.QUOTE]

That's not correct in a stock boat they spool up @ 1600-1800 RPM because that's where the engine gets closer to rated load. Turbo's react to the heat that's created by the load not just RPM. If you increase prop size you lower the RPM at which the engine is coming near rated load that creates heat and causes the turbo to provide boost at a lower RPM.

You need only to look at the engines power curve and you will see that @ 1200 RPM you are using a lot less power than the engine is capable of safely producing (without lugging).

Brian

[Genesis]

[QUOTE=Genesis;68637]This cannot be done successfully with turbo engines as they will not spool up until 1600-1800 RPM. Running without boost under heavy load at low RPM will lug the enigne.QUOTE]

Genesis,

I plan to run pretty much at displacement speeds most of the time. If I am running at 1100 to 1200 rpms is this not considered lugging the engine down? I understood your advice was to run at these lower rpms and leave the engines alone. Is harm done by not allowing the turbos to "spool up" while underway?

Also, in your last paragraph you mentioned something being a waste of time. I believe you were referring to making changes thru the props but wanted to be sure.

Dan

No, running there is fine. But I would not change the props with your intended operational regime. The gains from doing so in terms of fuel consumption will be modest and you will destroy the ability to run the engines up if it ever becomes necessary (e.g. to get away from a storm)

My point on pitching DOWN (depitching the wheels) is that if (1) you intend to run on plane and (2) are unhappy with your engines being too high-strung the cheapest way to address this is to depitch the props. This will unload them and effectively "turn them down" in maximum output without changing anything else (e.g. injectors, etc)

[ADaily3224]

Removing Pitch:

It's easier on the engines, if you drive your standard shift car up a hill you'll need to down shift to allow the engine to run at a higher RPM but with less load.

If you remove pitch from a propeller it's like shifting down a gear. You won't save fuel but your engine life will be extended.

Your boat is going up hill all the time, the more weight and growth on the bottom determines how steep the hill is. At low speed it has little consequence, it's the opposite at high speed.

Tony D

[Trojan]

Turbos don't operate on heat expansion. They operate on exhaust gas pressure and volume... That is why you have such a difference in exhaust size, input verses output. You can't drive up a turbo at low RPMs (spool up). But you can lug a diesel at the low end and build up enough heat to cook everything and never spool up the turbo. Lugging at low RPMs is bad for any diesel. Unless you run your boat by thermal couples. Loading your engine by over propping at low RPMs is asking for trouble. As Genesis states the fuel use will not get better but most likely get worse. Because of the linearity that is the reason they tune at WOT. This prevents over loading through the entire HP range. HP does not move the boat Torque does, but to maintain the torque you must have the HP. Look at the torque range and then try to balance the prop to the torque/HP. curve. Then you might save some fuel at a lower range RPM.
BILL

[doc g]

I would take the 1271tis over the 892tis any day . The 61 is a great boat and with a liveaboard I would not want to be with out the cockpit. We spend more time in the cockpit than almost anywhere else on the boat. The access to the water ,line handling,fishability,diveabilty etc are things that make the CMY's so nice. I personally will never own another motoryacht without a cockpit . Besides what's difference in cost of a rebuild 8 vs. 12 cylinder. The 12s will last longer before rebuild anyway and most that are on the market have been rebuilt at least once by now. Prices are about equal 56 vs 61 also. Of course I'm biased and all this is just IMHO!!!..........................Pat

[krush]

Turbos don't operate on heat expansion. They operate on exhaust gas pressure and volume... That is why you have such a difference in exhaust size, input verses output. You can't drive up a turbo at low RPMs (spool up). But you can lug a diesel at the low end and build up enough heat to cook everything and never spool up the turbo.

Your statement is almost 100% incorrect. Turbos operate off excess heat energy in the exhaust gas stream.

The turbine allows the exhaust to expand and extracts work from heat. The enthalpy (simply put, heat energy) change is do the the change in temperature and expansion of the gasses against the turbine (modeled as isentropic expansion usually). Mass flow is constant....velocity has almost no effect on the whole thing.

A turbo can spool at low rpm.

[MikeP]

"Turbos operate off excess heat energy in the exhaust gas stream."

Only incidentally...Heat alone won't turn a turbo at all; heat a turbo to any temp you want and it won't turn. However, direct cold air from an air compressor against the impellor and off she goes!

What moves the turbo isn't the TEMP of the exhaust gas, it's the velocity and volume of the gas. The turbo could care less if the gasses impinging on the impellor are hot or cold; if they are sufficient, the turbo will spin.

Certainly the hotter the gasses in the combustion chamber, the more they are expanding and the greater the subsequent pressure. But it is not the heat of the gasses that make the turbo turn, it's the pressure of the gasses. The fact that they are HOT is a product of how they are created.

[Trojan]

They will spool only if the primary is designed for low gas flow. If you have low gas flow and low velocity you have no movement. Heat expansion is not the only thing that turns the turbine blades. The expansion adds to the drive force. Without gas flow you have no turbine. The different size of pipes from the inlet to the exhaust lets the gas expand. It just happens to have the turbo blades in expansion chamber. In most cases until the engine is producing a pre designed exhaust flow there is not enough gas velocity to create an expansion to turn the turbo.

BILL

[krush]

The "pressure" of the gas is the result of the heat.

If you take the exhaust pipe from the head and somehow cool the air to ambient before it reaches the turbo, you will build no boost. You just sucked away all the energy that resides in the gas stream.

The kinetic energy associated with the velocity of the gasses moving through the pipe is negligible and is often just ignored when making calculations. The fallacy that the turbine is a windmill that operates off the velocity of the exhaust gasses is a widespread one. Turbines (gaseous type) extract mechanical power from the controlled expansion of gasses. No heat, no expansion. No expansion, no boost.

edit: if ya'll want to have a nice discussion, I can gladly explain the thermodynamics.


edit 2: Bill, if exhaust flow is what made boost, you would build boost at high RPM under no load (eg, in neutral with the throttle wide open)....which is not the case.

[MikeP]

I understand (I think) but I still have some issues...

Sure, if you cool the exh temp to ambient it won't spin the blades because you have reduced the VOLUME of the gasses and, subsequently the pressure as you do anytime you cool a mixture of just about anything.

So in this situation, the heat of combustion creates the expanding gases (pressure) to drive the turbo. But again, in my mind, HEAT per se doesn't drive the turbo, pressure does. To me that's easily proven by the fact that, as I said earlier, you can heat those impeller blades to any temp you want and they won't turn. Put 40PSI of cold air pressure out of an air compressor hose on them and they will.

I understand that the HEAT is what creates the pressure in this situation but since heat alone won't move an impeller and pressure will, it seems to me that it is more correct to say the turbo is driven by the PRESSURE of the exhaust gases, not the HEAT of the exhaust gases.

Lets say we measured the pressure/volume coming from our exhaust manifold at full load and we then had an air compressor generate that same pressure/volume. If we hooked the turbo to either of the sources, engine or compressor outlet wouldn't the boost be exactly the same?

[Avenger]

We've already pretty much beaten this to a pulp:

http://www.samsmarine.com/forums/showthread.php?t=6225

But if you guys want to keep at it, be my guest. :D

[yachtsmanbill]

Turbos must / most definitely provide something for someone, but I sure am glad the sub's got the Ns !! I could just about imagine what 2 or 4 new turbos would have cost me just to make the thing run, then all that cooler cleaning BS, and oil soaked blankets, and ER temps and on and on. ALL that for a few miles an hour???
YGBSM. ;-) ws

[neosin]

The props people have on these boats are made for getting up to 18kts or so, by putting more pitch in the prop or dia for (holding) you get more water moved for X fuel burned.

You lose your 18kts because you'd overload your engines, BUT you gain more speed at a lower RPM without (or little) extra fuel (it's kinda like putting a higher or lower differential gear in your car, thats what your pitch and dia on a prop do for a boat. ) , if any would be worth if because of the speed you'd pick up with a bigger dia/pitch prop push and holding water better than your smaller (go fast props). I'd bet you could still get 2 - 5 kts over your hull speed without over heating if you just had to. Depends on how you build it

It's works like this

FAST SPEED
Smaller DIA w/ more pitch

HULL SPEED
Larger (i would go biggest you can for holding power in waves) Med. - low pitch.

You could play with pitch to find that sweet spot for 1200-1300 rpms for hull speed, maybe even a little over. My hull speed on my 44TC is about 8.6kts, but i do very well at 9kts in deep water, deeper than 12 feet i get 9.5-9.8kts out of 1300-1400 rpm if i change my props to HULL speed props i would get 9kts with less fuel burn because DDs are built to be under load. When I'm at hull speed my DDs will burn more fuel per mile than if i put a little load on them, say 9-10kts... just depends on a few things like, wind... current, etc.

i'm not gonna get into all the details of how a 2 stroke or 4 stroke diesel works but on your 2strokers DDs mine being 330ish-hp 8v71s it's always WIDE OPEN throttle your just controlling how much fuel is put into the monster. there is no "throttle body valve controlling air flow".

It's built wide open, you pour more fuel in, it makes more horse power... Injector size being your limiting factor. IN theory you could just keep adding more fuel and keep making more power until you reach the point the crank/rods can't handle the load and BOOOM! lol ... don't do it however... when rods come out the side of the block it's nasty... trust me I've done it. :D

The point is, DDs your 8v71 and other flavors like to have some load on them at least. You'll find if you change your props to hull speed props, running 1/2 - 2 kts over your hull speed will give you the best fuel economy using DDs... other engines will be a little diff... but DDs must have X% load to become efficient to a given RPM & speed ...

I hope i put that all correct... i confuse it myself sometimes... I can care less about going 18kts with fuel at $2.20 gal for me... 8-10kts is just perfect... If i'm wrong on something guys point it out to me... this is what i've picked up after talking to DD people, prop people, people people, green people, old people, young people, and my dad... but he's just nuts anyways...


I'm no expert, but isn't this advice backwards? I thought to lower the load on an engine, you need to make it reach over max rpm (50 to 100) by decreasing pitch or diameter. I thought any engine that doesn't reach max rpm is overloaded throughout it's rpm band, not just at the top.......

Or did I read this wrong...?

[krush]

But again, in my mind, HEAT per se doesn't drive the turbo, pressure does.

I understand that the HEAT is what creates the pressure in this situation but since heat alone won't move an impeller and pressure will, it seems to me that it is more correct to say the turbo is driven by the PRESSURE of the exhaust gases, not the HEAT of the exhaust gases.

We do agree! A turbo does operate off of a pressure differential (not velocity). This pressure differential is caused DIRECTLY by heat. I'm just being an anal engineer and a fundamentalist :) I want people to get it right so they can show us cocky engineers up!

They beauty of the whole thing is we can calculate the power output of the turbo merely based on MASSFLOW (of the gas) and the change in temperature...with a couple safe assumptions thrown in there. This is why I say and people that really know turbos say that HEAT is what spins a turbo...semantical, maybe.

So fundamentally, the heat energy in the exhaust creates a pressure differential (ideal gas law) and the turbine does an isentropic expansion of these gasses and spins a shaft. Moving a step ahead, we can say the hot gasses cause a pressure increase and this pressure is what spins the turbo...both are correct, but the former is less ambiguous. Note that no where in there is the word velocity hehe.

[krush]

We've already pretty much beaten this to a pulp:

http://www.samsmarine.com/forums/showthread.php?t=6225

But if you guys want to keep at it, be my guest. :D


Trust me, I've had this discussion MANY times hahah. But I always bring the big ammunition to the table (formulas, cycle analysis, etc). Sadly, people loose interest when you trump their thinking with the laws of thermodynamics :( Are there any other mechanical engineers in the house? (FYI, I really "learned" most of this stuff on my own...classes just teach students to do book problems).

[Genesis]

Thermodynamics is one of the most useful sciences to understand.

Once you "get it", 99% of the BS you hear spouted about this or that "great idea" goes right in one ear and out the other, as it violates one of those laws - and they ain't suggestions. :)

[MarioG]

I believe that a turbo operates with the principle of velocity, volume and density of air. I agree that the hotter the gasses, the more efficiently the turbine turns. It is here that I believe this principle applies. The hotter the gasses are results in more velocity through expansion of the hot gasses which equals increased volume and density at the inlet of the turbine.

BTW, we hijacked this thread. Moderators, maybe we should put this in another thread?

[krush]

Thermodynamics is one of the most useful sciences to understand.

Once you "get it", 99% of the BS you hear spouted about this or that "great idea" goes right in one ear and out the other, as it violates one of those laws - and they ain't suggestions. :)

haha, true. Isn't it amazing that people will always refute the LAWS and 200 years of science behind them for some internet engineering theories learned from online forums....go on some car forums, it will make you cry!

[MikeP]

"people will always refute the LAWS and 200 years of science"

Cmon, surely you can't be comparing ancient stuff from people like Issac Newton to the up-to-date science that can be found on the internet in places like wikipedia? Hey, what did those old guys do that's at all relevant to the modern world? Did they develop reality TV, Entertainment Tonight, or even the World Wrestling Federation? Nope!

Just a bunch of old dinosaurs!

;)

[Trojan]

You can't have MASSFLOW gasses through a tube without velocity.


BILL



If you can't dazzle them with brilliance, Baffle them with BS.

[nor'easter]

I had a feeling that I should have made popcorn!

[Passages]

haha, true. Isn't it amazing that people will always refute the LAWS and 200 years of science behind them for some internet engineering theories learned from online forums....go on some car forums, it will make you cry!

Wait, you mean that magnet I put on my fuel line WON'T reduce fuel consumption 15%? What about the water injector I put on my carb? The latest was a "Turbulator" the spins fuel molecules which excites them so they burn more efficiently.

The snake oil industry is alive and well.

[krush]

You can't have MASSFLOW gasses through a tube without velocity.


BILL



I really shouldn't reply, but I may as well. One can certainly have very high gas massflow thru a tube with low velocity if the temperature is very low. The density of the gas will be high and thus the massflow can remain fairly high with a low velocity. Massflow is NOT velocity.

It's quite obvious that you don't understand the laws of thermodynamics, but if you want to keep arguing, go ahead. I personally don't argue with neurologists about how nerves in our body works, but if you want to argue with a mechanical engineer about thermodynamics, go ahead :)

[Trojan]

No body said it was. Velocity is the movement. Without directed movement your gasses won't do a darn thing. I retired from engineering 10 years ago after 35 years in engineering. I also have a high pressure boiler licence and a steam engine licence. I also was the chief engineer in a steam power plant with 14 boilers, each bigger than most houses. I do know how a turbine and turbo operate and gas expansion. Been there there done that. So don't try to impress me with your fancy talk. I believe in the KISS system. So do most of the none engineers on this forum. Piece my friend. You still have a lot to learn.

BILL

[yachtsmanbill]

No body said it was. Velocity is the movement. Without directed movement your gasses won't do a darn thing. I retired from engineering 10 years ago after 35 years in engineering. I also have a high pressure boiler licence and a steam engine licence. I also was the chief engineer in a steam power plant with 14 boilers, each bigger than most houses. I do know how a turbine and turbo operate and gas expansion. Been there there done that. So don't try to impress me with your fancy talk. I believe in the KISS system. So do most of the none engineers on this forum. Piece my friend. You still have a lot to learn.

BILL
Touche', Bill, touche'!! Kinda reminds me of da' bird in a way ! ws

[Trojan]

Sorry Bill. You know Me. I just don't like to be zoomed. He is not wrong, just a typical young engineer with lots of energy. Glad he stayed awake in class.

BILL

[Brian Degulis]

The original poster wanted to know how to maximize fuel efficiency at or around hull speed I suggested a larger prop turning slower would do that. Then the discussion expanded into turbo's and weather or not they spool up or create boost at lower RPM's. The following numbers are taken directly from the DD power curves for a 1271TI 650 HP @ 2300 RPM. The left column is the HP available at various RPM's In other words the most the engine can safely deliver without over loading or lugging. The right column is the HP required to drive the propeller at that RPM. This assumes the boat is propped out the standard way or fully loaded at WOT.

1200 RPM 355 HP 73 HP

1400 RPM 450 HP 150 HP

1600 RPM 525 HP 225 HP

1800 RPM 570 HP 325 HP

2000 RPM 610 HP 430 HP

2200 RPM 635 HP 575 HP

2300 RPM 650 HP 650 HP

This should put a few things to rest. First you can see that at lower RPM you could take an awful lot more HP out of the engine than the standard prop is drawing. So boats don't run up hill if you want to use that analogy they run down hill until they are at WOT Second you don't have to be @ 1600 RPM for the turbo to spool up obviously if you can safely get 450 HP @ 1400 there would have to be boost at that speed if the load is there. So turbo's respond to load wait excuse me turbo's respond to velocity that's created by expansion that's caused by heat that's created by load. (sorry I couldn't resist).

This is all about the inherent miss match that exists between the way an engine produces power and the way a propeller absorbs it. This is why large tugs and ships use variable pitch propellers to close that gap at lower RPM. In other words to load the engine closer to it's rated load at that RPM and they do it because it's more efficient and better for the engine to be running near rated load for any given RPM.

The other thing that was suggested was to de pitch the prop to reduce load on the engine. The only time that makes any sense at all would be if you are not able to reach full rated RPM when everything on your boat is right good props clean bottom etc. Maybe because you have added weight then if you want to be able to use all the power available you would have to remove pitch. In any other case removing pitch only makes the miss match worse. You will have to turn higher RPM to get the same speed and now you will be under loaded even at WOT and more under loaded at anything below WOT. This will be less efficient and worse for the engine. If you run on plane and want to reduce load your already doing that when you pull back on the throttle. @ 2200 RPM your using 575 HP of the 635 HP available @ 2000 your using 430 HP of the 610 HP available.

If you where to take the 1271TI used in the example and de rate it to 570 HP @ 1800 RPM and changed the props for the new rating the larger prop would now be absorbing around 225 HP @1400 RPM you would now have around the same thrust that took 1600 RPM with the original props and you would probably be able to make the same speed at around 1200 RPM as you did @ 1400 RPM. You would save fuel and load the engine closer to rated load but you would be limited to total of 570 HP instead of 650 HP.

Is this worth doing probably not I'm thinking around $6000 for the props and labor to install then you should de rate the engine also. I think the best fuel savings you could hope for might be around 1.5 GPH at hull speed. So 1.5 x $250 per gallon is a savings of $3.75 an hour so you would need to run for 1600 hours to break even @ 10 knts that's 16,000 nm just to break even.

Brian

[Genesis]

No, you de-pitch the wheels to reduce the stress on an overstressed motor.

Fact is, "high output" marine engines are designed on the bloody edge of the curve. The numbers you cited are for an engine set up for commercial service - 650 hp out of 850 cid, 0.76hp/cid.

"Recreational" engines are typically set up close to or even over 1.0 hp/cid, and if you try to pull a corresponding amount of power out of them at 1500 RPM, you will find out what happens in a big hurry, and not in a way you will like.

[krush]

Sorry Bill. You know Me. I just don't like to be zoomed. He is not wrong, just a typical young engineer with lots of energy. Glad he stayed awake in class.

BILL

We disagree over semantics from the sounds of it. Heat makes pressure, pressure turns turbine. Velocity is in the pipe and spins the wheel, but the initial energy comes from heat. A boiler makes heat not velocity hehe.

I worked at a multi unit steam plant too and I spent many hours with the operators learning how they ran the thing. They were perplexed as to why I was always greasy and dirty because they think engineer means desk jockey. I'm a little different than most young engineers :) I know I have lots of learn, but EVERYBODY has lots to learn...I realize everyday that there is more and more to learn.

I think where we got off track form each other is that I think you said that the engine acts as an air pump and this is what spins the turbo. An engine is just a positive displacement pump that is going to put out about the same volume of air per cycle regardless of RPM.

[Brian Degulis]

Fact is, "high output" marine engines are designed on the bloody edge of the curve. The numbers you cited are for an engine set up for commercial service - 650 hp out of 850 cid, 0.76hp/cid. (Quote Genisis)

Wrong the numbers I cited are" MAXIMUM Pleasure craft" ratings taken directly from DD. The comercial numbers are much lower. 1271TI 650 HP @ 2300 RPM Typical engines found in lots of typical boats like the the hatts being operated by the folks on this list.

Brian

[Genesis]

Maximum pleasure ratings from WHEN?

Detroit rated 6V92s at 535. You're telling me that a 12-71 was rated only slightly higher in "maximum pleasure craft" use? Maybe back before people got enamored with hotrodding 'em.

[Avenger]

From a 1988 Detroit ad:

Marine Pleasurecraft Models:

12V-71TA 900 BHP @2300 RPM

Marine Crewboat Models:

12v-71TA 585 BHP @ 2100 RPM

Marine Workboat Models:

12V-71TA 440 BHP @1800 RPM

I don't have my HP/ torque/ RPM formula handy, but it could be the same engine with different RPM settings.

For comparison the Industrial Model 12V-71T (not TA) was 633 BHP @ 2300 RPM

[Brian Degulis]

Maximum pleasure ratings from WHEN? Quote Genisis

I used one I had handy from 1985 and that was I think around the vintage the original poster was looking at a 1271 TI rated 650HP @ 2300 is very common Pleasure craft rating.

The Rating makes absouloutly no diffrence anyway whatever it is it will be rated for safe maximum outputs at diffrent RPMs. And it's still going to be linier and the prop is still going to be expotential. So if the engine (whatever the rating) is under full load (but not overloaded) at WOT then as soon as you bring it off the pin the propeller is not requiring the full rated load at that lesser RPM. And the more you come down the greater the diffrence will be between what the engines capable of and what the prop requires. So by removing pitch you acomplish nothing or at least nothing good you unload the engine even more than it's already unloaded. So you will now have to turn an engine that's not fully loaded faster to make the same speed. It's more about the prop and the way it absorbs power than it is about the engine.

Brian

[Genesis]

Look, one more time:

Many marine engines (NOT one with a 0.76hp/cid ratio, but DEFINITELY one with a 1.0+ hp/cid ratio) are running way beyond the safe limit for long, productive service lives.

Marine "pleasure" ratings are done this way because engine builders KNOW that most pleasure operators run 100 hours a year or less.

Therefore, if you get 1,000 hours before the engine is smoked, that's 10 years, and far beyond the warranty. Ergo, the engine manufacturers do not care if the engine comes apart at that point.

Now those of us who actually use our boats, however, DO care, because for us we run 200, 300 or more hours in a single year. Ergo, "1,000 hours" means we're smoking engines every THREE YEARS.

Therefore, if we can live with the lower performance levels that derating the engines produces, rather than changing injectors AND props, just change the props.

It does the exact same thing as changing the injectors (reduces the maximum fuel burn as the amount of power demanded drops) and yet is quite a bit cheaper than dorking with the motor itself.

That was my entire point.

There is no reason whatsoever for the original poster to contemplate such a thing because he is intending to run in an RPM zone where it does not matter.

HOWEVER, if he had engines that were 1.1hp/cid AND intended to run on plane, then it WOULD matter and, assuming he could live with the reduced performance, might double the expected engine life before overhauls on those engines would become necessary.

[Brian Degulis]

Therefore, if we can live with the lower performance levels that derating the engines produces, rather than changing injectors AND props, just change the props.

It does the exact same thing as changing the injectors (reduces the maximum fuel burn as the amount of power demanded drops) and yet is quite a bit cheaper than dorking with the motor itself. (Quote Genisis)

Heres my point Yes reducing the amount of power you demand from an engine will extend it's life no argument. Manufactures want to sell big HP often to big for reasonable life no argument

But you seem to think that turing an egine faster with less load (de pitching the props) is the way to acomplish that not true. You will acomplish the same thing in a better way by simply throttling back. Or if you wish re setting the governer for a slightly lower high idle if you want it to be a permanent change. The moment you come off the pin the the power required to drive the prop becomes less than the power the engine can safly produce pull it down further and the gap gets wider reducing load for that RPM more and more. That holds true regardless of the engines rating it's all about the way the prop works. So there is simply no point to turning an engine faster than it needs to turn for a given speed to acomplish reducing the load to extend life. It takes fuel for the engine to run itself the faster you run the more fuel is used simply to overcome the frictional losses so you won't be as eficient. All things being equal turning at a higher RPM also increases wear.

If the power required to drive the prop was in proportion with the way the engine produces it then de pitching would make sense. But engines ar pretty linear double the RPM and you double the HP. Props are expotential doubling the RPM requires around 4X the HP. So when you reduce prop RPM the power required to drive it goes down around twice as fast as the engines ability to produce it so the engine becomes under loaded as soon as you reduce RPM from WOT more and more as you reduce further.

BUT DON'T BELIEVE ME CHECK OUT THE POWER CURVES FOR YOURSELF AND IT WILL BECOME PAINFULLY OBVIOUS.

Brian

[krush]

Ahh, but what if they make those engine HP curve by keeping full throttle and varyign the load to bring the RPM down?

This is the way constant speed props work on airplanes, right? You sent the throttle (MAP) and then control the RPM with the prop. The HP goes down as the RPM goes down, but the throttle is still wide open.

I'm gonna design an in-boat dyno to settle this argument! I just need something that can get torque from the shaft (or mount the engine on load cells).

[Southern Comfort]

What a great thread!!!

We run Southern Comfort at 1300 RPMs along the ICW with pretty good fuel burn and 10 knots, trimed out and clean bottom. Outside and to the Bahamas we run 1700 and get closer to 14 knots. Our 12v71t's have 4500 hours and are still running strong putting about 300 hours each of the last three years on the boat.

3 of our guests have bought Hat's after chartering Southern Comfort. 58' LRC, 63' MY and a 71' (63' w and extension) After our winter trip to the Bahamas this year we'll most likely part with Southern Comfort in favor of a larger Hat and continue our charter biz focusing on Bahamas and Island farther than we care to go with our 1200 gallons.

We also own a 58 MY that is being refit in Ft. Lauderdale and is for sale as will be the Charleston operation.

Best thing about running a Hat as a charter boat no one has to think twice about the boat they are chartering.. Hatteras says it all.

[krush]

Ok, this is driving me nuts....it is RPM, not rpm's, rpms or rpms'!

Rotations Per Minute...not rotations per minutes thoguh you could do that but the convention is to keep minute fixed at 1.


Rotations/minute (rotations divided minutes)

[jim rosenthal]

OK, as long as you all are talking about thermodynamics with regard to diesel engines of the kind we all know and love...could someone explain to me how to interpret the readings of boost pressure and EGT that I am about to be looking at with my new engines? Or tell me where to find out HOW to look at this information? Like: what is the relationship between boost and EGT, and how should I expect the gauges to act at various rpm/load levels....thanks in advance.

[Genesis]

The more boost, the lower the EGTs.

Boost is GOOD on a diesel. Many people think its bad - nope. Boost means more airflow, which means a leaner mixture, which means you get the "bang" but not the heat dissipation problems.

There is a cirtical temperature for all engines beyond which things start to deform (melt!); you need to know what it is for the particular engine in question. In addition EGT probes AFTER the turbo mean less; what you really want is ones BEFORE (and ideally, one for each cylinder!) but you'll almost never see that on a recreational boat.

What engines are we talking about and what application? I can give you my "rule of thumb" numbers for Detroits in certain applications but need to know the engine and horsepower they are set up for, and where the thermocouples are.

The highest value application of both pyrometers (EGT) and boost gauges is in seeing changes early, marking deteriorating conditions (e.g. a deterioration in a turbo, airflow problems, etc) before it results in things like burned valves. Of course for that to do you any good you need a baseline.

[jim rosenthal]

OK, these are Cummins 370Ds, so 370 hp (355 SAE hp, I think); the thermocouples are right after the turbo outlet (I didn't have a choice on that).

[Genesis]

4-strokes run hotter than Detroits; I'd check with Cummins for the "absolute maximum" EGTs you should see.

Use 'em more as a monitoring tool - if you see differences between the sides or a change, something's up.

[yachtsmanbill]

Ok, this is driving me nuts....it is RPM, not rpm's, rpms or rpms'!

Rotations Per Minute...not rotations per minutes thoguh you could do that but the convention is to keep minute fixed at 1.


Rotations/minute (rotations divided minutes)

The things that IRKS me more than anything, is when I was in third grade and Mother Anastasia was paddling my ass for being a wise guy, the other kids that couldn't spell got to go on special field trips.
I suppose that being able to read at age 3 helped, but all these professionals here that have masters' and graduated magna cum laude can't figure out how to use their spell check function !! ws

[Top Shelf]

I've got 720 HP Covingtons in a 48 CNV. I can run at 16/17 knots at 52 gallons an hour or at 10 knots at 15 gallons an hour. That's a big premium for 7 knots. In fact we just came back from San Sal and with Fuel close or over $4 a gallon in most of the out islands it's hard to justify.

Ninety percent of the time we we ran at 10 knots. But had the option to bump it up and run through weather or slop when we needed to. Always blew them out once or twice a day. They've got 2780 hours on them and are running strong, I'm the third owner and don't have a lot of history from the 2 PO's. But they seem to have been well cared for and have not been rebuilt before (according to a couple of mechanics that have worked on them).

I've got some pending issues with the oil analysis and I'm smoking some and burning a little oil, so I'm close to a rebuild. But with close to 3000 hours and 18 years old on the original build I don't think they owe me much.

So to answer the original posters questions and some others advice, don't be afraid of them and if you run em lightly they're fairly efficient, but still have the juice to get you out of way when you need it. my 2 cents. Thx.

[Boss Lady]

are you draggin a sea anchor behind that thing? LOL I just ran a 54 bertram with 12v92 TIs (1050 hp each) and it only burned 55 gallons an hour at 20 knots and it weighs about 20,000 lbs heavier too.

[Top Shelf]

There's a whole other thread on the 48 cnv, bout does it really plane or not? I think it did once. This boat is way under tabbed from the factory (pocket tabs). 18" tabs just don't do it. Bennet says I should have 36 to 48". Full tower, marquipt, 13 ft tender doesn't help. 52K in the slings

But I did just see these new tabs that Volvo (I think) came out with. Looks like a composite blade deploys straight down from the transom. With this set up I think I could fill in the pockets and get to 36" or longer without a bunch of hull mods.

[Trojan]

My boilers produced steam not heat. The fire box did that. What happened to the torque ranges? The HP does not move the boat the torque does. You should prop for the best torque if you want to be efficient. In all cases you are loosing torque at WOT and are trying to make up for it with brute HP. That is where the over load comes in at and why you prop for WOT.


BILL

[Brian Degulis]

Props are always sized based on RPM Reduction Ratio and HP. Boats are driven by HP which is a total measurment of mechanical force very much like watts is a total measurement of electricity. RPM is similiar to volts and torque is similiar to amps. If you want to know the torque you can easily calculate it if you know the HP and RPM. Just like electricty if you know 2 of the values you can always calculate the third.

Boats are proped for WOT (or not) for all the reasons stated in previous posts it has nothing directly to do with torque.

The power curves don't take into acount prop size or reduction ratio because for the purpose of calculating what an engine is capable of it doesn't matter. It could be a large wheel turning slow requiring high torqre delivered from a deep reduction. Or a small wheel turning fast requiring little or no reduction. It's still based on a total measurment of power expressed as HP.

If for some reason you had an engine that was low on torque at a certain RPM range then that would be reflected in the HP rating in the curve at that range. If the torque drops off and the RPM remains the same then the HP must drop.

Brian