engine torque

[aerobat]

Re: engine torque

Towards the bottom usually results in the highest top speeds anyway.

thats interesting ! when we trust GM the engines tend to peak in power at 4800 . volvo and mercruiser usually give 4200-4600 at their carbed engines , so when you prop for 4200 and the engine peaks in power at 4800 you loose some horses. but maybe the higher pitch a little more slowly turning prop catches this up in a slightly better efficiency ?

 

[QC]

Re: engine torque

I don't know why, but over the years it is something I have noticed with most data. Look at most prop comparisons. Not dramatically obvious, but definitely a trend.

 

[E4ODnut]

Re: engine torque

QC,
With all due respect I disagree.

Automotive based gasoline engines were used for years as industrial and marine power sources before the popularity of the modern light weight diesels. I have had personal experience with them in generator set and water pump applications. They were typically run at RPMs close to peak torque, about 75% of full load on a continuous basis and gave very good service life.


It all depends on the application. Light weight high speed planing boats, heavy cruisers and displacement hull boats all have different power requirements. It's simply not correct to say that one size fits all and all must be propped for max HP RPM.


If the engine is sized correctly for the application then I would agree that method would probably be best, but very often a boat is over powered for it's practical use. In this case especially, if economy and engine life are the prime concern, as in a lot of commercial applications, it is better to prop so your maximum cruising speed gets your engine RPM close to peak torque RPM at a relatively high manifold pressure.

If you know the torque curve of your engine, you can experiment with different propeller power demand curves to match it so that the cross over point above your cruising RPM will give you a comfortable safety margin.

As I mentioned before, this assumes that you do not need your engine to reach full rated power and are prepared to accept the fact that this new cross over point is now your maximum power.

 

[QC]

Re: engine torque

I actually agree with you. I fought this for a long time here and gave up. I think you are fundamentally right, IF you have such an oversized power plant that would allow you to prop at high load at Peak Torque. So if planing speed required 175 horsepower, and you put in a 400ish something then you maybe at the magic combination of 75% load at peak torque RPM. That is, however, a rare combo unless the original intent was to go fast. No more of a disagreement than that.

All here swear you will lose valves and/or valve seats. I think a lot of that is a mistake as they aren't completely considering what you said "75% load at peak torque RPM". Let's try it on your boat. I'll bring the fuel flow meter. I agree on commercial use of gasoline engines which is more common than most know. They are often big blocks with little carbs/mixers. I see this a lot with natural gas engines which happens to be kinda what I do for a living

Edit: I stand by my full load comment though. Most heavy duty diesels are designed to run at full load/peak torque 24/7, I would not say the same about most gasoline designs. 75% changes my comfort level. I operate my tow vehicles in much deeper lugs than most would, and I prefer a manual shifting auto that will not downshift at full load for this exact reason. I just don't think full load at peak torque indefinitely is a good idea despite the fuel economy benefit. And, again, the 75% thing changes my answer. Annnnd, I still think the ultimate answer is a CVT. Then you can do whatever you want with whatever you have

 

[Tail_Gunner]

Re: engine torque

Marine engine's are setup for warranty precaution's reliabiltiy (reversion) and off the shelf tech which happen's to be old smog cam's....no overlap... no smog issue's.... nor reversion issue's...and very cheap part's. As to why they run so rich again i would think its long term reliabilty...and only when a new exhaust system for marine engine's are deveolped not much will change. That may very well be the reason for lack of published spec's they would not look to sterling.

 

[E4ODnut]

Re: engine torque

QC,
We're on the same page. I too have had a difficult time trying to convince others of the merits of this practice, probably because it is contrary to what has been preached by the "gurus" and the engine suppliers.

I had a very interesting conversation with a very well respected marine engine mechanic and dealer about it. He told me that the main reason the manufacturers insist on propping for max HP is strictly for warranty purposes. It assures that the engine cannot be overloaded or over revved. Makes sense from their point of view, but not from the point of view of engine economy or engine life. This is especially true if you are in a constant under load condition with a diesel. We talked about my practice of propping for the torque range I wanted. He agreed and said, "we do it all the time in commercial installations where economy and engine life are priority". I asked him about the warranty issue. He said the sea trials are done with the specified prop, this information goes on record. Then the prop is changed to the one that best fits the application. He said he's been doing it for years and has never had an engine failure that could be attributed to over loading.

All of this can only be done if one has the full performance data for the engine you are working with and fully understand the relationship between the torque, HP and propeller power demand curves. Most important of all one has to fully understand how to determine to what extent an engine is loaded and what the safety limits are. I use manifold pressure to determine load and exhaust gas temperature to see when things are getting into danger territory. I also use a wide band exhaust gas oxygen sensor to tell me what my air/fuel mix is, and because all the engines I work with are knock limited I keep a keen ear for pinging when setting up my timing curves and go on the conservative side for everything in the interests of engine life.

 

[aerobat]

Re: engine torque

in displacement hulls / diesels i saw intentional overpropping since the engine was powerful enough to bring it to max hull speed in any case , a larger prop pushed down rpms and brought the diesel more consequently into a range it loves : medium rpm,s with a relativly high load. the result was the same speed ( going beyond hull speed in a displacement boat is not possible even with more power output) , lower noise, better economy.

on a gasoline powered planing hull significant overpropping ( to lets say 3700-3800 rpm) might be bad since you can end up having a sitting dog struggeling to get out of the hole at all and you will sacrifice top speed since the engine definitly builds up power after peak torque.

further you most probably save only peanuts on a naturally aspirated gasoline engine. we do not have muscheldiagramme ( 3d bsfc maps) for our blocks but i bet the difference in sfc and overall fuel flow between a cruise at lets say 3800rpm/65% load (prop to rated rpm ) and 3500/75% (overpropped) is only minor.

like mentioned before - i,m asking myself if we will some day see cvt transmissions or controlled pitch propellers on boats- like some day powertrim was introduced to optimize boat performance for various situations.

 

[E4ODnut]

Re: engine torque

Aerobat,
Agreed. The advantage of over propping usually has the best benefit when a boat has considerably more power than the operator wants to use.

Take for instance my boat. She has a semi-displacement hull, weighs about 17000 lbs and has twin 351 Fords rated at between 235 and 240 HP each at between 4200 and 4400 RPM, depending on what spec you are looking at. I have her propped for ~3900 RPM WOT. She will do ~23 knots wide open and I can cruise her at ~17 knots at ~80% load if I want to, but I don't. My normal cruise is ~8 knots, 1800 RPM ~15" manifold vacuum. I have what I consider to be a compromise over prop. I could probably add a bit more pitch and get slightly better 8 knot economy if I was prepared to sacrifice some top speed, but I prefer to have that in reserve for emergency use only.

How much fuel does this save me, you might ask? I don't know for sure, but I have relatively accurate fuel flow indicators on the boat and in my motor home. I have done experiments with the motor home running a constant speed on a constant grade, shifting between direct and overdrive. The higher gear consistently resulted in ~3% less fuel burn even though the mixture was richer at the higher manifold pressure. This might not sound like much, but the savings are there and they are real.

My best improvement economy came from the conversion to EFI, which was done after the prop change. I did one engine at a time so I could compare data. Both engines were healthy and in a fine state of tune. At 8 knots the EFI engine burned ~20% less fuel than the carbed engine. I found no measurable difference in top end power.

In a road vehicle my feeling is that there is no such thing as too many gears. I agree that a controllable pitch prop, or some kind of variable speed gear box should be beneficial for economy in a boat that needs a wide range of speed, but I don't know just how practical that would be. For now it looks like we will have to live with the limited options we have to tune the engine, prop and hull combination to our operating style.

 

[QC]

Re: engine torque

The issue with CVTs for a boat are cost and weight. Auto manufacturers are adding more gears every day, and they are playing a lot with CVTs. They are all doing it for the same reasons we are noting: maximizing throttle position for best efficiency, but preserving performance as well.

 

[aerobat]

Re: engine torque

1800 RPM ~15" manifold vacuum.

you have a manifold pressure gauge installed in your boat ?

 

[Tail_Gunner]

Re: engine torque

The issue with CVTs for a boat are cost and weight. Auto manufacturers are adding more gears every day, and they are playing a lot with CVTs. They are all doing it for the same reasons we are noting: maximizing throttle position for best efficiency, but preserving performance as well.

Cvt's would not withstand the stress of that much mech load...300hp is the limit of a alpha, one rentry and snap...However think of cvt's with jet drive's....Now that has some real potential.

 

[QC]

Re: engine torque

I am not suggesting that anyone install a CVT today on any marine application. Just the concept as we were discussing matching speeds and loads.

 

[Tail_Gunner]

Re: engine torque

Yes it would greatly increase performance and economy. Doing a jet drive with cvt wouldn't take much I think it would be quite simple and OS sensors could be used for a closed loop system. Allowing for tuning discussion's like this one to have real implication's...or practical.

 

[E4ODnut]

Re: engine torque

you have a manifold pressure gauge installed in your boat ?

I had manifold vacuum gauges when I was still running the carburettors. Now that I'm on EFI I use manifold pressure which is used by the ECU and also shows on my real time display. As you probably know, manifold pressure references atmospheric pressure. Both are just a different ways of expressing the same thing.

 

[aerobat]

Re: engine torque

you should read athmosperic pressure at engine shut down ( about 30 inch ) , then below 10 on idle and again about 30 (on a non charged engine - minus suction losses ) on WOT , do you ? installed the gauge after the throttle body ? never heared of that in a boat, aero piston engines use this on constant speed props as i remember from school time on small pistons.

 

[E4ODnut]

Re: engine torque

My read out is in KPA because it's easier to do the math for fueling. The boat is at sea level so when the engine is not running it will normally read 100 KPA. The resolution is 1 KPA which is good enough for the purpose. If atmospheric pressure is really high it will read 101, if really low 99. I idle at about 31 KPA in neutral, 35 or so in gear. WOT is atmospheric. I have barometric pressure correction but it is really not required at sea level.

The sensor is in my ECU with tubing tapped into the intake manifold plenum.