So called Overpropping

[sunchaser]

LarryM

It seems you did not over prop, you right propped since your engine could reach rated max rpm with the last prop setup. Without fuel burn and %load for each rpm change it is hard to tell what is optimal though.

Therein lies the beauty of new diesels where prop changes, fuel burn, load etc are all available. Eschulman and Twisted have the new boats, now if we could only talk them into spending many thousands on changing props for the benefit of more data, for what purpose again?

 

[LarryM]

LarryM

It seems you did not over prop, you right propped since your engine could reach rated max rpm with the last prop setup.

You are absolutely correct, I should have been more precise. My second prop/pitch combination was the one that was over-propped. My third/final combination is as close to perfect for my boating needs as I think I will ever get.

Now, for some noise-cancelling headphones . . . . and another couple of layers of Soundown . . . .

Larry
M/V Boomarang

 

[Nomad Willy]

FF wrote;
"Most engine cylinders wear out in total piston mile"

I would think the amount of force sideways on the piston to cylinder wall interface would have much more to do w it.

An engine working harder has more combustion force and side loads on the piston due to the offset of the crankpin or rod journal, downward forces of conbustion, piston crown area, stroke ect ect. An overpropped engine having fewer power strokes would need to generate more combustion chamber force and greater side loads on the piston. And the sideloads would produce more wear. A lot more I would think.

The concept that the overpropped boat would have lower fuel consumption at some engine speeds can probably be assumed as having overdrive on a car (or a higher gear ratio) produces the same benefit. The question is how much benefit is there to be had. And of course that needs to be balanced or compared w the benefits of propping the way engineers and engine manufacturers recommend.

But even "good enough for all practical purposes" numbers will be hard to come by as there are so many variables. I wonder how much truth could be had just looking at power output v/s fuel consumption curves? Fuel burn at slightly different engine speeds will have slightly different efficiencies and is obviously one variable but there are probably many more. Fuel burn as reported by a skipper is full of variables and usually not very accurate. The reporting skippers will profoundly disagree of course but they usually won't even know what half the variables are.

To me the amount of reduced fuel consumption is a small benefit to have but I am not the skipper of a Hat w big DD engines. Nore is my boat extremely overpowered. So if I had a motivation to overprop it would be very small.

One catergory of boats that would have large motivation to overprop for reduced fuel burn would be passagemakers. I know little about passagemakers and have no idea if they overprop but knowing would seem appropriate information for this thread. I would think they would need to be powered for low fuel burn so the power loading would fall into a small range. No over powered boats here. Since really rough going would definitely come to pass and at least some extra power would be needed for that so underpowering would not be attractive at all. So .. a narrow range of power loading. Since one would need full power at times I would think overpropping on a passagemaker would be too undesirable to consider.

The KK 42 and W40 are both overpowered w the Lehman so some observations and running numbers could be had here and some passagemaking has probably been done w the Lehman. Fuel burn numbers in those boats during some passagemaking should be better than most. I suspect boats of this type probably just run slower for better fuel burn rather than overprop so they may not have much information re overpropping. And perhaps like me they don't overprop for other reasons.

 

[bayview]

indeed and with overdrive and carbed engines a big loss was pumping losses associated with creating that vacuum in the manifold. Lower RPMs with overdrive reduced those losses.

Diesels run unthrottled so pumping losses are reduced. The idea of running at the point of maximum BSFC has merit but within most reasonable operating ranges it probable is a nearly flat curve.

Over loading increases head and piston temps as well due to lower air flow vs fuel supplied to increase HP for rev up.

 

[Nomad Willy]

Bay,
As well as less coolant circulated and less lube oil circulated.

 

[FF]

>As well as less coolant circulated and less lube oil circulated.<

So what?

The mfg gives a HP and Torque curve that works for his engine, usually from 1200-1500 up to flank..

As long as all the limits are observed , the engine should lube and cool properly for the life of the engine.

Owners hardly need to second guess the engine mfg to obtain normal service life.

Usually the only hassles are engines that are auto transplants that do not have 4 or 5 service ratings and are light duty at best.

There simply loading to 1 hp per 3 cubic inches of eng disp. will keep the engine intact. when used 24/7.

 

[bayview]

Owners hardly need to second guess the engine mfg to obtain normal service life.

Exactly and for that reason owners should not second guess manufacturers about propping either.

 

[Delfin]

As a matter of fact, I do have some real data to share. Last year I switched from a 17x13 DynaQuad 4-blade prop to an 18x14 DynaJet 3-blade prop. According to all of the prop calculators, Michigan Wheel and my local prop specialist, the new prop should have been 2" over-pitched. Otherwise, it was an ideal prop.

I installed it and ran for several months, then had 1" removed and found it to be perfect.

My engine's pleasure duty specs are 56 hp @ 3,800 rpm. The hull speed (SL=1.34) is 7.02 kts.

Before (17x13DQ 4-blade):
6.90 kts @ 3900 WOT
6.80 kts @ 3500 High Cruise
6.60 kts @ 3000 Cruise
6.00 kts @ 2500 Cruise
5.00 kts @ 2000 Economy

After (18x14DJ 3-blade) Overpropped:
7.20 kts @ 3650 WOT
7.20 kts @ 3500 High Cruise
6.80 kts @ 3000 Cruise
6.60 kts @ 2500 Cruise
5.90 kts @ 2000 Economy

After (18x13DJ 3-blade) Re-Pitched to Optimal:
7.30 kts @ 3950 WOT
7.10 kts @ 3500 High Cruise
6.85 kts @ 3000 Cruise
6.55 kts @ 2500 Cruise
5.70 kts @ 2000 Economy

Obviously the first prop, the DynaQuad 4-blade, was not well suited to the boat at all.

The last two sets of numbers for the DynaJet 3-blade are really interesting. Overpropping by just 1" of pitch reduced WOT by 300 rpm, but the improved performance at lower speeds was minimal. Each prop was evaluated with 2 2-way passes of 1/4 mile each in protected waters. This really is comparing apples to apples. This was the same prop with 1" of pitch removed. What a difference, and now the engine operates within in the manufacturer's recommendations and I sleep better.

Larry
M/V Boomarang

Larry, that is extremely interesting data. Do you recall if there was any difference in resonance with the 3 blade over vs optimally propped? What I find very encouraging about your numbers is that the increase in rpm relative to speed is only indicated outside cruising speeds. I intend to have my prop re-pitched this fall and was worried that instead of cruising at 1300 rpm to achieve a certain speed, I would have to cruise at 1550 with the increase in noise, etc. But your numbers indicate not, so thank you again.

 

[Nomad Willy]

A realistic cruising speed for a Willard 30 is 6 knots. At six knots on Larry's boat 2100rpm is required whether or not the boat is under propped 150rpm or over propped 150rpm.

So 6 knots is produced w an inch less pitch and less prop load. Less prop load equates to less fuel consumption.

So Larry's boat is more efficient w a 13" pitch prop (a tad underpropped) than with a 14" pitch prop that is a tad over propped.

So Delfin perhaps you'll only increase a small fraction of prop speed you anticipated gaining or possibly none at all.

I always assumed over propping got a bit better fuel burn but perhaps not. Or perhaps I did the number interpolation wrong.

 

[Delfin]

A realistic cruising speed for a Willard 30 is 6 knots. At six knots on Larry's boat 2100rpm is required whether or not the boat is under propped 150rpm or over propped 150rpm.

So 6 knots is produced w an inch less pitch and less prop load. Less prop load equates to less fuel consumption.

So Larry's boat is more efficient w a 13" pitch prop (a tad underpropped) than with a 14" pitch prop that is a tad over propped.

So Delfin perhaps you'll only increase a small fraction of prop speed you anticipated gaining or possibly none at all.

I always assumed over propping got a bit better fuel burn but perhaps not. Or perhaps I did the number interpolation wrong.

Eric, I don't think it works that way. More aggressive pitch equals a higher "gear", so to speak, so rpm required to reach a certain speed should be lower with more pitch than less. Flatten the prop and spin it at 1600 rpm and you go nowhere. When I flew, once at altitude I'd feather the prop back, rpm goes down, fuel consumption goes down and speed is maintained. Maybe I'm confused on this, but I thought that was the way it worked on props in air or water.

What's interesting about Larry's numbers is that within the margin of error of measurement, there isn't much difference between the right pitch and one that was more aggressive at the rpm range I care about. But even with his boat, if he put on a 18 x 0 prop, it wouldn't matter what rpm he ran at - he wouldn't go anywhere.

 

[bayview]

That data doesn't tell us anything about efficiency. His engine is capable of producing the hp to push his boast at 6 knots at many different rpms. It would use about the same fuel to produce that hp under most settings until it was way overfueled at which point efficiency would drop. There are also an infinite number of prop settings that will put the same hp into the water but the hp to go six knots is always the same and the prop is delivering that same hp

 

[eyschulman]

That data doesn't tell us anything about efficiency. His engine is capable of producing the hp to push his boast at 6 knots at many different rpms. It would use about the same fuel to produce that hp under most settings until it was way overfueled at which point efficiency would drop. There are also an infinite number of prop settings that will put the same hp into the water but the hp to go six knots is always the same and the prop is delivering that same hp

Agree it is the HP and engine load that has great importance. The manufacturer wants propping to spec to make sure there is no overloading at any point on the power curve. Aside from the science there is the consideration of warranty-insurance and resale and over propping can significantly become an issue. So for those who don't want to believe the motor will be harmed if they over prop and manage load the benefits if any gained do not appear to me worth the risks.

 

[psneeld]

the whole point of my thread o several weeks ago and this one of FF's...is that some hard data supports the idea that overpropping some engines on some boats some of the time has MORE pros than cons.

Kinda like every discussion here...."it depends"....

 

[Brooksie]

Delfin (& others) are quite right on the fact that RPM's don't tell the fuel burn. But if RPM's and loading (prop/gear) put the engine in the "sweet spot" of that engine's fuel map some rather good fuel savings can be had, as Calder discovered. All below the manufacturers overload line on the map and below this line they are not overloaded. But, as mentioned b/4, you can't get a manufacturer to set up an engine this way if it is to have a warranty, at least not the size engines we a talking about. They will only warranty an engine that reaches full RPM at WOT.

 

[psneeld]

Delfin (& others) are quite right on the fact that RPM's don't tell the fuel burn. But if RPM's and loading (prop/gear) put the engine in the "sweet spot" of that engine's fuel map some rather good fuel savings can be had, as Calder discovered. All below the manufacturers overload line on the map and below this line they are not overloaded. But, as mentioned b/4, you can't get a manufacturer to set up an engine this way if it is to have a warranty, at least not the size engines we a talking about. They will only warranty an engine that reaches full RPM at WOT.

Not according to a one time poster here (I pretty sure I remember correctly)...he was adamant that ALL Grand Banks left the factory for quite a few years overpropped. His intel was usually pretty good so I have no reason to disbelieve him.

I'm pretty sure that someone had the warranty on those engines whether engine or boat manufacturer...can't believe that many sold new without engine warranties.

I just think that engine manufacturers are like many other manufacturers...they warranty and publish guidelines for the lowest common denominator they can expect..not some end user that is educated on issues and can overcome the con's that some think outweigh the pros.

 

[markpierce]

Seems to me the subject of overpropping for possible efficiency is more relevant to ships rather than our small boats where the financial benefits will be minimal, if any.

(Ship clearing the Coot's path through a bunch a sailboats.)

 

[psneeld]

Seems to me the subject of overpropping for possible efficiency is more relevant to ships rather than our small boats where the financial benefits will be minimal, if any.

When I first posted the Pro Boatbuilder article..I quickly received input just as you said....long haul guys that run certain kind of engines and speeds would benefit the most and trying to nail it down on a small boat would be a waste of time.

I might agree that it is a waste of time to search..my only horse in the race is that if you buy a boat that is knowingly overpropped and it works for you...no need to panic with the chicken little crowd.

PLENTY of real world info out there on what works and doesn't work.

I'll also be the first to say that if you have certain engines with known histories of overpropping issues...take heed....if you buy a boat with engine (s) that have regularly been overpropped with no ill effects...then take a cold one out of the fridge.

and as FF points out...even if there's no short term efficiency...as long as you are with design limits and can run at a lower more bearable RPM...so much the better right there and maybe even longer service life (don't make me explain this one..the die hards can't be convince that it's even a possibility so I'll let FF or someone else beat their brains out).

....Mark...only the first para was with your post..the rest is just generic....

 

[Nomad Willy]

Delfin and others,
What I was trying to say in post #39 is that going 6 knots takes the same amount of power over propped, right propped or under propped. It should take more rpm under propped and less rpm over propped. Larry's numbers don't support that. Seems to take 2100 both ways. Shouldn't.

Over propped one should burn more fuel per stroke and under propped less. But under propped the engine takes more strokes and over propped less. Over propped the injectors are injecting more fuel to keep the boat at 6 knots. But the total fuel injected may not be as much as the under propped engine. Or it may be more. It depends on how efficient the engine and prop is making the required power to drive the boat 6 knots. The same amount of work must be done. With the over propped engine there are fewer power strokes and so those power strokes must make more power and thrust per stroke.

Bayview is right though. More information is needed to predict efficiency. I knew I must have been wrong in post #39 as what seemed to be ... didn't make sense. I was thinking that under propped there was less load and reduced fuel consumption. But it takes more strokes so it's not apples and apples.

Delfin when you flew you leveled off when reaching altitude thus greatly reducing power required. So you reduced pitch and rpm at the same time. That wouldn't be possible if you didn't change your angle of incidence.

Yes Scott the GBs were over propped as well as all the Willards. Willy w her original Perkins engine was 250rpm over propped and the boat was only capable of producing 33hp at the resulting rpm of 2750.

So I'm back to square one whereas most boats that are over propped have lower over-all fuel consumption. I think there is a fuel savings but the amount is about 5%. Someone said it was 15% and that may be possible but to get that reduction in fuel burn extreme over propping would probably be necessary. I prefer to be right on or about 50rpm under propped.

And Larry's Willard should get more speed at the same rpm w an inch more pitch.

 

[markpierce]

If not able to make maximum RPM at WOT, I'd suspect something was wrong (like time to change fuel filter.) If over-propping means the engine's maximum RPM isn't possible, regardless, I'd look to changing to an appropriate propeller.

 

[dimer2]

"Delfin when you flew you leveled off when reaching altitude thus greatly reducing power required. So you reduced pitch and rpm at the same time. That wouldn't be possible if you didn't change your angle of incidence. "

Once in straight and level flight, I would choose my cruise RPM and then increase pitch until I saw a drop of 100 rpm and then nudge it back a hair, giving maximum speed at a given rpm. This optimized pitch is obviously only for the given RPM and will prevent WOT.

I am having difficulty understanding why the same principal doesn't work for boats. If my cruising RPM is 1800 and I prop to achieve that RPM with the same throttle setting, disregarding WOT, I am not over loading the engine nor am I over propped....for that RPM. Purely IMO. Now how you would select the maximum pitch prop for that RPM, I have no idea.

 

[psneeld]

"Delfin when you flew you leveled off when reaching altitude thus greatly reducing power required. So you reduced pitch and rpm at the same time. That wouldn't be possible if you didn't change your angle of incidence. "

Once in straight and level flight, I would choose my cruise RPM and then increase pitch until I saw a drop of 100 rpm and then nudge it back a hair, giving maximum speed at a given rpm. This optimized pitch is obviously only for the given RPM and will prevent WOT.

I am having difficulty understanding why the same principal doesn't work for boats. If my cruising RPM is 1800 and I prop to achieve that RPM with the same throttle setting, disregarding WOT, I am not over loading the engine nor am I over propped....for that RPM. Purely IMO. Now how you would select the maximum pitch prop for that RPM, I have no idea.

Guess it depends on exactly what (and possibly who) you give credence to.

I flew for many years but I'll plead ignorance on the fixed wing side and the helos I flew don't have useful info from what I remember.

I'll say it again...the article had hard info that led a fairly well respected guy to form an opinion...one that disagrees with many TFers.

But unless you read the article and the source of the info...and put it all into context...this thread seems to be a lot of little pinpoint of light in the darkness....

Anyone that claims an absolute of the "overpropping" theory shoud be taken for what you think absolutism is in a pretty darn gray world sometimes.

Whether buying or already owning a boat that is overpropped...investigating is the order of the day..not panicking and reacting as some would have you believe.,,,unless of course you own one of those diesels that has a rep of poor longevity with an overloaded prop at WOT and immediate action should be instigated with probable prop alteration.

 

[Delfin]

"Delfin when you flew you leveled off when reaching altitude thus greatly reducing power required. So you reduced pitch and rpm at the same time. That wouldn't be possible if you didn't change your angle of incidence. "

Once in straight and level flight, I would choose my cruise RPM and then increase pitch until I saw a drop of 100 rpm and then nudge it back a hair, giving maximum speed at a given rpm. This optimized pitch is obviously only for the given RPM and will prevent WOT.

I am having difficulty understanding why the same principal doesn't work for boats. If my cruising RPM is 1800 and I prop to achieve that RPM with the same throttle setting, disregarding WOT, I am not over loading the engine nor am I over propped....for that RPM. Purely IMO. Now how you would select the maximum pitch prop for that RPM, I have no idea.

Well, I think it is the same concept and Brooksie explained it well.

I could fly straight and level with the prop as flat as it could be set, and burn more fuel without going any faster than if I "shifted into high gear" by feathering the prop and reducing the throttle. The thing that happened is the rpms went down just like it does in a car with you hit 5th gear, and fuel consumption along with it. If you tried to take off with the prop feathered, you'd overheat the engine. I had an Insight gauge that let me monitor the EGT of each cylinder (it was a turbo aircraft) and I could see it happen in real time.

I know I am overpropped now, by about an inch. I also know that cruising along at 1450 rpm the engine is loafing because the EGT downstream from the turbo runs at 475 degrees and oil temperature is around 175 degrees. To get 850 degrees, I have to hit 1750 rpm, and that is still quite cool with the oil temperature still around the same. As noted, when I run her up to that rpm I do so only to get everything nice and toasty. For the first 60 seconds, I can detect exhaust (dry stack) if I look closely. After a minute or so, you can't see anything, so I assume the engine is not overloaded at the rpm, even if she is a bit overpropped.

Now, whether I get the slightest benefit from the current pitch is a mystery to me. Maybe, but Larry's numbers would suggest not. I'll know in October when I re-pitch, and won't I feel like a dope if it doesn't work.

 

[dimer2]

"Now, whether I get the slightest benefit from the current pitch is a mystery to me. Maybe, but Larry's numbers would suggest not. I'll know in October when I re-pitch, and won't I feel like a dope if it doesn't work."

No, you are going to the trouble and expense to try something that most of us wouldn't, because in your mind it should work. I also believe you will keep us appraised regardless of results, which will be much appreciated.

 

[LarryM]

A few final thoughts and ideas.

When I reduced the pitch by 1", I was surprised that I picked up 300 rpm and an extra 0.1 kt. at WOT. I didn't think it would make that much difference, and certainly didn't expect to pick up boat speed.

The taller (over-pitched) prop actually did produce slightly better speed up to about 3000 rpm, and then again above 3250 rpm to 3650 rpm where it had absorbed 100% of the engine's power, 150 rpm below the WOT rated 3800 rpm. Also, there was no measurable increase in speed between 3500 and the WOT of 3650 rpm. I suspect there might be more propeller slip in this speed range than with the flatter (ideal) prop.

The flatter (ideal) prop with 300 more rpm and perhaps less slip, consistently delivered 0.1 kt. better top speed. It did this at 3950 rpm which is 150 rpm above the rated 3800 rpm.

The Yanmar is rated at 56 hp, but only about 35 hp is required to to produce the hull speed (SL=1.34) of 7.02 kts. This occurs at 3250 rpm which is right on the Yanmar prop power curve for this engine. The additional 21 hp only produces an additional 0.3 kts at most, which is entirely expected in a full displacement hull like the Willard/Fales 30. At this speed, the bow is high and the stern squats. The wake is enormous.

All of the speed trials were conducted with half fuel, half water, 2 pax, a fresh bottom job and clean prop and rudder. The engine was recently serviced with new fuel filters and a valve adjustment. It produces the factory specified high idle of 4140 rpm.

That's about all I can think of that might have any bearing on my results.

 

[FF]

>Over propped one should burn more fuel per stroke and under propped less<

This makes the wrong assumption that fuel burn is linear.Or that the engine is being operated overloaded.

Diesels come with fuel maps that show the highest efficiency , HP produced for fuel burned, is always at higher loadings, regardless of the operating RPM.

Yes an engine can make 50 hp at 2500 or 50 hp at 1500 but the amount of fuel required will be substantially less at the higher low RPM loading.

Of course the engine should be within its mfg power graph at the low rpm to preclude operating overloaded .
This setup may not reach the published,rated WOT with no harm at all.

Overloading in this case would only be theory, as the engine could not cruise at rated RPM at WOT.

The engine would not be overloaded , it would be operating with in its rating
just as a 1200, 1500 or 1800 noise maker would .

Actual overloading is more fuel than there is air to burn , the belch of black smoke seen when a plaining boat hits the throttle and waits for the RPM to come up.

No big deal its only a few seconds , and does not harm the engine as the air limits the HP produced.

Overloaded operation is not what is suggested buy overproping , having a setup that can not reach WOT is not a problem so long as cruising at WOT is not contemplated.

 

[bayview]

The funny thing about all this propping discussion is that slowing down a little will probably make much more change in fuel required than any propping or moving along the BSFC cure could ever do.

 

[Nomad Willy]

Good point Bayview!

Larry they say on WBO that 18hp is required for a cruise of 6 knots. Thanks for the details. It all makes sense to me.
Larry I wonder if your findings are good evidence of how poorly a 4 blade prop performs on small boats? Or an indication of how effective a larger dia prop is. Your blades were only 1/2" longer on the 4 blade. Perhaps the blade design is heavily involved. For some reason your 4 blade was a looser.

Fred I did say the over propped one should burn more fuel PER STROKE since it will be running at a slower engine speed and the same amount of power to push the boat is required so more power per stroke would be a given. That dosn't say the over propped boat would burn more fuel in an hours time or overall. So since it burns more per stroke it can't be burning much less fuel over time. Over propping burns less fuel but not much less. The fuel burn element of over propping is fly stuff but the other elements are not ... excluding noise reduction.

When I had a diesel car (Maxima) I liked going up hills as the engine got considerably quieter. Going at a steady 40mph on the level it was very noisy. Diesel engines running at a high load and low rpm are much quieter but a diesel running at relatively high rpm and less load despite the noise are probably enduring much less wear but many engine parts are excluded from that .. Like rocker arms and cam chains. The pistons and cylinders should be experiencing much higher wear at heavier loads and lower rpm ... like when over propping.

 

[bayview]

I doubt anyone here has ever worn out a diesels bore or bearings . Burned pistons cracked cylinder heads blown head gaskets, dropped valves from overload are a different story.

 

[psneeld]

The funny thing about all this propping discussion is that slowing down a little will probably make much more change in fuel required than any propping or moving along the BSFC cure could ever do.

Assuming that's the only reason you might overprop.

 

[Brooksie]

Here is a fuel map. The dashed line is a vehicle coming up to top speed in 4th gear (like a boat with the "proper" size, manufacturer recommended propeller.
The solid line at the top is the overload line. This vehicle comes to the overload line at top speed, HP, & RPM. If the owner lived in a country where the top speed limit was 40mph (like a trawler with limited speed) he might want to gearup (prop-up) higher so his dashed line went through the 245 g/Kwh portion of the map and hit the overload line at maybe 3200 rpms thus saving 20% on fuel 245 vs: 300 g/Kwh . Never passing the overload line or being anywhere near it at his cruise speed.