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post #1 of 46 Old 12-14-2012 Thread Starter
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overpropped vs underpropped

Hi,
I just had a new diesel engine installed, which seems to be working well.
I also have a max-prop propeller where you can change the pitch in 2 degree increments.

The target rpm at max throttle is apparently 3500-3600 for this engine according to the mechanic. I can make about 3260.

After signing off on the engine, and sending the warranty info to the mfr, the manufacturer is saying that I need to decrease pitch.

Doing the math from the prop manual, which reports about 7% rpm increase per degree, I would expect to be then underpropped, at about 3725?

What is the point? I can hit close to the target above and below the target rpm.
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post #2 of 46 Old 12-14-2012
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re: overpropped vs underpropped

The math is the math, you would/should be able to over rev the engine, assuming your 3260 is under normal load. Personally, I would not change it unless it somehow voids the warranty.
You should never run at more than 3/4 throttle continuously, and at WOT never for sure more than your engine manufacturer specifies (usually less than 30 minutes).
If you are making hull speed, leave it be - if you are not making hull speed in reasonable wave action at 3/4 throttle then your engine is too small, IMHO.
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post #3 of 46 Old 12-14-2012
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re: overpropped vs underpropped

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Originally Posted by groundhog View Post
Hi,
I just had a new diesel engine installed, which seems to be working well.
I also have a max-prop propeller where you can change the pitch in 2 degree increments.

The target rpm at max throttle is apparently 3500-3600 for this engine according to the mechanic. I can make about 3260.

After signing off on the engine, and sending the warranty info to the mfr, the manufacturer is saying that I need to decrease pitch.

Doing the math from the prop manual, which reports about 7% rpm increase per degree, I would expect to be then underpropped, at about 3725?

What is the point? I can hit close to the target above and below the target rpm.
You need to listen to the manufacturer since they will be the ones to void or not void your warranty. Most engine manufacturers want to see you within 100 RPM of max rated.. I know Westerbeke/Universal are pretty strict on this.

Most would rather see it slightly under propped than over propped. Add a load like a HO alternator, cruising supplies, engine driven refrigeration, head wind or head current and you'll be glad you're not over propped..

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post #4 of 46 Old 12-14-2012 Thread Starter
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re: overpropped vs underpropped

well, the engine seems to effortlessly move the boat as set up.
It reaches hull speed well before full throttle.

I just dont understand what being 250 or so rpm off makes any difference.
If I was well over propped, I could see re-adjusting. But they are going to force me to re-lift the boat now, well after they signed off on the boat when we could have easily made an adjustment.
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post #5 of 46 Old 12-14-2012
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re: overpropped vs underpropped

In my example my boat was underproped with a Maxprop.

@20 degrees
WOT no load 3400rpm
WOT under load 3400rpm

@22 degrees
WOT no load 3400rpm
WOT under load 3250rpm

I gained a good 3/4 knot at cruising 2600rpm.

Last edited by kellysails; 12-14-2012 at 01:52 PM. Reason: typo
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post #6 of 46 Old 12-14-2012
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Re: overpropped vs underpropped

I too have a max prop and a 4-108 Perkins that has a max continuous rating of 3600 rpm. One thing you need to check is the calibration of the tach using a photo tack. I found that mine was off quite a bit.

The standard wisdom is to set that pitch so that you can achieve that rpm rating at WOT.....I have not followed that wisdom and not suggesting that you follow what I did. I'm just relating what I have done. After changing that pitch and seeing how the boat performed under various setting(it's a 37 ft Tayana), I decided on a pitch that let me cruise at 6 knots at 80% of WOT. However note that my WOT is only 3000 and not the 3600 ....The only thing IMHO that I'm doing is not achieving the max hp that is available if I could go to the 3600rpm. I fail to see how that I'm lugging the engine or glazing the cylinders. I could be wrong, but I've been doing this since the engine was new back in 1985 putting an average of 60 hours per year for the past 28 years.
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post #7 of 46 Old 12-14-2012
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Re: overpropped vs underpropped

Just reread my reply and need to explain it better. I wanted to cruise at around 2500 rpm and lower pitch settings on the max prop would not let me achieve my desired 6 knots.
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post #8 of 46 Old 12-14-2012
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Re: overpropped vs underpropped

The easiest way to explain why you don't want to be over propped or pitched is by comparing your boat to a large truck pulling a heavy load. Your boat is NOT like your car which glides effortlessly down the highway where you can select the highest gear available and just loaf along. Your boat is traveling through the water which has a good deal of friction, and it takes a set amount of horsepower to overcome that friction. Your boat is more like a truck climbing a grade, all the time, and like the truck you have to be in the correct gear (prop/pitch) for the engine to be at the correct rpm in order to develop the needed horsepower. If you are over propped or pitched it's the same as being in too high a gear which puts much more load on the engine and builds a lot more heat. Your engine will not last as long and will be much more prone to a catastrophic failure (bearing failure, head gasket problems, turbo failure, etc), especially if you get into a situation where you need to run at WOT for an extended time. If you have lots of horsepower you can get away with it, possibly for a long time, but sooner or later it will bite you. Deciding to set your pitch in order to achieve a desired speed through the water at a certain rpm is throwing everything known about engine/prop theory out the window. You can't expect an engine to climb a hill in overdrive (which is what over prop/pitch is). Ask any good marine mechanic if over propping or pitching is a good idea, they'll undoubtedly have numerous horror stories to tell you.
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post #9 of 46 Old 12-14-2012
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Re: overpropped vs underpropped

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Originally Posted by jrd22 View Post
The easiest way to explain why you don't want to be over propped or pitched is by comparing your boat to a large truck pulling a heavy load. Your boat is NOT like your car which glides effortlessly down the highway where you can select the highest gear available and just loaf along. Your boat is traveling through the water which has a good deal of friction, and it takes a set amount of horsepower to overcome that friction. Your boat is more like a truck climbing a grade, all the time, and like the truck you have to be in the correct gear (prop/pitch) for the engine to be at the correct rpm in order to develop the needed horsepower. If you are over propped or pitched it's the same as being in too high a gear which puts much more load on the engine and builds a lot more heat. Your engine will not last as long and will be much more prone to a catastrophic failure (bearing failure, head gasket problems, turbo failure, etc), especially if you get into a situation where you need to run at WOT for an extended time. If you have lots of horsepower you can get away with it, possibly for a long time, but sooner or later it will bite you. Deciding to set your pitch in order to achieve a desired speed through the water at a certain rpm is throwing everything known about engine/prop theory out the window. You can't expect an engine to climb a hill in overdrive (which is what over prop/pitch is). Ask any good marine mechanic if over propping or pitching is a good idea, they'll undoubtedly have numerous horror stories to tell you.
Great analogy


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post #10 of 46 Old 12-15-2012
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Re: overpropped vs underpropped

I think the analogy is flawed in the sense that as I drive my Jeep up a hill in 5th gear I can tell that I'm lugging the engine seeing the rpm reduced to below 1000 no matter how much throttle I give it. I then down shift to 3rd and the rpm goes up and more power is available. Now on a boat there are no gears to shift from considering that the tranny is set in my case at 2.1:1 and the pitch is pre set with no means of adjusting underway. Under calm conditions I can maintain 6 knots at 2500rpm but as the seas and headwinds increase my speed will be reduced but that rpm will remain at 2500 plus I can still wot it at 3000. I'm just not using the available hp if I had preset the pitch at a lower setting. My 2 cents.
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