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post #3 of Old 12-14-2012
Maine Sail
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re: overpropped vs underpropped

Originally Posted by groundhog View Post
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..

-Maine Sail / CS-36T

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