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post #39 of Old 12-18-2012
MedSailor
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Re: overpropped vs underpropped

Quote:
Originally Posted by jrd22 View Post
I agree with Minne. It's not just that you can't reach WOT, it's that at 2500 rpm you are asking the engine to do what it normally would at 2800 rpm (approx) with less hp so it puts more strain on everything. That's not a huge difference and your engine might do OK, but if you get into a situation where you are motoring into a stiff wind and large waves and you need to run near full throttle that is when you might blow a headgasket or overheat, etc.
Well put. From the mechanics I've talked to, being ABLE to reach max RPM at WOT is the only real way you can tell if you have the correct amount of stress on your engine at any given speed.

How many hundreds of RPM can you fudge it and get away with? That depends on a lot of things and many unknowns. If you adjust your new prop so that you can reach max RPM at WOT and you can still hit your cruising speed at a reasonably efficient RPM then there is no downside.

At what RPM do you currently reach hull speed/cruising speed? What engine do you have?

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