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post #9 of Old 12-14-2012
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Re: overpropped vs underpropped

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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