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post #26 of Old 12-16-2012
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Re: overpropped vs underpropped


Not trying to pile-on here, but I also disagree with your analogy. The OP's problem is NOT being able to reach max rated RPMs whereas in your analogy any gear other than D would result in more RPM's.

Also, your analogy seems to point to fuel efficiency as the goal. No mechanic, or holder or a warranty gives a rats ass about MPG. I think they are worried about wearing out (or breaking) the engine prematurely, which is also what I'm worried that the OP will do.

On the topic of MPG, for any given hull/boat, there is an optimum engine HP rating. In a perfect world you would choose an engine that is exactly sized and propped to reach hull speed (or your desired cruising speed) at the engine's maximum torque and efficiency curve.

Now we enter the real world of the boats, props, and information that we have. The OP has an engine of a certain HP installed in his boat, and presumably can't/won't change that. Now he has a choice; he can either choose a prop pitch/size that will optimize MPG or engine life.

I'm with jrd22. I think he should ask SEVERAL mechanics that are preferably familiar with his engine and ideally his boat. He should ask them exactly how close to rated rpm at WOT is close enough, and then prop to suit.

From what he said earlier, he can reach hull speed without being anywhere near full throttle, so he likely has an engine that is well sized to his boat. Once he de-props it a couple inches he'll still likely be hitting hull speed easily and hopefully it'll be closer to where it should be on the mileage curve.

On my boat I solicited the opinions of several mechanics, and the one I trust the most told me that the rating on my engine (4000rpm) is actually bogus and could back this up with some good reasoning. He recommended that I prop mine to hit 36-3800rpm at WOT and I've done that. My circumstance is a bit different as I have a screwy, older engine (Perkins 4.108) and this mechanic had messed around with my particular engine on my particular boat before. My engine is also undersized for my boat, but old, sturdy, and heard to break, so I've pitched for a slightly shorter lifespan in exchange for being ABLE to reach hull speed at 75% throttle. I mention this because I've given the topic a LOT of thought and time over the years.

Speaking to the physics again, here is another thought experiment. Imagine the OP fitting a tugboat prop to his boat. He can hit hull speed at 500RPM and WOT gets him 550RPM. He looks at his fuel curve in his manual and shouts with joy because he's only supposed to be burning 0.5gal/hr at 500RPM. The only problem is that that consumption curve is assuming a regular load. What he will actually find is that he's burning more diesel than an appropriately size prop (due to inefficiencies in overly-rich combustion) and he will be putting so much stress on his engine as it labors with all that torque that he will wear out his engine in no time.

TQA, in the interests of continuing our dialog, can you give us specifics on which information you feel is inaccurate?


PS To the OP: all of these discussions about reaching WOT are intended for a boat with a CLEAN BOTTOM AND PROP. If your bottom and prop are foul don't re-pitch based on your experiments. That is, unless you expect to always have a foul bottom and prop.

I have a sauna on my boat, therefore I win.
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Last edited by MedSailor; 12-16-2012 at 11:24 PM.
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