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Old 15-07-2012, 13:07   #16
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Originally Posted by Blue Stocking

This is my whole point, I am nowhere near WOT to attain 2100 rpm.
Engine, when head was lifted, showed no carbon, no ovality on the bores, doesn't consume lube oil.
As I conjectured, if the engine is happy and shows no signs of being stressed, all I will do is accelerate wear--and burn more fuel.

I put this question out there to motivate the intelligent feedback I have received to date.
Recently there have been a few threads about re-powering sailboats, and it seems we err on the side of bigger engines.

--and Phil Rhodes' pedestal was built a long time ago.
By conventional wisdom the boat is overpropped and overpowered.

If as, as you say the engine is turning 2100 rpm and the boat is making hull speed, and advancing the throttle further results in no more RPM this doesnt make complete sense...

- we are hull speed. More hp wont help push the boat faster unless we get lots more horesepower. But the engine is not at full hp so advancing the throttle something should happen. The engine should start passing unburned fuel (black smoke and not a good result) or the rpm should rise, the prop will probably cavitate but the boat goes no faster. Cavitating props erode so that is also not a good result.

- you basically have horsepower on board you cannot convert to propulsion. By lowering the pitch you raise rpm. This will make idle forward slower, good for maneuvering around marinas, give better acceleration, good for plowing into and through rough seas and getting the boat moving and outs the engine at the proper power band

The drawback is for a given boat speed the engine is running faster and may be more noisy. The question about how much horsepower you are generating and how much fuel you are burning is sort of a lame duck. The power curve the manufacturer shows is ideal state, i.e. ideal torque load - which you currently dont have. Barring inneficiencies it takes X horspower to push your 26k boat at 7 knots. Creating X horsepower requires basically the same fuel. So while the lower pitch prop will require the engine and prop turning faster they will be more unloaded than the higher pitch prop at same hull speed.

If you just want to tool around at the lowest rpm, get the highest pitch prop you can, you might get hull speed at 1500 rpm. At some point you are so low on the torque curve the engine will produce black smoke and you wont be able to get the rpm into a usable band.

Having too much horsepower on a displacement boat creates its own set of issues, which you are mulling over...

For me its having a matched system, boat, engine and prop. Experience says some extra horsepower (10-20%?) and little lower pitch for acceleration, handling chop and power take off.

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Old 15-07-2012, 15:05   #17
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Re: Sailboat propulsion engines generally oversized?

Have you considered that maybe you arenít getting full throttle? Most engines will produce some smoke when overloaded like yours appears to be. What is the max rpm in neutral? If you are truly over proped will most probably lower your fuel consumption slightly. Just my two cents, Mike.

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