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post #13 of Old 07-07-2009
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Originally Posted by L124C View Post
Yikes, thats not a drive train, it's a reef Poor little Yanmar! I've sometimes noticed that my motor (same one) will smoke at 3000 RPM if I'm fighting a strong tide and/or headwinds. If I back off the RPM, it seems fine. The motor purrs at 3000 otherwise, so I assume it's not a prop issue. This has never made sense to me, as the motor shouldn't "know" what the opposing forces are. Right? They should simply make the boat go slower. Or, are the forces somehow "lugging" the motor? Hypothetically, if the boat was tied to the dock, shouldn't the motor turn effectively at 3000 RPM? Dock, line, boat and dock cleat failure aside of course Someone please explain
Depends if you mean having the motor turning the prop when at the dock or running in neutral. If in neutral then it should theoretically run to it's max governed speed.
However in the context of turning the prop, forces come into play.
If you are stationary and turning the prop, then the blades angle of attack to the water are at maximum so it has maximum load and it's static (the boat not moving rpm will be lower).As the boat begins to move forward the props angle of attack relative to the water is decreased so the load on the prop is less and the motor can spin it easier, faster the boat through the water the lesser the AoA on the blades.
So if your are motoring into a headwind your boat speed will be less so the AoA will be greater.
For instance if I am motoring my boat and try to go to full power too early then I will get smoke but if I allow the boat speed to increase and I incrementally open the throttle I can achieve full power and no smoke from the engine.
Same with a propeller driven aircraft, on the ground your max static rpm will be lower then what you will achieve when you are flying. In an aircraft this effect is more easily seen as if you place the aircraft in a dive to increase air speed you can actually over speed your motor if you do not reduce power.

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