I have a quick question. I have installed a plastic prop on my 1983 Hunter 27 with a Yanmar 1GM. I have not checked speed with a GPS yet but it seems lower that with the two bladed bronze wheel. Here is the question regarding the transmission and its relation to the new prop. I do not seem to have any problems with the transmission. If it is overfilled with oil or not filled with clean oil will the transmission slip and deliver less that rated shaft RPM output. My impression is that it will turn until it dies and there are no plates to slip. It is straight gearbox similar to the read end gearbox in an car. What is the answer before I go changing the prop? Appreciate the community of Hunter owners, Keith
83 27 Ft Hunter, Transmission Question
- Thread starter boldseaman
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dito Landsend
The prop has too shallow (low numerically) a pitch. The prop and engine are matched to drive the boat at hull speed when the engine is at max continuous RPM. Assume the prop is 50% efficient. A sailboat should have a hull speed of around 1.3*sqrt(LWL). For your boat LWL is 22' so hull speed is 6.1 knots. Imagine that the prop is 100% efficient. Each revolution the prop advances by an amount equal to its pitch (inches/revolution). Each minute the prop will advance by pitch*RPM (inches) convert inches to NM, factor in the efficiency and you have the speed that that pitch prop would drive the boat at max continuous RPM. Since you want the pitch and not the speed rework the equation with max continuous RPM and 50% efficiency or pitch*RPM*conversion_factor*efficiency=knots to start. Reworking you get knots/(RPM*conversion_factor*efficiency)=pitch. Substituting you get 6.1 / (max_continuous_RPM*(1/1216)*0.5)= pitch of ideal prop to buy. Since there was lots of guessing about efficiency, hull speed, etc and props generally only come in a few pitches should you go for one with more or less pitch. More pitch will put the engine at a lower RPM at hull speed while less pitch will put the engine at a higher RPM at hull speed. I'd go for a higher than calculated pitch as you can't run the engine over speed for long and get away with it. This is the situation you have now, a low pitch prop running at max continuous RPM and not reaching hull speed. If you give me the max_continiuous_RPM and a small fee I'll calculate the pitch you need. ;-)
The prop has too shallow (low numerically) a pitch. The prop and engine are matched to drive the boat at hull speed when the engine is at max continuous RPM. Assume the prop is 50% efficient. A sailboat should have a hull speed of around 1.3*sqrt(LWL). For your boat LWL is 22' so hull speed is 6.1 knots. Imagine that the prop is 100% efficient. Each revolution the prop advances by an amount equal to its pitch (inches/revolution). Each minute the prop will advance by pitch*RPM (inches) convert inches to NM, factor in the efficiency and you have the speed that that pitch prop would drive the boat at max continuous RPM. Since you want the pitch and not the speed rework the equation with max continuous RPM and 50% efficiency or pitch*RPM*conversion_factor*efficiency=knots to start. Reworking you get knots/(RPM*conversion_factor*efficiency)=pitch. Substituting you get 6.1 / (max_continuous_RPM*(1/1216)*0.5)= pitch of ideal prop to buy. Since there was lots of guessing about efficiency, hull speed, etc and props generally only come in a few pitches should you go for one with more or less pitch. More pitch will put the engine at a lower RPM at hull speed while less pitch will put the engine at a higher RPM at hull speed. I'd go for a higher than calculated pitch as you can't run the engine over speed for long and get away with it. This is the situation you have now, a low pitch prop running at max continuous RPM and not reaching hull speed. If you give me the max_continiuous_RPM and a small fee I'll calculate the pitch you need. ;-)
To answer the question
Your tranny will not slip. It may suck up some HP if overfilled and not allow the engine to reach top RPM but it will not slip.
Your tranny will not slip. It may suck up some HP if overfilled and not allow the engine to reach top RPM but it will not slip.
The KM2A transmission does have a clutch
so it is not correct to think of it as a direct drive like the rear end of a car. Excess oil or dirty oil( that would be cause for concern) would not cause it to slip. Chances are that if your boat has slowed down that it is due to the change in prop. The prop's pitch has to be large enough to allow the engine to reach the rated top RPMs at wide open throtle. A GPS is not the best device to measure boat speed as it measures speed over the bottom and does not take into account speed or direction of wind and currents. A knotmeter would give you a better indication of boat speed. The "feels slower" gauge is not very accurate when it comes to boats.
so it is not correct to think of it as a direct drive like the rear end of a car. Excess oil or dirty oil( that would be cause for concern) would not cause it to slip. Chances are that if your boat has slowed down that it is due to the change in prop. The prop's pitch has to be large enough to allow the engine to reach the rated top RPMs at wide open throtle. A GPS is not the best device to measure boat speed as it measures speed over the bottom and does not take into account speed or direction of wind and currents. A knotmeter would give you a better indication of boat speed. The "feels slower" gauge is not very accurate when it comes to boats.
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