RPM

[dsw]

Would decreasing the pitch have a noticeable impact on the rpm or would the diameter of the propeller change the rpm ?

 

[VicS]

Decreasing the pitch will for a given speed through the water increase the RPM. In theory halving the pitch will double the revs.

 

[roly_voya]

Changing the diameter changes the amount of power required to turn a prop with a given pitch so if 40hp will drive a 7 ton boat at 6kn using a 10" pitch and a 10" dia prop then it should drive a 14 ton boat at 3kn using a 5" pitch and a 14" dia prop (double the area). This assumes rev are kept the same, doubling the revs as the same effect as doubling the pitch.

 

[boatmike]

If you are restricted on the RPM your engine will reach compared with its theoretical maximum then this may well be caused by too course a pitch. Reducing the pitch will allow higher revs. The relationship is not directly linear as slippage will be greater at higher pitch more than likely but roughly so. The diameter does affect the torque required to drive it but is far less directly related to RPM. If you were to give us the engine HP at max RPM (theoretical), the pitch and diameter of the prop and the gearbox ratio. Then tell us what boat it is (waterline length especially) and what speed you obtain at whatever RPM you actually achieve I am sure a more useful answer could be given.

 

[jimbaerselman]

For a given revs and boat speed, a bigger propeller must have less pitch to absorb the same power.

However, efficiencies change, so it's difficult keep the same speed with the same revs and the same power and a different prop setting! Ok if you're tied up!