Engine RPM, shaft speed and propeller diameter

Sailboat propeller shaft speeds are typically between 850 – 1650 shaft RPM (they are not speed boats).  Faster shaft speeds have too high a tip speed and blade loading that will produce noise, vibration and cavitation. The propeller sizing is made for this approximately maximum range of 1700RPM.

The combination of power from the engine (hp and RPM) and shaft speed, as determined by the gearbox ratio, must be appropriate for a sailboat installation.

Consider the example of a very high engine RPM, such as 3800 at maximum power, running through a 2.682:1 reduction gearbox.  The propeller tip speed will be very high and so too is the blade pressure.  A combination of these factors will induce cavitation on the propeller, which will manifest itself as a noise or possibly even a vibration at the top RPM.

In order to counteract the potential cavitation, the propeller diameter and consequently the blade area ratio must be increased. This means that the propeller must be oversized for the engine and therefore maximum attainable RPM will be about 3300-3400 RPM at full throttle. In this scenario, the propeller will hold down the engine from reaching the maximum RPM of 3800 at full power.  Therefore, to ensure low cavitation levels, the gearbox reduction ratio would need to be changed to 3.0:1 or larger.

Reducing the propeller diameter will increase shaft RPM.  However, a reduction in diameter will result in less thrust throughout the RPM range.