Resources // Michigan Wheel Marine
A propeller with a diameter larger than will fit under my boat was suggested. Can a smaller diameter still be used without hurting my performance significantly? What other adjustments to the propeller should be made to compensate?
If the difference between the optimal and allowable diameter is minimal, say 1 or 2 inches for a medium size pleasure boat, a corresponding increase in propeller pitch will be effective at maintaining reasonable propeller performance. For some situations, it may be suggested to increase blade number, say from 3 to 4, to provide more blade area on the diameter constrained propeller. For some rare cases, particularly on re-power projects, a engine-gear combination is selected with the wrong gear reduction ratio. This can result in a significant propeller selection problem with no real solution other than changing to a different gear reduction ration.
You suggested a 13” diameter x 14” pitch propeller for my boat. Would a smaller diameter propeller with more pitch provide the same performance?
A propeller with smaller diameter and larger pitch might provide the same load on the boat’s engines but may not have the same efficiency as the suggested propeller. Propeller diameter is chosen to be optimal for the boat’s particular combination of horsepower, RPM and speed. Deviating significantly from the best diameter may result in slower acceleration, reduced cruise speeds and efficiencies, and more difficulty in slow speed maneuvering.
Can one propeller be best for both maximum boat speed and maximum pulling power, say when pulling a skier or fish nets?
In general, no. If maximum pulling power is desired, the propeller size, particularly the pitch value, is often reduced to allow higher engine RPM (where more power is produced) at lower boat speeds. For maximum top boat speed, the optimum propeller pitch value will typically be higher.