PROPELLER TECHNICAL DATA
Standard series propellers are designed to function in a non-cavitating to a partially cavitating environment. Cavitation is a water vapour cavity which forms on the surface of the propeller hub or blade as a result of low pressure due to water flow over the blade surface.
Stable cavitation is quite common on smaller performance propellers and often results in no adverse effects.
Unstable cavitation can result in vibration and noise problems or in extreme conditions, blade surface erosion. Cavitation is not necessarily bad but needs to be controlled to avoid problems. A primary effort in sizing is to qualify an application as to the amount of blade loading (pressure in pounds per square inch) and what propeller area ratio is required. Too much blade area can reduce the efficiency of a propulsion system because the more the area the more drag.
There are ranges of loading that will predicate which style of propellers can be used. Typically, this ranges from the 3 blade, on moderately sized boats (through 40 feet), 4 blade on mid-range to larger (40 to 100 feet), with 5 blade coming into play where there is extreme blade loading and compromise of diameter. There may be overriding considerations in selecting a 4 or 5 blade over a 3 blade such as maximizing vibration reduction.
On moderately sized boats, generally speaking, if optimal diameter is possible with adequate tip clearance, a 3 blade will yield the best top end speed. However, the choice of a 4 blade may provide similar cruising speed and may offer a more comfortable ride with less vibration. With an increase in blade number, the “blade rate frequency” increases for a given shaft RPM.
In general, the higher the blade rate frequency, the less problematic vibration is. On the larger, heavier applications, with higher gear ratios, the loading requires greater area ratio, and 4 or 5 bladed propellers have a better speed potential. Properly matching propeller area ratio to an application will optimize propulsion and reduce the possibility of destructive cavitation.