A propeller can be defined as follows: A mechanical device formed by two or more blades that spin around a shaft and produces a propelling force in boats (or airplanes). There are several technical terms to define the propeller’s characteristics such as: diameter, pitch, disc area relation, hub, bore etc. All these characteristics are calculated to design the optimal propeller accordingly to specific needs of the customer and the boat characteristics.
Pitch: Is the displacement a propeller makes in a complete spin of 360° degrees. This means that if we have a propeller of 40” pitch it will advance 40 inches for every complete spin as long as this is made in a solid surface; in a liquid environment, the propeller will obviously slide with less displacement.
The pitch concept is not exclusive for propellers; other mechanical devices like screws also use it. For instance, a screw with 10 mm of pitch will advance 10 mm for every complete turn when hit by the screwdriver. In fact, the “screw propeller” concept is literally making reference to that the propeller works exactly like a screw. It is very important that both, pitch and diameter are properly calculated. If for any given HP the pitch is too big, the propeller becomes heavy and demands more power than the engine can reach and vice versa, if the pitch is too small then we have a light propeller that wouldn’t absorb the engine’s full power.
So, what would be the appropriate pitch? Certain parameters need to be checked like power, rpms, gear reduction, size of vessel, vessel application (i.e. a trawler or a tugboat needs power while a yacht requires velocity).