A sailboat prop can have two blades or three. A 2-blade prop is more efficient (because there is less interference between the blades) and creates less drag while sailing. A 3-blade prop gives somewhat less vibration and better control while docking.
Diameter
and pitchProps are sized first by diameter and then by pitch, both measured in inches. The pitch measurement is the theoretical forward travel during one full revolution, neglecting slippage. A 14 x 9 prop, for example, is 14 inches in diameter and has 9 inches of pitch. A prop should be sized such that The other critical dimension is, of course, shaft diameter.
Prop
rake and
cupRake is the angle between the center line of the prop blades and the shaft, expressed in degrees aft of a line perpendicular to the shaft. Cup is extra curvature added at the trailing edge of a powerboat prop blade, or at the tip, primarily to reduce cavitation at high RPM. Cupping increases rake, when added at the tip, and it increases pitch, when added at the edge. It is usually described as low, medium, or high cupping. Most sailboat props have no cup except for some folding and feathering props.
Direction
of
rotationLike engines, almost all sailboat props are right-handed and rotate clockwise when viewed from astern, which means that their prop walk turns the stern right (facing aft), or to port, in reverse. Left-handed props do exist, however, because they are often paired with a right-handed prop in boats with two engines.
Diameter and pitch: the propeller should be sized such that the engine just reaches max RPM at full throttle in calm water when lightly loaded.
A propeller shop can change the pitch, rake, and cup of a bronze prop by heating it and beating the blades against formed cones or mandrels. The shop can also increase the shaft size and reduce the prop diameter. As a rough approximation, one inch of diameter equals two inches of pitch and medium cupping adds one inch of pitch. Online calculators are handy for playing with the numbers.
Inspect the prop at every haul-out for nicks, bent blades, curled blade edges, pits, corrosion, or marine growth, any of which will detract from performance.