Propeller Specifications: The Basics
Here is some basic information about propellers that, if nothing else, will make you sound like you weren’t born yesterday down at the dock.
Direction: Engine/propeller units can turn either way. The direction is determined by observing the rotation from the stern looking forward. A RH (right-hand) prop turns clockwise, a LH turns counter clockwise. Some multi-engined boats will have counter-rotating propellers to cancel out any side thrust that naturally occurs from propeller rotation.
Diameter: The overall distance across the circle created by the blades. Do not confuse this with distance from one tip to another across the hub. Measure from hub center to blade tip and multiply 2x.
Pitch: the distance the propeller moves through the water in one rotation. Think of the propeller as a screw or threaded bolt. When you turn a bolt, for instance, into a threaded hole the distance it moves into the hole in one rotation is its pitch. The higher the pitch (meaning greater angle to the blades) the farther the prop moves in one rotation. As a rule, all props slip a little, so the stated pitch and actual pitch will be different by a small amount.
Cup: The curling, either by forming or casting, of the trailing edge of the propeller blade. Cupping generally provides more top speed and midrange efficiency with reduced slippage. A cupped propeller will reduce top engine RPM by 100-200 versus a non-cupped prop. Propeller shops can adjust cup to maximize an engine’s performance.
Rake: Not to be confused with pitch, rake is the angle of the blade’s projection fore or aft from the hub when looked at from the side, If the blade comes off the hub at a square angle, then rake is 0 degrees. A low rake angle provides the most power, while a high rake angle provides more speed and lift. General-purpose propellers come with a 20- to 30-degree rake.
Other things to look for on a propeller are shaft bore diameter and composition. Not all propeller shafts are the same. Also, propellers come in stainless steel and are generally more efficient due their strength and thinner specifications. They bend and don’t break when hitting something, as well. Some say this can cause more damage to a shaft, but that has not been this writer’s experience (note implied admission that I’ve hit bottom a few times). To soften the blow, cushioned hubs are fairly common, but are not found on high performance props.
So, diameter, pitch, rake and cup make a propeller. There are a number of programs on the internet for assessing your propeller’s performance, and professional shops will do the same, and make changes if you want them.