Parting Thoughts

Propping Basics

by Scott Huber

The purpose of a propeller is two-fold: the first is to move the boat, the second is to properly load the engine. The first, most people understand, the second, a lot of people do not understand.

The maximum obtainable RPMs of a marine engine is basically determined by the size of the propeller. A propeller that is too small will allow an engine to go beyond its rated RPM range. This is not good. A propeller that is too large, will cause the engine to run shy of its rated RPM range and therefore overload the engine. This is also not good. By changing the propeller we change the RPM range of the engine. Prop size is determined by two independent factors: the first is the diameter of the prop. The overall diameter, from blade tip to blade tip is the first number used in sizing a prop. The second factor is pitch. The pitch is the distance that the propeller would travel through the water with no restrictions after one revolution. Example: If a propeller has a 16” diameter and a pitch of 16 (16 X 16) would travel sixteen inches through the water with one revolution. It is important to note that propellers are also available in left or right hand rotation for forward. When adjusting your propeller, changing either the diameter or the pitch will change the RPM range of the engine and the performance of the boat.

Differing compositions can improve the performance of a propeller. A stainless prop is a more efficient (less slippage) than an aluminum propeller. The drawback to a stainless prop is that when you strike an object, there is a greater chance that you will do damage to your outdrive because it is a less forgiving material than aluminum and transfers any shock directly into the drive.

If you have a slightly damaged prop, you can fix small dings with a file. A bend in a blade or a folded blade requires replacement or reconditioning at a propeller shop. In determining the proper size prop for your boat there are four things to consider. The design speed of the hull (a knots or mph rating from the manufacturer), the horsepower of the engine, the RPM rating of the engine and the gear ratio. By calculating these four numbers you can determine the appropriate prop size. Call us for an consultation if you have questions on your prop size.

Categories: The Express

Tags: Marine Parts Express, prop size, propeller, propeller shop, props, RPM

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