DIY Network

Propeller: Choosing the Right One

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Diameter and pitch are printed on the inside hub of the propeller. Here, the diameter is 14", while the pitch is 19". The pitch distance -- as shown in this illustration -- is the distance the propeller could travel if moving through a solid. When determining the propeller that will give you the best performance, trim the engine and run at full throttle, then check the RPM.

Here are a few terms to know to when selecting a propeller:

• Diameter: If you drew an imaginary circle around the tips of the blades and measured the area the blades scribed, this would be the diameter of that particular propeller.

• Pitch: The theoretical distance the propeller would travel if going through something solid (but keep in mind, in water, there's slippage).

• Cupping: To thwart the slippage that can occur, manufacturers bend the ends of the blades a fraction to "cup" the water as the propeller blades move through the water. This improves performance by decreasing the slippage.

Selecting a Propeller

• Start by looking in the owner's manual. The manufacturer will indicate a certain RPM range that the boat should deliver. When running at top speed with the engine trimmed up, the boat should be able to run within that RPM range. To test this, run the boat with no gear on board, only one passenger, and about a quarter of a tank of fuel. Run the boat at full throttle with the engine trimmed up to your boat's best running attitude. Trim the engine up until it cavitates a bit. Trim the boat back down until the cavitations are gone -- this is the optimal engine trim. Look at the tachometer to check the RPM. Replace the propeller if needed to get within this range -- keeping in mind that it will be different with passengers, a full tank of gas, and any type of load you might carry. For every two inches down in pitch, you'll pick up 300-400 RPM in speed.