How to Determine Boat Prop Size

A boat's propeller has two numbers that relate to its diameter and pitch. If a prop has a diameter of 12 inches and a pitch of 23 inches, for example, it's called a "12/23" prop. Most props have this information inscribed on the hub, but use or wear can render the inscription unreadable. Still, obtaining this information is a process even a novice mariner can perform.

Measure the distance from the center of the propeller hub to the outermost point on one of the propeller blades. This measurement represents the radius of the propeller. The first number used in describing a propeller is the diameter of the circle described by the propeller, which is twice the radius. Hence, a propeller that measures 6 inches from the center of the hub to the outermost point on any one of the propeller blades has a diameter of 12 inches.

Determine the pitch of the prop by observing the performance of the boat on the water at half-speed, taking note of the indicated speed and the number of RPMs indicated on the boat's tachometer. The pitch of a prop is the distance, in inches, one revolution of the prop will move the boat forward. Any calculation of a propeller's pitch is approximate, since the actual performance of a propeller is affected by factors such as hull shape, water salinity, cleanliness of the external hull surfaces, air and water temperature and water depth.

Refer to your boat operator's guide to determine the gear ratio of the boat's power train. If the ratio is 3:1, you will only need to use the first number, "3," to calculate the pitch of the prop.

Calculate the approximate pitch of the prop by dividing the number of RPMs indicated by the boat's tachometer by the gear ratio of the boat's gear box. Multiply the result by 1.2 and divide that result by the speed indicated on the boat's speedometer. If the answer is not a whole number, round the answer up to the next largest whole number.