The size of a boat prop is described by two numbers, the diameter of the prop and the pitch of the prop. If a prop has a diameter of 30 cm (12 inches) and a pitch of 57.5 cm (23 inches), it is called a "12/23" prop. Most props have this information inscribed on the hub of the prop, but use or wear may render the inscription unreadable. If the boat has a previous owner who installed a custom prop, the prop may not be marked at all. Even so, the prop size can be determined during the boat's next outing.
Measure the distance from the centre of the propeller hub to the outermost point on one of the propeller blades. This is 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 which measures 15 cm (6 inches) from the centre of the hub to the outermost point on any one of the propeller blades has a diameter of 30 cm (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 defined as the distance one revolution of the prop will move the boat forward. Any calculation of a propeller's pitch will be approximate, since the actual performance of a propeller is affected by hull shape, water salinity, cleanliness of the external hull surfaces, air and water temperature and water depth.
Refer to the 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.
Measure the prop radius before you launch; everything else can be done in the boat while you enjoy the day on the water.