How to Measure Boat Prop Diameter

For a propeller to give a perfect fit on a boat, it should be of correct diameter. When a prop does not help the boat to achieve its optimum performance, a simple increase or decrease of diameter will solve the problem and provide increased performance. The correct diameter will depend on the length of the boat, the weight of the boat when it is half loaded, and the maximum permitted rev of the engine and the planing or displacement of water caused by the movement of the boat. The diameter is also useful to determine disc area and the disc area ratio. 

The ideal diameter of the prop is usually stamped on the side of the hub closest to the prop shaft hole. There will be two figures given. The first figure will be the diameter of the prop. For instance if the figure given is 12×11, the diameter will be the first figure in this case-12.

The straightforward method of measuring the diameter of the prop is to use a measuring tape.  The distance between the edge of the blade and center line of the shaft hole should be measured to give the radius. Multiplying the   radius by two will give the diameter of the prop. The general view is that if the diameter is big the performance is better. This is not always the case. The hull underbody could create limitations on boat performance based on prop diameter.

The other method of measuring the overall diameter is to measure the circle surrounding the prop blades from tip to tip. When a propeller of increased diameter is fitted on the boat, it increases the torque to run at a specific speed.  Propellers are available with diameters that are usually in one inch increments.  Therefore all measurements should be made in inches.

The Hub diameter is another diameter that impacts the size and performance of a propeller. The hub diameter is measured by subtracting two times the blade length of the propeller from the root of the blade to the tip.

The use of the boat is an important consideration when increasing or decreasing the diameter of a boat prop. Boats that carry or tow heavy loads or are used for leisurely cruising need props with large diameters. Cruise boats, pontoons and some fishing boats are examples of boats that require props with large diameters. Large diameter propellers give greater thrust for slow speed boats. For high speed boats used for racing or other forms of recreation, a prop with a small diameter is more effective. For slow or calm waters in lakes and rivers, large diameter props give good performance. For the high seas small diameter props perform better.

Following the manufacturer’s specifications in the manual for the best diameter for the prop for the engine from the manual is another method of getting the diameter of the best performing prop.

The diameter of a prop is an important measurement that impacts the perfect propeller fit for optimum performance of the boat.