How to Figure a Boat Propeller
- 1). Consult your boat owner's manual for the stock propeller that mounts on your engine. Write down the numbers that indicate the propeller diameter, pitch, cupping angle (if included), and metal makeup, such as aluminum or stainless steel. Note whether your propeller has a two-, three-, four- or five-blade configuration.
- 2). Look on the propeller hub. You will see two numbers, something like 10 X 16, for example. The first number is a size increment which indicates the diameter of the propeller. The diameter number equals twice the distance from the center of the hub to the outside tip of the propeller blade. It can also be envisioned as the distance from one end of the propeller to the other as it forms a circle while rotating. Use a tape measure to measure your propeller diameter with this formula.
- 3). Look at the second number on the propeller hub. This number indicates the pitch, which is the distance a prop moves forward during one revolution. When the blades have sharp angles (more tightly wound) on their inside surface toward the hub, they have a higher pitch number. If the blades appear more splayed out and flatter, the pitch angle will be lower. Remember that the actual travel of the prop through the water produces some slip, which accounts for about 10 to 20 percent less pitch than the actual stamped design number.
- 4). Look at the trailing edge of the propeller to see if you can detect a "cupped" design. A cup appears as a curved lip on the end of the propeller blade. The cup allows the propeller to "bite" more through the water, which results in less slipping. A cupped propeller blade has more power and faster response from a standing start, but allows fewer engine revolutions per minute. It performs better near the water's surface when the craft has been trimmed out. You can hold a straightedge ruler over the blade tip to detect a cup.
- 5). The angle or degree at which the propeller blades slant forward or backward, when looking at the hub, determines the rake. Rake generally determines the boat's performance characteristics. Aft rake, or a backward slanting propeller, helps bring the bow of the boat up during trim. Aft rake also keeps the forward section of the boat drier, while providing higher top-end speed. When the blade has a forward or negative rake, it forces the bow of the boat down. The forward rake propeller is suitable for heavy pulling.
- 6). Determine if you have an aluminum propeller. They have a duller finish and weigh less than steel props. Aluminum props, although more expensive, have more flex and poorer durability than stainless steel. An aluminum prop with four blades will be more stable and will have better acceleration from a dead stop. Three-blade props will have a better top speed, but less acceleration from a standing start.
- 7). Determine if you have a stainless-steel propeller. This will be heavier and much shinier than aluminum. It will provide better overall performance, last longer and have no blade flex. The three-blade steel prop will perform adequately during standing acceleration, and has a good top speed. The four-blade variety will have better acceleration from a standing start, have better stability and provide better fuel economy. The five-blade steel prop will have great standing-start acceleration but will suffer in the mid- and high-range speeds, with lower fuel economy.