Propeller Basics

Diameter

The distance across the imaginary circle that is made when the propeller rotates.

Pitch

The theoretical distance, in inches, that a propeller travels forward during one revolution. Think of pitch as speed. The higher the propeller pitch that the engine can turn, the faster the boat will usually go. You only need to choose the correct pitch propeller for the boat and engine. The correct diameter for your required propeller has been predetermined by the manufacturers.

Determining the Correct Pitch

Consult the engine owner's manual to find the recommended wide-open-throttle (WOT) range for your engine. If the current propeller is at WOT RPM within the specified RPM range, select a replacement or upgrade propeller with the same pitch as the current propeller.

If the current propeller isn't at WOT RPM within the recommended RPM range, select a replacement or upgrade propeller with a larger or smaller pitch using the following tips as guidelines:

• Adding 1 inch of propeller pitch will reduce WOT RPM by 150 to 200.
• Subtracting 1 inch of propeller pitch will increase WOT RPM by 150 to 200.
• If you're upgrading your existing 3-blade to a 4-blade propeller, remember that a 4-blade propeller generally turns 50 to 100 RPM less than a 3-blade propeller with the exact same pitch.

Propeller Materials  

The most common materials that propellers are made out of are aluminum or stainless steel. Most outboard and sterndrive propellers are made from aluminum. Aluminum is inexpensive and is suitable for general purpose use.  Stainless steel is much more expensive, however it's over 5 times more dirable than its aluminum brother. Surprisingly the material the propeller is made from also affects performance.

Fitting a stainless steel propeller will increase performance. The reason for this is simple. Stainless steel is a much stronger material than aluminium which enables the propeller to be manufactured with a thinner blade section. If the leading edge is thinner less air bubbles will be formed on impact and thus cavitation is reduced. A thinner blade is much more efficient that a thicker one. For racing applications the blade section can be thinned to such an extent that you could literally use the propeller to have a shave. For most applications such a thin blade section is not desirable as it is more prone to failure under stress. For the enthusiastic boat owner, simply changing from an aluminium propeller to a stainless steel one will give improved performance.

4-Blade Propeller Facts

• Plane the boat faster than 3-blade propellers
• Keep the boat on a plane at a lower speed
• Give improved mid-range speed at the same RPM as a 3-blade propeller
• Provide quicker acceleration than most 3-blade propellers
• Run smoother than 3-blade propellers
• Have better holding power in rough conditions
• Are less likely to ventilate in sharp turns
• Provide better low speed handling
• Are not quite as fast on the top end as a comparable 3-blade propeller