What's the right size propeller for my boat?

The correct propeller size (pitch) will allow your boat's engine to operate at the engine manufacturer's maximum recommended engine RPM at full-throttle. It is very critical to be sure your engine is propped correctly for your boat/engine combination and for the type of boating you will do. An engine that does not reach the recommended RPM at full-throttle is in an "over propped" condition, resulting in "lugging". This severe strain can lead to detonation, piston seizure and engine damage. An engine that revs past the manufacturers recommended RPM will have higher than normal wear and can also be damaged by fatigued parts breaking and passing through the engine.
To help with your selection process for a replacement or upgraded propeller with a larger or smaller pitch, use the following suggested guidelines:

> Adding 1 inch of propeller pitch will reduce full-throttle RPM by 150 to 200.
> Subtracting 1 inch of propeller pitch will increase full-throttle RPM by 150 to 200.
> If you're moving from a 3-blade to a 4-blade propeller, a 4-blade generally turns 50 to 100 RPM less than a 3-blade propeller with the same pitch.

Diameter
Diameter is the distance, in inches, across the imaginary circle made when a prop 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.

What is the difference between aluminum and stainless?

Many pleasure boats are equipped from the boat manufacturer with an aluminum propeller. Aluminum props are inexpensive and easy to
repair, and offer good all-around performance. Stainless steel propellers are over five times more durable than aluminum props, which mean that stainless steel propellers are less likely to be damaged when they encounter minor underwater debris. Stainless steel is much stronger than aluminum so stainless steel propellers can be made in a wider variety of shapes and sizes to increase performance on specific boat/engine applications.  

What's the difference between a 3-blade and a 4-blade propeller?

Three-blade propellers are good for general purpose use and, on many boats, are faster than 4-blade props. However, 4-blade propellers offer significantly different performance characteristics compared to 3-blade props. A 4-blade compared to a 3-blade propeller usually:
> planes the boat faster
> keeps the boat on plane at a slower speed
> gives improved mid-range speed at the same RPM
> provides quicker acceleration
> is less likely to ventilate in sharp turns
> offers better low-speed handling
> is not as fast on top end

If I want to switch to a 4-blade propeller, what size should I get?

If your current three-blade propeller allows your boat's engine to operate within the upper end of the engine manufacturer's recommended RPM range, choose a 4-blade propeller of the same pitch (or next lower pitch, depending on propeller selection) as your 3-blade propeller. Keep in mind that the 4-blade should decrease your engine 50 to 100 RPM.

What is propeller ventilation?

Ventilation occurs when air from the water's surface or exhaust gases are drawn into the propeller blades. The normal water load is reduced and the engine over-revs, losing propeller thrust. This action occurs in turns, when trying to plane the boat in a sharp turn or with an excessively trimmed-out engine or stern drive.