Two numbers are used in describing the size of a propeller. One describes the diameter and the other describes the pitch. Diameter is always the first number listed. A 24 X 20RH prop has a diameter of24". The "RH" stands for right hand rotation of the propeller. Viewing this like a screw, a "RH" propeller would push the boat forward when turning clockwise. Likewise a "LH" propeller would push the boat forward when turning counter-clockwise.
Diameter is the distance across the circle the prop makes when it rotates. The diameter is two times the distance from the center of the hub to the tip of the blade.
Pitch is the theoretical forward movement of the propeller during one revolution. Slippage occurs in most boats, so the actual forward movement is less than the design pitch. Pitch is the second number listed in the propeller description.
Many of today's propellers feature a cup at the trailing edge of the blade. This curve causes the propeller to get a better bite on the water. This reduces ventilation and slippage. The cup can result in higher top speeds where the motor can be trimmed so that the propeller is near the surface of the water.
Ventilation occurs when surface air or exhaust gasses are drawn into the propeller blades. Tight turns, over-trimming the engine or mounting the engine high on the transom can cause this. It can also cause loss of speed or rapid rise in RPM's.
Cavitation is a phenomena of water vaporizing or "boiling" due to the extreme reduction of pressure on the back of the propeller blade. Many propellers partially cavitate during normal operation, but excessive cavitation can result in physical damage to the propeller's blade surface due to the collapse of microscopic bubbles on the blade.
There may be numerous causes of cavitation such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blade edges, etc ...
Rake is the degree that the blades slant forward or backwards in relation to the hub. Rake can affect the flow of water through the propeller, and has implications with respect to boat performance.
Correct propeller size for your boat and engine combination is based on a wide open throttle. Your operator's manual will tell you the highest RPM for your motor. The best propeller for your boat will allow your engine to operate in the recommended range and maximize the performance of your boat.
You should never boat without a spare propeller as you should never drive your car without a spare tire. Stores are few and far between out on the water.
