When describing a prop you'll always hear two numbers:
something like 13 x 17 .
The first number is always the outside diameter of the propeller in inches (outside edge to outside edge measured across the center).
The second number is always the Pitch in inches of the blades.
What this means is:
If the propeller was in a large vat of soft butter:
One revolution of prop would move it that far forward in inches through the butter.
Of course this would be in an ideal situation with no slip factor.
Everyone in the prop business seems to have a different formula for figuring prop slippage and boat speed to motor RPM ratio.
Most of them are sort of close but all of them are only theoretical rules of thumb.
A good prop shop will usually let you buy and try a prop for suitability with the option of exchanging it for a different Dia. X Pitch Prop.
Most of today's props have a cup at the trailing edge of the propeller blade.
This curved edge on the propeller lets it get a better "grab and go" thereby reducing slippage.
You'll also hear about double-cupping which is about the same deal:
improved for running in a trimmed out position at or near the water surface.
Air from the water surface or exhaust exiting from the exhaust outlet being drawn into the prop blades causes Ventilation.
When this situation occurs, boat speed is lost and engine RPM climbs rapidly(seems to rev up or breaks traction).
This is often the result of trimming the engine too far up or out, sharp turns, and outboards mounted too high.
Cavitation; often confused with ventilation, is a result of water boiling because of extreme reduction of pressure on the surface of the propeller blade.
Propellers often cavitate during normal operation, but extreme cavitation can cause a cavitation burn on the prop's surface which looks like little pits on the blade.
Cavitation is caused by incorrect prop selection, bent or distorted blades, bent prop shaft, and the list goes on.
The propeller should be checked regularly to be sure all the blades are in good condition.
If any of the blades become bent or nicked, this condition will set up vibrations in the drive unit and the motor.
Remove and inspect the propeller.
Use a file to trim nicks and burrs.
TAKE CARE not to remove any more material than is absolutely necessary.
For a complete check, take the propeller to your marine dealer where the proper equipment and knowledgeable mechanics are available to perform a proper check of pitch, rake, and diameter at modest cost.
Inspect the propeller shaft to be sure it is still true and not bent.
For the best prices on a new prop click here.
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