A propeller converts the engine’s power to thrust that drives the boat.  As the propeller turns, low pressure is created on one side and high pressure is created on the other side.  The propeller moves toward the low pressure, moving the boat with it.  How well a propeller moves toward the low pressure depends on several factors. 

  • Thin blade (low resistance)
  • Diameter (thrust)
  • Pitch (load)

No one single propeller design will work for all applications.  Hull weight and available horsepower dictate what will work for best for you.  You need to keep in mind the operating range of the engine and select a propeller that will allow the engine to come close to its maximum RPMs at WOT (wide-open throttle). 

Two important terms to become familiar with are ventilation and cavitation.  Both are issues that can occur from poor rigging, improper prop selection, or a damaged propeller.

Ventilation is a problem that means air is being pulled into the propeller from the surface of the water or from exhaust gasses while in reverse.  The ventilation plate on the lower unit tries to limit this problem.  Prop design or high mounting height can be possible sources for this issue.   It is not unusual to vent the prop with some hull designs during hard turns. 

Cavitation describes water that is actually boiling on a surface because pressure has dropped so low.  A damaged prop blade will cause a very low-pressure area behind the damaged area.  Water boils at 212 degrees at sea level.  Dropping the pressure on a prop blade enough will cause the water to boil until it moves across the blade to a high-pressure area.

March 23, 2017 by Sheri Reeves
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