Ventilation

Stainless steel propeller with vent holes

What is Ventilation?

Ventilation is often confused with cavitation. For information about cavitation, see our What is Cavitation post.

Ventilation is when surface air or exhaust gasses are drawn into the propeller blades. When this occurs, boat speed is lost and engine RPM increases rapidly.

What causes Ventilation?

Ventilation can result from:

  • excessively tight cornering
  • a motor that is mounted very high on the transom
  • over trimming the outboard motor or sterndrive
  • concave hull damage causing the transom to lift and pulling the propeller toward the water’s surface

Can ventilation ever work to your advantage?

Occasionally, venting a propeller can be used for positive results. In a stainless steel outboard propeller, vent holes behind the blade can be used to introduce bubbles which add turbulence that allows the propeller to spin a bit faster. This will raise RPM for a quicker hole shot that does not affect top-end speeds. It can be helpful for boats that have difficulty getting onto plane, since it allows the engine to turn at higher RPMs. If your prop did not come with pre-drilled vent holes, we can add them! This is not always a recommended option. Bring your prop to our propeller professionals for advice.