Cavitation or Ventilation?

To fix it, we must first consider the differences.

Published: Tuesday, May 18, 2021
By: Professionals of Henry H. Smith Co

Cavitation and ventilation are two terms used interchangeably but are actually very different. The difference is especially important when they are discussed in the context of your boat. Both involve water and air but the sources of the air and the solutions are very, very different.

Ventilation is the introduction of air onto the propeller from the surface of the water. This can be introduced from under the hull, from the engine exhaust, or directly from the surface. Design ventilation (like propeller vent holes) can help increase RPM when needed but unintentional ventilation will cause performance issues.

Ventilation of the propeller reduces its contact with the water, diminishing its thrust and increasing the engine’s RPM. The boat will not accelerate or maintain speed as it should, a condition that is more obvious when cornering. Just as a vehicle will not perform well when it does not have traction (think ice and snow), a boat will not perform if the propeller is not well-connected with the water.

Cavitation, on the other hand, is air in the form of bubbles that are created by extremely low pressures in the water. These bubbles can cause surface damage when the vacuum decreases and the bubbles collapse or implode. Evidence of the implosion can be seen in pitting of materials and surfaces, often called cavitation burn.

Cavitation is most noticeable as noise and vibration, sometimes experienced at specific speeds or RPM’s (frequencies). The visible damage accumulates over time and an investigation into the source (upstream of the damage) is required to locate the source of cavitation. The source is often something new or changed, like propeller damage or a new through-hull fitting. Resolving cavitation requires eliminating the source of the extreme low pressure.

Ventilation solutions involve changes to your boat’s set-up and operating conditions. With outboards particularly, the motor mounting height or trim can be adjusted relatively easily. Remembering to trim-down when heading into a corner will also avoid ventilation and keep your propeller… propelling.