23 w/ twin 225 OPTIs....

[peterb]

When you say that you are cavitating, do you mean that your prop's are coming out of the water as you go from wave to wave?

[Trey1096]

I expect that is the case Peter. My 23 SF has twin Merc 200’s on the transom and it’s bad to suck air into the props when the seas kick up.

What’s the best way to prevent it?

Trey

[Fr. Frank]

When you say that you are cavitating, do you mean that your prop's are coming out of the water as you go from wave to wave?

That would be "ventilating" not cavitating. If you're ventilating unintentionally, you need to get the propeller deeper in the water during the conditions wherein the ventilation occurs. Cavitation is the production of air bubbles on the backside of the blade caused by the pressure differential between the front and back surfaces of the blades in their relationship with the surrounding fluid pressure. Ventilation is the Introduction of air onto either front or rear surface of the blade, but usually the front side.

Here's an easy way to understand the difference: a submarine that's 500 ft underwater can cavitate when too much shaft speed is applied too quickly. But it must be at the surface to ventilate.
A powerboat whose propellers are piercing the surface of the water is ventilating.

Another note, Most new stainless props are designed to both cavitate AND ventilate. Their designed to cavitate at the sudden application of power at slow speeds to allow the propeller shaft rpms to rise rapidly, getting the engine quickly into it's power band. They are also designed to ventilate slightly at high speed, running right at the surface, to eliminate as much fluid drag (or friction) as possible. That's why the trailing edge of each blade is cupped. The cupping allows the blade to run at the surface, and the cupping holds water against the rear face of the blade, minimizing ventilation.

End of lesson.