Switch 10x7 4 blade prop to a 3 blade

[Whassup]

I have a 15 hp Merc Pro Kicker with a 14 spline 10x7 4 blade prop that is on a 16' pontoon. At full throttle the motor cavitates but at slower speed it is fine.
The pontoon mfr suggests switching to a 3 blade prop. The motor has an extended shaft.
What pitch should I get for a 3 blade prop?
Attached photo is from the 4 blade prop that was on the newly purchased motor

 

[Chris1956]

It is likely you mean the prop ventilates. Ventilation is much more common. It is when the prop is able to suck air. The Anti ventilation plate above the prop is designed to prevent that.

Cavitation is when the blades create a vacuum or low pressure area on the prop blades as the blades push the water. Cavitation causes the prop to pit. My high-powered speedboat has that issue, but it would be hard for a 15HP motor to do that.

You should first see if you can id the source of the ventilation. Sometimes a speedo pickup or transducer can cause it. Is the motor perpendicular to the water when running at speed? Is it deep enough in the water? The A-V plate should run at the surface or just above it.

Different blade shapes ventilate more or less. I never heard it was caused or cured by changing the blade number.

 

[Texasmark]

I'm going to say that what's in front of your lower unit and the engine's depth at low speeds are areas for investigation. Disturbing oncoming water has an effect. Too high a load to be pushed and the prop being too high in pitch or too close to the surface are others.

One thing that has helped people with "ambient air suctions" is the Whales Tale or such that it provides a large rigid surface between the prop and the ambient air to enhance what the anti-ventilation plate on the engine just above the prop is supposed to accomplish.

 

[racerone]

Sounds like motor is mounted too high.