Boat Journal

Chronicling a love affair with sailing

Month: January 2012

Prop Pitch and Cavitation

After all this time, I’m still having prop problems. When the engine reaches about 800 RPM, I can hear cavitation. How do I know that’s what it is? Well, one of my regular crew is a Navy boy and he said that’s what the sound was. Not only that, when we revved it up more, there what a strong vibration to the boat – more than I’d expect to be normal.

So I asked Scott at Electric Yacht what he suggested. He said to try decreasing the pitch on the prop.

Pitch is the distance a propeller will advance with one turn. So, a 10″ pitch means that the propeller will move forward 10″ with one revolution.[1]

The prop is a 13″ 2-blade Max Prop. If you check the specs on the thing, you can see that it’s fully adjustable: pitch and direction of movement. I had based the pitch I set it at to what it was before I took the old inboard off. Or so I think. I didn’t know that I could look inside and find the settings. So I measured the angle and found a formula online to calculate what the pitch was. The pitch was 12″. So we decreased it to 10.

What’s so bad about cavitation? What is it? Well Wikipedia defines it as

… the formation and then immediate implosion of cavities in a liquid – i.e. small liquid-free zones (“bubbles”) – that are the consequence of forces acting upon the liquid[2]

The problem is, those bubbles can cause pitting on the prop. Enough of this and you have a non-functional prop.

After decreasing the pitch to 10″, the vibration at high RPMs seemed to go away. But the cavitation sound was still present. So I turned to the wisdom of the CAL boat and electric boat forums. The comments attached are the responses I received.

What now? Well, we’ll try decreasing the pitch to 8″ and see what happens.