Outboard Motor Cavitation
Outboard Motor Cavitation
(OP)
Im not sure if this is an aero issue but can anyone tell me how to prevent motor cavitation whilst performing sharp turns on a small boat with an outboard motor. The inflatable craft we use for surf lifesaving struggles with cavitation during these maneuvers. I have heard that clockwise and anti-clockwise turns should produce different results depending on the rotational direction of the propeller...





RE: Outboard Motor Cavitation
- Add one of those plastic trim tab type things that bolts to and effectively enlarges the flange above the prop. I forget the name.
- Try a smaller prop with more blades.
- Shim the swivel stops so you can't turn the motor quite so far. Do it quietly, and stop when you get complaints about reduced maneuverability.
Mike Halloran
Pembroke Pines, FL, USA
RE: Outboard Motor Cavitation
dik
RE: Outboard Motor Cavitation
RE: Outboard Motor Cavitation
RE: Outboard Motor Cavitation
RE: Outboard Motor Cavitation
RE: Outboard Motor Cavitation
-Reidh
RE: Outboard Motor Cavitation
However, I believe the problem here is air being sucked from the surface. When in a sharp turn and under power, small outboard boats often "cavitate". What's actually happening is that the suction side of the propeller gets too close to the surface. This commonly happened on my Dad's outboard boats and smaller I/O boats. As he bought successively larger ones, it stopped happening. He now has a Formula SunSport 28' with a 502cid engine and a Bravo III drive. This lower unit has twin, counter-rotating in-line propellers. Because of its draft, he can turn it very sharply under full power and not cavitate.
This doesn't follow the strict definition of cavitation, but is a commonly used term in boating for this effect.
Three of MikeHalloran's suggestions can solve this problem; I don't think a smaller prop with more blades will do it unless its much smaller and you lose enough performance so that the boat doesn't tip as much in a turn.
RE: Outboard Motor Cavitation
You could try a set back jack that lets you alter the engine height easily so you can find a happy medium.
Another solution might be to try a surfacing prop like a cleaver style. This should be readily available and a good dealer might let you try before buying. It might not be as good for ordinary running though at normal engine height.
Tom Stanley
RE: Outboard Motor Cavitation
The best solution is either to lower the prop with a long-shaft outboard, or add an anti-cavitation plate, which attaches just above the prop and has the same effect as lowering the prop. This is the same technique as used to stop vortexing at a tank outlet. The plates are commercially available for the smaller outboards.