Outboard Motor Cavitation

[opskop]

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...

 

[MikeHalloran]

- Buy a long shaft motor.

- 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

 

[dik]

Does increasing the pitch of the prop reduce cavitation by ensuring a greater fluid pressure on the prop? This might also increase the speed of the boat...

dik

 

[btrueblood]

Increasing the pitch will just make the prop cavitate at a lower speed.

 

[dik]

btrueblood... thanks, I did't realize that. I thought that the added pressure would minimize the likelihood of 'bubbles' being formed. As long as a constant water pressure is exerted on the surface, wouldn't that eliminate cavitation? Is it that the geometry of the prop is such that it's causing the problem... I guess the radial velocity also causes problems, from near nothing at the hub to max at the tip.

 

[swearingen]

Remember that an increased pitch usually means a lower pressure on the back side of the prop, decreasing the speed required to cavitate.

 

[dik]

thanks... Dik

 

[reidh]

At a given RPM, a propeller with greater pitch will have a lower pressure on the back side. However, a greater pitch prop wouldn't have to spin as fast for a given vehicle speed. I would imagine that a greater pitch propeller would reduce engine (and prop) speed enough to offset the pressure loss caused by the increased pitch, thus reducing the chances of cavitation.

-Reidh

 

[swearingen]

That's why I said "usually". Reidh, you are correct that this could sometimes be the case, but not at lower boat speeds. By definition, this problem is at lower speed because he's in a turn.

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.

 

[tstanley]

The standard answer for this problem is to lower the motor on the transom so that it doesn't cavitate. It might not have to be lowered much, probably not as much as you'd get by using a long shaft, that would be too low.

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

 

[Itslogical]

Cavitation is probably not the correct description, since in this case it is not due to a pressure below NPSHR, but is instead due to sucking air in. It is really due to vortexing. This happens often on our boat, and to get rid of it we need to reduce rpm's, at which time a big bubble of air exits the prop area and off we go again.
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.