outboard & cavitation

[Shearwater]

On approaching the marina after a very slow potter for an hour trolling some lures, I decided to rev the engine up to a sustainable max boat speed - to see how fast it would go and to clear the 'tubes'. As the yacht (24 ft) approached 5 kts the engine started to race - or scream would be more accurate. I rapidly throttled down and repeated the experiment; presumably the prop is spinning so fast water separates away fro m the prop surfaces. However, this did not happen when repeated after mooring up; the engine boomed away quite happily & the prop stirred up a load of mud. Now - what do I do when I want to motor as fast as possible? Do I watch the log and keep below 5 kts or I've advertised bolt-on plates which are supposed to help. Any clues please?

 

[boguing]

How's the engine mounted?

If it's on the transom your stern wave will affect the depth of water that the prop is working in. When that depth approaches the critical point you may well be getting ventilation - when air can find it's way down to the prop.

Cure? Try increasing the revs slowly, to see whether you can outfox the wave. Or - can you get it any deeper? Lastly, there's a thingy called a 'Hydrovane' (I think) that's a rubber foil that clamps around the leg, just above the wrongly named cavitation plate. This increases the difficulty of sucking the air down.

 

[misterg]

Sometimes there is a rubber bush between the drive shaft and the prop. This can become un-stuck, allowing the prop to slip on the drive shaft under high power.

The o/b doesn't have a shear pin in the drive, does it?

Andy

 

[William_H]

It may be as simple as fore and aft trim of the boat. Most (small) yachts are really bad with o/b cavitation if someone is on the bow waiting to pick up a mooring.
OK if the trim is normal in both cases then the stern wave may be the answer. olewill

 

[jeremyshaw]

I've known people get good results with fins on dinghies. but don't think it would work on a yacht (being so much heavier). Other issues to consider are depth/length of shaft, angle of drive and of course the dreaded prop pitch.

 

[Talbot]

I had fins on the diesel outboard on my cat, and they made a significant difference at full power in bad weather.