Cavitation - what to do?

Koeketiene

Koeketiene

Well-Known Member
Joined
24 Sep 2003
Messages
18,433
Location
Le Roussillon (South of France)
www.sailblogs.com
Cut a long story short - last week I had left number 1 child at the helm and went below. She's normally fine, but this time she was "engrossed in conversation" with number 4 boyfriend (aka The Squid).

Sailed over a mooring. Rushed up - did 180 and sailed off - at first it seemed like no harm done - tho ego to a bashing.

This week we motored the final stretch home (wind on the nose). What's that noise? Sounded like a genny going. The faster we went - the louder it got. Cavitation!

Educated guess: what am I looking at? New prop? Bent shaft? What would be the tell-tale signs?
How much do these things cost these days? Last time I had to buy a new prop it set me back about 200.

If I would need a new shaft, or I it could be straightened: where do you recommend? (East Coast).

Thanks
 
Why should running over a mooring induce cavitation?

Cavitation may occur if the boat speed is not enough for the engine speed - eg motoring into wind and waves.

Different prop is the only solution to this - less pitch and more surface area - but at the expense of speed in flat water.
 
Doubt its cavitation ... may be a bit of rope still attached ? Do you have a rope cutter ? If it doesn't clear it unbalances the whole drive assembly.

I would suggest a dry-out and look at stern-gear .... it could even be a rope stuck across skeg to rudder ???
 
If chain mooring there must at least be a possibility that the chain has nicked a blade of the propellor. In this case, a prop repair is indicated, which - depending upon the size of the 'nick' - might be easy or impossible. Either way, suggest a swim to have a look (water temperature is a comfortable 19 on the east coast at present), or put the boat on a slipway and take a look. Doubt the shaft is bent, you would likely have more than a 'cavitation' symptom if so.
 
Re: Cavitation - what to do? Chain ...

OK - sounds like a "nicked" blade ....

Note that to run sweet a propellor has to be reasonably balanced ... nick a blade and it can sound like a buzzsaw, or literally house is collapsing around you !

Suggest as before - a dry-out and look.....
 
Agree with all who say cavitation is a red herring. It is however not as difficult as suggested to bend the shaft or indeed to bend a blade on the prop itself. I am afraid it generally comes down to the same thing though. Whatever it is you need to dry out and inspect.
 
Same thing happened to me but over a rope mooring riser. I got an irritating whistle rather than a knocking. Over the next couple of sails I wondered what to do about it and when I next took the boat out, the whistle had disappeared! I think that the cavitation, for that's what it was, had been caused by bits of antifouling being chipped off the prop and over time, the growth of some weed had faired over the chips and so smoothed the cavities. It doesn't take much to induce cavitation and if I were you and I found a nick on a prop blade the first (and cheapest) thing I'd try would be to fair the prop myself with a decent file. Send a small lad down in a wet suit to do it.
Good luck
 
Forget cavitation. Cavitation doesen't just suddenly start. It is caused by the water leaving the prop' blades faster than it can be replaced, causing a vacuum which plucks chunks out of the blades. Generally only found on high powered very fast boats. Your problem is either rubbish round the prop' or damage to prop' or shaft.
If the shaft is bent it is possible to straighten it using a jack under the boat and a dial gauge. In any other case unless it clears itself the boat will need to be lifted to check the problem.
 
Turned out to be...

Almost nothing to do with sailing over the mooring.

1. Fouled prop - piece of rope roud the prop (banging noise)
2. Shot cutlass bearing - caused the extra vibration

Now high & dry - new cutlass bearing on order. Hope they can knock the old one through the P-bracket.
If propshaft has to come out (and rudder removed - hydraulic steering) we're looking at some serious "labour" costs. /forums/images/graemlins/crazy.gif /forums/images/graemlins/frown.gif /forums/images/graemlins/mad.gif
 
You must log in or register to reply here.

Share this page

Top