Cavitation

[stewart]

THe boats out of the water this week for antifouling etc etc and the yard have just told me the props are beyond repair due to cavitation (for once there's nothiing in it for them on this because I've got a spare set that came with the boat!). I've not seen them myself yet.

What is cavitation, what causes it and how do I stop it happening again? The guy at the yard said that it's could have been caused by the poor state of the antifouling on the hull but I'm not convinced.

 

[kimhollamby]

What is the boat/engine arrangement?

See top of the thread for JFM's explanation of cavitation...better more than my first hurried attempt here

Outdrives and outboards can have a whole set of installation reasons as well as prop choice reasons.

On a shaftdrive the reasons are usually less easy to resolve although a fix employed in many cases is to switch from three blades to four to put more blade area in the water.

Incidentally cavitation is not just a fast boat thing although it is most likely on high speed full lock turns...even displacement boats running in a straight line can suffer if the prop type doesn't like the water flow coming off a keel ahead of it.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com<P ID="edit"><FONT SIZE=-1>Edited by kimhollamby on Wed Mar 6 09:53:41 2002 (server time).</FONT></P>

 

[stewart]

It's a Sunseeker Camargue 46 with 2x550 Detroit Deisels on shafts. I don't know whether they are 3 or 4 blade props, or whether this sort of thing tends to be a design fault with certain hull designs. Normally run at around 30 knots but not usually high speed turns. Can it be caused by the way a boat is driven?

 

[jfm]

Proper cavitation is where the water on the low pressure side ofa pump impeller (in this case, that's the forward faces of your props) vapourises. That causes the familiar overevving and loss of grip. The damage is caused when the vapour rapidly condenses again on the surface of the propeller, in tiny explosions that chip away at the metal.

There's another effect, ventilation, where atmospheric air is mixed into the water flow to the prop, eg in a fast outboard boat in a tight turn. That's harmless, doesn't create the above explosions. Yours must be cavitation, not ventilation. But you should be able to get the props fixed if you want?

Reasons for cavitation are mostly bad prop design (high "speed" of the prop), bad design of the tunnels or other areas past wchich the water dloes to the prop, plus warm waters (so water vapourises more easily.

 

[jfm]

Fluid mechanics

Kim, with respect, you are mostly describing ventilation, where atmospheric air is mixed into the water flow. This never causes prop pitting and damage. Cavitation involves "slippage" because of water vapour, not air, in the water flow. The pitting then occurs when the water vapour bubbles implode and re-liquify

 

[kimhollamby]

Much more so on outdrives and outboards than shafts.

It would be interesting to check with Sunseeker if the props are standard or whether they started using something different before the model run ended. Do you know if your boat was pulling pretty much the correct maximum revs for the engines at wide open throttle?

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[stewart]

Well it does run in warm water (Balearics) so that seems to make sense but obviously not much can be done about the design of tunnels etc. so it's down to the props I guess. Do certain boats have more of a tendency to do this than others?

 

[kimhollamby]

Sorry, I was trying for a quick pointer that drilled down to the source without too many pars and didn't do a good job. Offending pars removed now. That said, ventilation and cavitation can be close cousins as air can be a factor in disturbing the water pressure around the prop.

As you know (and as I found out to my cost even with a displacement boat) some of this stuff falls awfully close to black art as opposed to science. The way this thread is going I suspect we have a classic case here...

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com<P ID="edit"><FONT SIZE=-1>Edited by kimhollamby on Wed Mar 6 10:04:05 2002 (server time).</FONT></P>

 

[jfm]

Re: Not your driving

It's not caused by how the boat is driven. It is a pure fluid mechanic effect and happens because of a combination of prop design and "duct" design. The "duct" is the bottom of your boat and there's not too much to design there, the most likely cause is prop design. In a high speed boat like yours, especially in warm waters, some cavitation is inevitable. If you have only tiny pitting damage and it has taken years to happen, dont worry. But if your props are massively knackered in a short time then you need new and different props

Just to be clear, I'm talking about the real cavitation that causes physical damage to props, bad cases of which are quite rare on normal boats. If your props are just ventilating, ie over revving because of sucking in atmospheric air (maybe on tight turns trimmed out, or whatever) that's no bid deal, but it is affected by your driving style and is fine, more power to your elbow, keep openning up the throttles in those turns yee haaar

 

[kimhollamby]

Just a thought...do you have rope cutters fitted? RNLI researched cutter effect on prop efficiency a few years back and reported some cavitation burns on blades with some set-ups.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[stewart]

Re: Not your driving

Well I have a spare set of props which came with the boat and are unused so I'll put them on and see how they go. If there's problems I guess I'll have to change design when they need replacing.

The boat went down to the med last August, though I can't be certain what condition they were in before that.

 

[stewart]

Runs flat out at around 1800 - 2000 rpm, doing around 35knots which is the quoted maximum for the boat but not sure what the normal revs should be.

 

[stewart]

No, no rope cutters fitted.

 

[kimhollamby]

Ummm, not sure, but will try and find out for you if someone doesn't get there first. Your boat should ideally be within plus/minus 50rpm of rated engine maximum at wide open throttle if propped correctly, a touch faster on rpm is okay with clean bottom/light load/colder water on the basis that gives you a bit in hand for the season ahead.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[kimhollamby]

Damn!

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[kimhollamby]

Re: Not your driving

Other thing to check...are you sure that it is cavitation burns only and not a helping of electrolysis? As the boat is out you should in any event check the electrical continuity between the anodes, shafts, props and rudders. Always worth looking at the bond between the engine and the shaft...quite often whatever provides continuity over the flexible coupling fails after a few seasons and needs replacing.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[stewart]

Re: Not your driving

Anodes are apparently quite work down and the whole cathodic system is supposed to be being checked as part of the service. The whole process would be much easier if I was there with the boat rather than 1500 miles away!

 

[kimhollamby]

Re: Not your driving

This kind of stuff not always that easy if you are 1500mm away from it, let alone 1500 miles! Getting the spares put on, having the electrical bonding checked and then getting a second opinion on the originals from a decent prop repairer sounds like the best strategy. Plus, as said earlier, do try to establish if Sunseeker changed the prop type at all.

Associate Publisher ybw.com websites kim_hollamby@ipcmedia.com

 

[jfm]

Good thought Kim....

Good point, re electrolysis. Real damage by cavitation is pretty rare on leisure boats, so electrolysis a likely cause. Where is the pitting, near the tips or near the hub? And is it on the low pressure side (front face of props) or the high pressure side (aftward facing surface of props)? You never get cavitation damage on the high pressure side, so if it's there it's probly electrolysis

 

[stewart]

Re: Good thought Kim....

Apparently the pitting is along the hub mainly with only a little on the blades of the prop.