Can cavitation/ventilation cause prop pitting?

[Forktail]

I'm familiar with cavitation and ventilation. But I have an outboard/boat that continues to cause pitting on the blades (back side). It is so bad that on my latest Solas prop it has eaten away part of the aluminum and caused a pit about the size of a dime, in just 50 hours of use. The boat motor combo does require running at almost maximum trim to get best performance. I do not run a fin on this outboard.<br /><br />So for you prop buffs, is this normal or correctable? Any insight welcome.

 

[Jack Shellac]

Re: Can cavitation/ventilation cause prop pitting?

Yes. Cavitation is water vaporizing or "boiling" on the back side of the blade due to an extreme reduction of pressure. It can physically damage the blade. Some causes of cavitation are: wrong prop for application, wrong pitch or damaged blade edges. Additionally, distubances in the water flow around the prop can cause damage that looks like cavitation. Finding the actual cause takes some trial and error. Maybe some of the other guys can be more specific. Hope this is useful to you.

 

[Walker]

Re: Can cavitation/ventilation cause prop pitting?

You say only 50 hrs. But does the boat stay docked in the water. Stray electric current can rapidy accelerate galvanic corrosion. Also does your motor have a self sacrificing anode?

 

[ob]

Re: Can cavitation/ventilation cause prop pitting?

Cavitation pitting of rotating equipment parts is most prevalent on enclosed housing pumps and the like,where its hammering effect is confined .This happens particularly on centrifugal style pumps where there is water or fluid dischrge slippage.In most cases I have been witness to it was either due to an air leak on the suction side of the pump or a insufficiently restricted (sized)discharge line.This damaging cavitation sounds as if the pump housing is full of marbles.I personally have never seen this phenomena occur on an outboard propeller.I tend to go with Walkers angle and feel that in your case that the subtractive pitting that you are experiencing is charge or galvanic related.A casting flaw on an aluminum prop will give this type corrosion a place to start.I have run many hours using aluminum props in saltwater and have never experienced what you describe.You definately have a unique situation considering this prop has only been in service for 50 hours.I probably haven't helped much ,but the only thing I could suggest is that you utilize all of the optional zinc anodes available for your lower unit,to include the horseshoe zinc that bolts to the front of the anti-ventilation plate.Good luck with your problem.

 

[Forktail]

Re: Can cavitation/ventilation cause prop pitting?

The boat doesn't stay in the water (I trailer it), so electrolysis or galvanic corrosion probably isn't a big factor. I do have zink anodes on the boat and the outboard.<br /><br />ob might be hitting it on the head with a flaw in the prop casting. It was kind of a cheapo prop. The pit is only on one blade, but oddly the other blades have worn paint off in the exact same spot. This led me to believe that cavitation/ventilation was the cause. I don't know. <br /><br />Any more input?

 

[Hooty]

Re: Can cavitation/ventilation cause prop pitting?

I have seen cavitation eat on props before but it doesn't happen every day. From your post it sounds like it's happened with that setup before. You say you've got to run at maximum trim. Where is the cav plate in relation to the bottom when the l/u is parallel with the bottom of the boat?<br /><br />c/6<br /><br />Hooty

 

[Forktail]

Re: Can cavitation/ventilation cause prop pitting?

It's set up about an inch below the bottom, so it should be ok? The boat is a 22' custom built aluminum fisher with a modified V, so the hull could be doing some weird things or creating turbulence? Maybe a cav plate fin like a Dolfin would help? To give you an idea, trimmed down the F115 does 5200 and 27 mph at WOT. Trimmed up as far as I can without cavitating, she'll do 5700 and 34 mph. 15 pitch. A 17 pitch gives about the same speed, but cavitates easier. Gets on step instantly.<br /><br />Keep coming with the suggestions....

 

[Hooty]

Re: Can cavitation/ventilation cause prop pitting?

I don't know if that would help or not. I'd look for something forward of the prop. If you can, get underneath the boat with a straightedge and check the bottom for a hook or bulge or anything that's not true and fair.<br /><br />c/6<br /><br />Hooty

 

[jim dozier]

Re: Can cavitation/ventilation cause prop pitting?

The location of the pitting on the backside of the prop is consistent with the physics behind cavitation. That would be the low pressure side, if it was something like electrolysis it would be going on both sides. I don't have any experience with this however.

 

[ob]

Re: Can cavitation/ventilation cause prop pitting?

I still tend to lean toward it being charge related.This is an aluminum prop being spun by a steel propshaft.Dis-similar metals and movement through saltwater = current.I could be wrong,but that's my guess.

 

[ledgefinder]

Re: Can cavitation/ventilation cause prop pitting?

I'd try the same question on the "screamAndFly" website. They focus on high-performance boats, including a lot of offshore racing (salt), trim, surface props, etc..<br /><br />Sounds like a nice boat, BTW!