Cavitation

[lagoon500]

Hi
I have cavitation on my folding bronze propeller blades and I wondered how to go about finding the source of the cavitation?

[SV Bacchus]

Did you increase diameter to the point the blades are closer to the hull? That could possibly be the source.

[DougR]

Prop cavitation is a very complex subject....many different types of cavitation and just as many or more causes. There are numerous technical papers available on the Internet dealing with the subject.

That said, there are a few causes that are somewhat common in the sailboat world:
- inadequate blade area- inadequate blade area for the available power causes high blade pressures on the face (rearward surface) of the prop and high vacuum on the back( forward surface). This high vacuum causes the water to vaporize into bubbles, and then when the bubble moves to a higher pressure area of the blade, the bubble collapses and impinges on the blade. You can actually hear the cavitation noise begin and then increase as more power is applied. This can be solved by a larger diameter prop, or a prop with a higher BAR ( blade area ratio), i.e. wider blades within the same diameter, or go from a two blade prop to a three blade prop.

- Poor pitch distribution within the blade- A properly designed prop has higher pitch angle close to the hub and decreasing pitch angle as you progress outward on the blade, getting very flat near the blade tip. Props with poorly distributed pitch have different " angle of attack" in different areas of the blade which can cause differing pressure and cavitation. Very common in flat bladed feathering props.

- Disturbance of flow into the prop- Mostly a problem in higher speed craft such as sterndrive boats and sport fishing boats but can be found on sailboats where saildrives have heavy growth or eroded zincs, or large square cornered collar zincs located close to the prop on shaft drives.

- Dings or imperfections of the leading edge of the blade- this causes flow disturbances right at the front of the blade and can cause a stream of cavitation bubbles streaming back across the face or back of the blade. The bubbles usually collapse in a localized area of the blade and cause pitting in a small area. Sometimes on aluminum props it can burn a hole right thru the blade.

These are typical causes of cavitation, and a bit of reading will open up the subject more. Hope it's helpful.

DougR

[Jim Cate]

Good post, Doug... well done!

Jim

[LoudMusic]

A fun video to compliment Doug's information

.

[Pauls]

Buy the "Propeller Handbook" by Dave Gerr. He will give you formula for calculating blade area required for your engine and trans. If the blade area is too small you will cavitate. If this is the case your solution will be getting a different prop, and without the calcs you'll just be guessing.

[lagoon500]

DougR, thank you so much for your explanation.

[lagoon500]

Nice video. I'll take a look at that book. Thanks for all your help.

[boatpoker]

What makes you think your props are cavitating ?

It is extremely unlikely that your props are bronze. Most likely "manganese bronze" which is actually in the brass class due to very high zinc content. If the prop is turning pinkish or is pitted you are possibly suffering from dezincification due to galvanic current rather than cavitation.

Photos ?