cavitation?

[crackerjack]

I've got an 18' bowrider with merc 3.0 i/o alpha drive. Had a 19" aluminum prop when I got it. Ran pretty good, but I noticed that at wide open throttle, it would go beyond its 4800 rpm "redline" to almost 5k rpm. I decided it must need a 21" prop. Thought it would bring the revs down and maybe get a little more top speed.
Well, it brought the wot revs down to 4800 but didn't get me any more top speed. Engine seems happier with the 21" than the 19" when I'm cruising, usually around 3500-4200 rpm. But now I've noticed I'm getting cavitation burn on the prop! On the trailing edge back of each blade about an inch from the hub theres a spot the size of a penny where the powder coat is chipped away and the aluminum is pitted. What gives?

Both are aluminum Vortex, 19" didn't do that.. ???

 

[Frank Acampora]

Re: cavitation?

Pitch is the theoretical distance the prop moves through the water with one revolution. So Increasing the pitch by 2 and lowering the rpm by about 2-400 gives the same net forward movement--no increase in top speed.

If you look at the cross section of a prop you will note that it is shaped somewhat like an airplane wing. Just as an airplane lowers flaps to increase the curvature of the wing and increase lift, the 21 pitch prop has more curvature to the blades than the 19.

Propellers operate by "flying" through the water. The curve of the blade generates a negative pressure behind the blade and a positive pressure on the blade face: Thus, the boat is actually pulled forward.

When the negative pressure gets too low on the blade back, the water can "boil" --even at the cold temp of a lake or sea. It usually "boils" at or near the leading edge of the blade. When the boil bubbles travel to an area of less negative pressure, they collapse, exerting tremendous pressure on the metal. This results in metal being removed from the blade. If left unchecked this can destroy the prop. Your 21 pitch prop is generating more negative pressure than the 19 and exceeding the limit at which water "boils."

You have not said if you hear ventilation during turns. This will sound like gurgling, the engine will speed up and the boat will slow. Just like a car spinning the tires on snow. At any rate, ventilation is almost always accompanied by cavitation. However, cavitation can occur in straight travel also.

There are several cures for this: Trim down during turns, and / or cup the blades. The third option for outboards --not available on outdrives --is to lower the engine so the prop is deeper in the water. A fourth option is (if you can fit it) to go to a larger diameter prop in the 19 pitch. About 3/8 to 1/2 inch more diameter will give enough greater blade area to bring down the wot rpm.

If both these props are not cupped, cupping will have the same effect on engine rpm as about 2 pitches. Therefore, cupping the 21 will be undesirable. However, cupping the 19 will lower your rpm and also give better bite during turns. It will not increase the boat speed.. Also, check the bottom of your hull and the leading edge of your lower unit for any projections or roughness that might cause turbulent water being delivered to the prop. Check the leading edge of the prop for dings or roughness--this can lead to cavitation.

 

[crackerjack]

Re: cavitation?

Thanks for the detailed explanation.

I don't think I'm getting any ventilation. I've never heard the engine race or seen the tach jump up.
I may try trimming down in turns anyway, just to see how it reacts.
I don't think I could fit a larger diameter prop.
The Michigan Wheel "Vortex" props are advertised as having a cupped trailing edge.

I'll take a close look at the hull and outdrive for any roughness...

Thanks!

 

[walleyehed]

Re: cavitation?

I'm gonna jump on this one...engine height has effect on Ventilation and efficiency...not CAVitation.
Every prop design is different and "cup" is a very loosely used term anymore.
And then there is blade area...increasing to bring the RPM down rarely works, or it wouldn't be a 19 pitch anymore..
The Vortex is a fairly good, all-around alum prop, but it sounds like you'd be better off going to some higher technology...most CAV burns occure due to mis-application or a bad leading edge, or possibly a bent blade. Aluminum has thicker blades and the leading edge is more blunt. This in itself can cause Cav burns due to spinning the prop too fast in a mis-application.
Is there any reason you would not consider an SS prop?

With all due respect...a cupped prop, if done correctly in the right location can most definitely increase speed due to more lift and less wetted area, yet in the same pitch.

 

[Frank Acampora]

Re: cavitation?

I beg to differ on blade diameter. In the real world, when I reduced my blade diameter from 13 to 12 1/2, I gained 400 RPM so therefore the reverse must be true: If I increase the blade diameter from 12 1/2 to 13 inches I will lose that 400 rpm.

 

[Frank Acampora]

Re: cavitation?

Come to think of it I disagree with all your comments! Just because you have not seen a ventilated prop with cavitation burn doesn't mean it doesn't happen. Again when you ventilate a prop, cavitation also usually happens. If you lower the prop to reduce ventilation, if cavitation was associated, it will also be reduced.

Yes a cupped prop in high speed applications will raise the boat and make it run looser and faster. This 18 footer is a moderately powered, moderate speed boat. It is a bow rider and as such is built for comfort not all out speed. It runs fairly tight on the water. I stand by my assertion: in this case the cup will not increase speed. My comments were directed to his application, not theory.

 

[walleyehed]

Re: cavitation?

Obviously you became a member while I was gone...
I test props for all Major manufacturers and have data on ONE boat with over 400 props now...I ran nearly 100 props on a wellcraft cuddy just last season, and the 2 bassboats over the last 2 years, well..I'd hate to guess how many I've ran on them...Maybe you should talk with some of the folks here and see just how far off I've led them....right or wrong direction...maybe that will give you a better Idea as to where I'm coming from.
Did you take into account the blade design may be different on the smaller diameter prop? Is it the same model? It gained RPM when you dropped diameter because it was likely a less efficient prop design..I would guess your slip ratio increased dramatically...that's not how we make an educated recommendation.
I'm not talking theory...I'm talking by experience.

Another thought here..If we're running surface piercing props that are in a "controlled" ventilation process, why have I not seen Cavitation burns on all of these props??? Do you think a prop is not piercing the surface when trimmed out???

 

[Dunaruna]

Re: cavitation?

[colour=blue]Popcorn at the ready, begin......................

 

[walleyehed]

Re: cavitation?

No..I'll leave it alone now. The only tool I have is actual data..if that don't work, I guess I can't help...

Popcorn got butter?

 

[Frank Acampora]

Re: cavitation?

Won't clog up the BB by arguing with the expert. I took ten lashes with a Cat-O" Nine Tails across the back. I am properly chastised. But Please, Please, Please, don't confuse me with the facts!

Hope that got the twist out of your shorts.

 

[walleyehed]

Re: cavitation?

Only had 1 knot and it was a loose one so it came out without a wrinkle....8)