Prop Wear (No Beanies Invovled)

Sea Wolf · Posted: Tue Oct 06, 2009 2:50 pm Post subject:

Mike-

I just copied the image "as is" and figured the site would re-size it.

Will try to reduce it somewhat, if I can before the 30 minute "dead zone" sets in!

Joe. Laughing

NORO LIM · Posted: Tue Oct 06, 2009 3:11 pm Post subject:

oldgrowth · Posted: Tue Oct 06, 2009 4:19 pm Post subject:

_______
Dave

NORO LIM · Posted: Tue Oct 06, 2009 4:23 pm Post subject:

Cool, Dave. Thanks.

Sea Wolf · Posted: Tue Oct 06, 2009 9:32 pm Post subject:

Thanks for the reduction and comparison, Dave!

While the Viking 65 prop and the one in question both show cavitation burn, what's more striking is that the pattern of bubbles on the prop in the video and the one in question are much more alike than the ones in the side by side photos of the C-Dory's prop and the Viking.

I think we've established that we have a cavitation burn problem, but the question is why is it on one prop and not the other?

Is there any reason that the water speed across the port side prop would be any higher than the one on the starboard side? In turns with a right handed prop on the outside of the curve? Downside vs upside movement?

If not, perhaps there is something ahead of the prop causing this?

Time to switch props and test?

Joe. Teeth

journey on · Posted: Tue Oct 06, 2009 10:49 pm Post subject:

Well, here goes. Joe, your example propeller had paint removed in a broad area near the hub. That looks like cavitation to me. Looking at NORO LIM's propeller, I don't see the same pattern and think it's just bad paint or sand scouring.

Remember what cavitation is: per Bernoulli's principle an increase in the speed of the fluid occurs simultaneously with a decrease in pressure. So as the prop accelerates the water, cavitation occurs when the pressure falls below the water's vapor pressure. As the water then slows down, the collapse of the vapor produces a shock wave, hammers the propeller, removing paint and in some cases bits of metal.

So I think that the larger paintless area in your example is the result of cavitation, since the prop looks eroded, but NORO LIM's is too fine/regular. I think that maybe some sand passed by, or the paint wasn't properly applied.

Just a different view.

Boris

Sea Wolf · Posted: Wed Oct 07, 2009 12:48 pm Post subject:

Boris-

Hi, and good morning to you! Rainbow

I'm aware of the differences between the Viking prop and the one in question, just couldn't find a photo of a cavitated prop more like the one in from the C-Dory.

The pattern on bill's prop almost exactly matches the bubble formation pattern of the cavitating prop on the video, and is so regular that it doesn't seem to me to be a random pattern of oil left on the prop before painting or of abrasion occurring during usage. One would have to paint the oil droplets on carefully to get that regular of a pattern, I'd think. Wouldn't abrasion also show up on the front surface of the prop, too?

Studying the dynamics of and experiments with cavitation on propellers seems to indicate that patterns like the ones on Bill's props at the aback of the outer edges are the normal to be expected, though patterns differ as to whether the surfaces are polished metal or painted, as well as the specific props used and circumstances.

Joe. Teeth

journey on · Posted: Thu Oct 08, 2009 11:35 am Post subject:

Thanks, Joe. I'll accept that

Boris

Sea Wolf · Posted: Thu Oct 08, 2009 12:00 pm Post subject:

rjmcnabb · Posted: Thu Oct 08, 2009 12:35 pm Post subject:

Hi all,
This cavitation business is interesting stuff. On that five-bladed prop, most surface loss is on the leading edge, close to the hub. Does anyone know if that is the area of greatest compression? The tips are clear, but they would exhibit the greatest linear velocity. if velocity alone would exert the greatest compression. I guess that angular acceleration would be the same all along the length of each blade, but the chord varies with position on the blade. I hadn't thought about it before, but I guess the pitch of a prop is the sum total of its whole action, and not that each place on a blade has a constant pitch. Question

I don't even want to think about cupping.
Rod

NORO LIM · Posted: Thu Oct 08, 2009 1:48 pm Post subject:

Sea Wolf · Posted: Thu Oct 08, 2009 8:10 pm Post subject:

NORO LIM · Posted: Thu Oct 22, 2009 8:59 pm Post subject:

The hunt continues. Here is a picture of the seam between the Permatrim and the engine's ventilation plate.

The sealant (I'm guessing 4200 or 5200) has some gaps that allow water through. Could this create enough turbulence to cause the burn?

The port engine (the side with the burned prop) has more gaps, but the starboard side also is not completely sealed.

And Joe, I did check the angle of the engine torque tabs. The two are not the same. The port side is not turned at as great an angle as the starboard side. I'm assuming that with twin engines, the two should be the same. Now, which one is correct? The whole issue of this angle is complicated - depending on several factors as I understand it.

So I now have too many variables to easily assess this by experimentation. But I will persevere, even if it means making more than one adjustment at a time. It's kind of fun.

Bill

BrentB · Posted: Thu Oct 22, 2009 10:06 pm Post subject:

on flip side
Maybe backed into something on one side only?

tsturm · Posted: Thu Oct 22, 2009 10:58 pm Post subject: