Prop Wear (No Beanies Invovled)

[journey on]

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]

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 :thup

 

[journey on]

Thanks, Joe. I'll accept that

Boris

 

[Sea Wolf]

Thanks, Joe. I'll accept that

Boris

Boris-

We could well both be wrong, too!

Hopefully, the prop switch will reveal something!

Nice visiting with you!

've enjoyed reading your posts and travel stories!

Are you coming to the Seattle Boat Show and C-Brat Get Together?

Cheers!

Joe. :teeth :thup

 

[rjmcnabb]

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

 

[NORO LIM]

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

That makes at least three of us - you and me, and Chandler Bing after Joey and Ross clued him in about the tailor!

Things do just get curiouser and curiouser, though, don't they. I got the new props in order to boost my WOT RPM a little. They have slightly less pitch, but also lack the cupping that the previous props had. One more thing to put in the mix, but still doesn't seem relevant since the burn is occurring on only one prop.

Next time I feel limber enough to do it, I will swap port and starboard props, and then we'll see what happens.

Bill

 

[Sea Wolf]

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

That makes at least three of us - you and me, and Chandler Bing after Joey and Ross clued him in about the tailor!

Things do just get curiouser and curiouser, though, don't they. .......

Next time I feel limber enough to do it, I will swap port and starboard props, and then we'll see what happens.

Bill

OK, but just stay away from the tailor, or at least beyond arm's length from the curious........

Joe. :lol:

 

[NORO LIM]

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]

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

 

[tsturm]

Poor paint job on the port side????

X-2? Poor primer under that paint??

 

[helm]

Our permatrim plate on the Suzuki DF 140 on Chack Chack our CD 25 was only held on by the the forward two bolts, much to my chagrin, upon our arrival at Big Chute Marina last fall, after we had traveled from New York via the Erie and Trent Severn.

I got in touch with the gang at Shipyard marina who forwarded new bolts and suggested that the originals had been overtightened.

The aluminum prop on Chack Chack with its black painted surface was undisturbed.

We replaced the bolts, with less torque on the nuts and traveled via the Georgian Bay and North Channel to Mackinaw City arriving with permatrim still attached, and the bolts intact.

We repowered with a DF 200 minus the permatrim and truly don't miss it.

I don't believe you can pin Bills prop problem on the permatrim.

Perhaps the solution is as simple as stainless steel props?
Eric

 

[Dora~Jean]

How about sending your photos to a local prop shop (if they have web site) and get their opinion. There's a good chance they've seen this sort of thing before. Then...you'd have 'another opinion'!