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Lucky Day



Joined: 10 Oct 2006
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PostPosted: Fri Nov 06, 2009 11:29 am    Post subject: Cause cavitation? Reply with quote

The trailing edges of my prop have "rainbow" coloration on the trailing edges of the stainless prop. I am told this is the result of cavitation. I know of only one time when I knew cavitation was occuring. That was when the lower unit was fouled with seaweed.

The boat is an '04 CD-25. Engine is a Honda 135 with a permatrim.
I notice that there is a small fin mounted to the underside of the anti-cavitation plate -- and that the fin is cocked off center to starboard. Should it be angled off center? Could this be causing cavitation?

In this photo the fin I am referring to is marked with a green arrow. (The camera did not capture the coloration on the blades.) Thoughts?

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Sea Wolf



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PostPosted: Fri Nov 06, 2009 12:31 pm    Post subject: Reply with quote

Lucky Day-

That rainbow effect could be caused by cavitation removing some of the chromium layer from the top of the stainless steel.

Stainless is made up of iron (base metal),

carbon (to make steel for hardness)

nickel (ties up other atoms to help stop corrosion)

chromium (helps stop corrosion)

(This is over simplified, but so is everything else.)

After shaping and some polishing, the chromium atoms are brought to the surface of the finished object to make an especially corrosion resistant layer to protect the finished prop or other object. This can be done by passivation or electropolishing, etc.

Cavitation, where bubbles are formed and collapse because of low pressure areas created by prop thrust, among other things, can pull atoms off of the surface of the prop or other objects, resulting in missing paint or metal atoms, or pitting.

The rainbow effect is probably due to layers of chromium atoms being removed by cavitation. The chromium layers are actually somewhat transparent on an atomic scale, and the reflections we see in chrome objects actually come from reflections from the top several or even many layers of chromium atoms. Removal of some layers in a graduated sequence from trailing tip forward creates a rainbow effect from the interference of the different wave lengths of the light reflected at various levels. Much the same phenomenon can be seen from light reflected from oil spread on water where the oil thickness varies as the rainbow interference of reflected light occurs.

So yes, you probably have some form of cavitation occurring.

The fin is the trim tab fin, which is usually set even more off facing the port side of the boat. It is needed because the torque generated by turning the propeller is countered or opposed by the motor shaft (and motor) wanting to turn the opposite way. This torque on the motor shaft and motor, makes it easier to turn the motor (steer) in one direction than the other, something we all experience with mechanical steering. (Hydraulic steering doesn't transmit this torque very well.) The trim tab is needed to give the motor more of a "neutral" steering feel, though it can't totally neutralize that effect.

Since the trim tab is behind the prop (clearance varies), it probably doesn't cause the cavitation. (But Mother Nature works is Strange Ways, sometimes.)

Since the rainbow effect is new, it was probably caused by the seaweed.

I suggest you get some "passivation polish for stainless steel at a marine store, polish the prop by hand, and then see if the problem recurs.

Hope this wasn't too long-winded or boring, but the old retired science teacher gets loose sometimes!

Joe. Teeth Thumbs Up

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Lucky Day



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PostPosted: Fri Nov 06, 2009 2:20 pm    Post subject: Reply with quote

Joe - thanks for the great response. Polish the prop I will.
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ramos



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PostPosted: Fri Nov 06, 2009 3:19 pm    Post subject: Reply with quote

Joe,
I enjoyed your' informative and understandable response as well. Of course, I enjoyed science class in school.
Thank you, even though my prop seems to be fine!

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thataway



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PostPosted: Fri Nov 06, 2009 3:54 pm    Post subject: Reply with quote

A great explaination by Joe! I would not worry, as long as there is no real metal damage. If you were having serious ventillation (which would be air bubbles injected by turbulant water) you would be noticing erratic speeds near your top end, plus over revving of the engine as the ventillation occurs.

The following is a quote from L. J. Wallace:

Ventilation: This is the most common occurrence, yet most people incorrectly call it cavitation. Ventilation occurs when air enters the blade area, allowing the prop to spin freely, most commonly caused by the prop nearing or breaking the surface because of too much trim; in a tight turn when the engine or outdrive already is trimmed up; or from a through-hub exhaust. An over-revving engine and a louder exhaust note are sure signs of ventilation.

Cavitation: Poor machining, bends, dents, broken-off bits, uneven wear, pitting or poor propeller selection can create an area of unequal pressure on the back side of the prop - the side nearest the transom - that actually can make water boil. Just as water's boiling point can be raised when under increased pressure, such as in a car's radiator, a decrease in pressure can lower the boiling point - in extreme cases to temperatures as low as 50 to 60 degrees. When such boiling occurs, air bubbles form. As the boiling water passes to the higher-pressure area of the blade, the boiling stops and the bubbles collapse. The collapsing bubbles release enough energy to erode the surface of the blade.
The drop in pressure, boiling and bubble collapse is called cavitation. The damage caused by the collapsing bubbles is called "cavitation burn." Remem- ber that cavitation is caused by a decrease in pressure, not an increase in temperature.

Originally Published: August 1998
2009 Bonnier Corporation

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Thataway
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NORO LIM



Joined: 24 Apr 2008
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PostPosted: Fri Nov 06, 2009 5:02 pm    Post subject: Reply with quote

Having a related issue (Prop Wear) with my boat, I couldn't help but notice something in Lucky Day's picture.

I got no takers when I asked if a gap in the seam between the Permatrim and the engine anti-ventilation plate might be a cause of the burn on my prop. Now I see what looks to be a much more dramatic gap on the Lucky Day. (The seam between the two plates on my boat is partially filled with what I believe is 5200.)

How many of you with Permatrims have a tight seal between the two plates - specifically right at the aft most portion of the Permatrim's cut-out? I believe the installation instructions from Permatrim call for a sealant all the way along the seam. The instructions also specifically caution against positioning the Permatrim too far forward in order to avoid the curvature that occurs where the horizontal anti-ventilation plate merges with the vertical lower unit. Putting the Permatrim too far foward forces it to bend.

Could an open antiventilation/hydrofoil seam cause cavititation burn?

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Lucky Day



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PostPosted: Fri Nov 06, 2009 6:12 pm    Post subject: Reply with quote

The installation instructions for the permatrim did not mention sealing anywhere, except to "Dab a bit of the marine sealant around each of the bolt holes on top of the AV plate." (The particular permatrim model for the Honda 135 is the M9.)
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ramos



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PostPosted: Fri Nov 06, 2009 6:41 pm    Post subject: Reply with quote

I sealed mine all the way around. I don't know much about this but, I would think it could create a vacuum. Seems like that could cause ventilation but not cavitation. Just a guess though.
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NORO LIM



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PostPosted: Fri Nov 06, 2009 6:59 pm    Post subject: Reply with quote

Lucky Day wrote:
The installation instructions for the permatrim did not mention sealing anywhere, except to "Dab a bit of the marine sealant around each of the bolt holes on top of the AV plate."


I just looked up the Installation Instructions. You are correct. They only mention a dab around each bolt hole. My installer, however, did run a bead all the way around. The bead either was not adequate to completely seal the seam, or the plates have parted a little since installation. The picture in the Installation Instructions does appear to show a completely flush installation. The instructions note that AV plates vary in their flatness.

On another forum (The Hull Truth), the issue of a gap is discussed. The Permatrim dealer responds, in part . . . "when you put the Permatrim on don't push it all the way forward so the end of the throat in the Permatrim is hard up against the leg of your motor, it is ok to have a small gap at the back of the throat. That way the Permatrim won't ride up on the curve in the AV plate where it joins the leg and rock too much . . ." (emphasis added). The instructions themselves also say, "The Permatrim should be mounted as far forward on the AV Plate without the Permatrim riding up on the curved portion of the AV Plate." I find these statements ambiguous. Is a "vertical" gap (that's what I would call what I and Lucky Day have, and what lets water through) or a horizontal gap (that's what the installation instruction picture shows) that's OK?

And "OK" for what purposes? None of the discussions or instructions I have been able to find address the possible cavitation issue. My sense is that the mounting instructions and the advice from the dealer are mainly concerned about the structural integrity of the installation.

So, my question remains - does anyone think the gap could be a cause of prop wear?


(Ramos said . . . "Seems like that could cause ventilation but not cavitation." I'm no expert, either - by any stretch of the imagination. It's strictly learn as I go. I thought I was using the distinctions laid out by Dr. Bob. Maybe I got it backwards. Anyway, Yawl know what I mean.)
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Sea Wolf



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PostPosted: Fri Nov 06, 2009 8:01 pm    Post subject: Reply with quote

Suppose you were building an airplane, especially a fast one. Would you leave gaps and incongruities between the fittings and parts of the plane?

What do you think that motor, anti-ventilation plate, and hydrofoil would look like in a flow chamber (wind tunnel) with smoke streams in the air, or with tracers in a liquid flow test tank? Water is much thicker than water, but they're both flowing (fluid) substances.

If I were racing, I'd fill in and fair out every part of that anti-ventilation plate and hydrofoil I could to reduce drag, turbulence, and vibration, which would reduce both ventilation and cavitation. Should be done anyway, just do it with something you can get apart later (!)

Fortunately, my brand of hydrofoil is flat, and fits snug again the plate of the motor. I did use acorn nuts to smooth out some of the flow issues.





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Lucky Day



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PostPosted: Sat Nov 07, 2009 9:50 am    Post subject: Reply with quote

Joe suggests I hand polish the prop with a "passivation polish for stainless steel" and keep an eye on further discoloration. I googled "passivation polish" but got very technical hits. Any suggestions for a suitable easy to find polish? Thanks.
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Doryman



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PostPosted: Sat Nov 07, 2009 3:26 pm    Post subject: Reply with quote

Lucky Day wrote:
Joe suggests I hand polish the prop with a "passivation polish for stainless steel" and keep an eye on further discoloration. I googled "passivation polish" but got very technical hits. Any suggestions for a suitable easy to find polish? Thanks.


Yes, I got a tube of it, made in Germany I think, from West Marine. Check there.

Warren

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Sea Wolf



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PostPosted: Sat Nov 07, 2009 7:41 pm    Post subject: Reply with quote

Lyucky Day-

Click on the magic tube!



Joe. Teeth Thumbs Up
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Lucky Day



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PostPosted: Sun Nov 08, 2009 7:41 am    Post subject: Reply with quote

Magic tube indeed! Thanks for all the great support, C-Brats.
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Wefings
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PostPosted: Sun Nov 08, 2009 7:43 pm    Post subject: Reply with quote

Flitz or Collinite make metal polishes .Jewelers Rouge will do the same .
Its not a big deal or problem .Probably has to do with the fin . The torque tab is to compensate for prop torque .Its in the correct position . Go boating .......
Marc

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