Cavatation Burn
 

So I am getting some cavatation burn on both sides of my outdrive. It is rite behind my water pick up. I was told that it is from air bubble actually burning and eating at the drive. So i took a piece of sand paper and sanded it just in case there was some burs or something. What else could really cause this.?

the cavatation cant be good on the drive, what kind of prop

I am running a 26p mirage. It was lab finished and now it need to be redone. I know it cant be good for it.

My cat was real bad with cavatation until i put a bravo 1 prop on

I have heard of the air bubble burn before thought it was bull until you posted

Cavitation burn is no joke. Over time, it can eat right through and does wonders for streamlining waterflow. :rolleyes:
It occurrs in areas of extreme low pressure, usually behind some kind of protrusion - like water intakes, or just behind leading edges or drives and props. This low pressure actually "boils" the water turning to steam very briefly. When it returns to its liquid state the implosion of the bubbles erodes the metal because they're actually mini sonic booms.

Check out this photo. The "air" you see is not really air sucked from the surface, rather it's water turned to steam right behind the leading edges of the prop where the lowest pressures are seen.

It's eating my Imco w/Bravo 1. Word on the Donzi forums is that there is not a thing I can do about it.
http://i763.photobucket.com/albums/x...s159Medium.jpg

I have the same burns just like you described

So how do you take care of cavatation burn?

Mine is getting worse. I wonder how thick the aluminum is there. Pic is in post #7, now it is twice as deep :(.

Your prop has nothing to do with it, there is something ahead of the drive that is causing it. Anything that breaks the flow causing bubbles will do it, carefully look forward section of the gear case and the bottom for rough areas. I have seen one side of a drive burned from the owner having an extra gasket on the fill screw, also improperly placed through hull fitting can cause it.

Is it right behind your water pick up holes? Mine is on both sides and is right behind my water pick up holes.

Mine is right behind the water pick-up on the Imco (post 7). Now it is twice as deep.

You have to grind a notch to eliminate the low pressure area.
Take note of your drive trim angle at speed. With the boat on the trailer, put the drive at the same trim angle.

Sit and consider the water flow off the hull onto the drive. Look at the angle of the water pickups. Consider how the water flows across this area. Look at how excess water tries to go into the pickup, then dumps out along the side of the drive at speed.

You will be able to reshape or notch the area to rid the low pressure area and stop the erosion. Each X dim height/trim angle combination will need a slightly different solution.

Also, note your WOT water pressure IN FRONT of the water pump. Sometimes installing a high volume pressure dump can let more water thru the inlets, with less being forced out of the pickup cavities which also reduces erosion.

MC

very good thread .....

How fast are you going? I've had similar problems with an Outboard and my Bravo. In both cases I added a nose cone and it eliminated it on the Outboard, the Bravo has been reduced. I now get a little burn about the size of a nickel at the rear of my skeg on both sides. I'm running in the 80's with the Bravo with a Hydro-Motive nose cone.

Since this thread has been dead for awhile I'm going to ask about my specific cavitation burn issue. An Imco rep told me that they are familiar with this problem and that there is nothing I can do to stop it and that the only way to repair it is to have it welded up. The weld would not be a big deal because it could be done with the shorty off but still assembled. Meanwhile some people think that JB Weld will hold up there at least for awhile so I have filled it with JB Weld and have not tested the "repair" yet. I did do some minor mods on the water inlet to improve water pressure at higher trim angles when I'm really airing it out. They were done after consulting with an Imco Technician. The cavitation burn started before the mods and continued after. I'm attaching three pictures. One showing the current extent of the burn, one showing the water inlet and one showing the water inlet with a black line painted on the trailing edge of the inlet. What do you guys think about grinding a small bevel around the area that I "painted". Would that likely break up the turbulence?

http://i763.photobucket.com/albums/x...s750Medium.jpg
http://i763.photobucket.com/albums/x...s102Medium.jpg
http://i763.photobucket.com/albums/x...s601Medium.jpg

That last picture hints of the burn source: Note the slight angle of the burn. Now, look upstream at the left edge of the inlet. See how the metal comes out into the opening as you trace from leading to trailing? I would try filing that straight and then sand a 1 mm radius around the sides of the opening.

With Machinist blue dye (or paint) spray the whole the bottom of the bullet (where the burn is now and to both sides). When you run next, make one pass. The dye will quickly erode . See if the pattern moved or went away.

That whole casting is very rough. After you test, weld up the burn. Then smoothly fill and sand the whole surface. All those little holes and gouges are sources of flow interruption and bubbles that burn. (Look at the skeg on a Merc #6 or M8 drive to see what the surface should look like when you're done,)

Thanks. After posting these pics I did really see the off-set of the burn and the irregular shape of the rear of the water inlet. I will correct both and grind a very small radius and try the paint thing. I can't do it right away since I have a "pool" going as to how long the JB Weld will last. My bet was less than one hour on plane. Yes, the casting is pretty rough.

Based on my experience, you may win that bet. I've tried similar products and they didn't last long at all.

have tried the JB Weld and it was gone in a weekend ,don't bother

I have had similar problems with my IMCO lowers and cav burning behind the water pickup. My friend owns a marine repair buisness that specalizes in fast boats...so he has a lot of IMCO lowers sitting around. What I noticed looking at a lot of them is that IMCO is not consistant about where they place the water pickup notch fore and aft on the underside of the lower. Some start at the very front...and some start two or so inches back from the point...point being...unless the pickup notch starts as far forward as you can get it...notching out under the pointed part...most of the lowers I saw had cav burns on them.

My suggestion would to be to open up the notch as far forward as you can...and make sure that the front part of the notch is parrallel to the prop shaft...or even slightly pointed up...not down as your entrance ramp is...and then shorten up the notch by that same amount by welding up the rear of it....basically moving the notch as far forward as possible while retaining its original length. Water than can come in straight from the front of the drive instead of getting slightly pushed out of the way by the nose or downward sloping entrance ramp.

I did this on my current lower...and the cav burns stopped completely.

Chris

P.S. I will try to go over to the storage place and take some pics of what I did.

P.P.S You should also weld up the current cav burn area so it doesnt get worse...it is like cancer, and the rougher it is...the faster it will get eaten away

CB-BLR, that IS interesting and explains some things. I have elongated my inlet after consulting with an Imco rep about losing water pressure at positive trim. The cavitation burn started before I did any grinding and continued after. I just cleaned up the rear of the opening and filed a slight bevel on the sharp edge. I also did the JB weld fill in. Now I'm waiting for some warm weather. Here is what my inlet looks like now:
http://i763.photobucket.com/albums/x...s756Medium.jpg
http://i763.photobucket.com/albums/x...s751Medium.jpg

You need the front of the opening deeper into the nose...the taper where it gets narrow at the front...should be the same width as the rest of the notch...not narrowing.

ie...the water should be able to go straight in from the front...the way you have it now...will still produce cav burns.

Chris

So is this just an IMCO phenomenon, or does this happen to standard Bravo's, too?

Michael

Thanks Chris, I will look into that.

Michael, this thread started out about burns on a Bravo. I kind of hi-jacked it because it was dead.

Considering what IMCO charges for a SC lower, you would think that they did their hydrodynamic homework but alas, NO.

I have been dealing with the same issue on my cat and am just about ready to weld the hole shut and use another thru hull to feed my inter cooler.

I would love to see pictures of the water inlet on other Imco shorty lowers. Please state if the inlet is stock or if not, what you did to it. Pictures of any kind of cavitation burn on any drive would be interesting too. Thanks.

I'm getting ready to pull the trigger on a Imco and to be honest this issue concerns me, I read a thread a while ago that a person drilled two holes on the top of the nose or tip of the imco that faces the rear of the boat I wonder if that would help reduce cavitation burns?

Also does anyone know if this problem exist with IMCO's that are painted black?

I don't mean to hi-jack this thread by any means just trying to do my homework before I shell out the cash.

thanks,
Gregg

Gregg, you're not hi-jacking any more than I did! Your boat does need the shorty and worse case scenario is that I will need mine welded up a little every couple of years. I service the drive every fall anyway so popping off the lower and taking it to a welder will not be a big deal. I do hope to find the solution to my problem though. I will probably widen the front of my inlet mod as suggested unless more info comes out that says otherwise.

And hopefully we'll get a chance to run together again this summer on the big lakes.

Here are some pics of my current lower....finally...no cav burn!

Now that is very interesting. Thank you. Why TF would Imco put the inlet on the BOTTOM of my drive?:angry-smiley-038: I will attach two pics of my lower from similar angles for comparison. I'll have to use photobucket...I'm going to do some work on the front of my inlet but any welding may have to wait.
http://i763.photobucket.com/albums/x...s759Medium.jpg
http://i763.photobucket.com/albums/x...s760Medium.jpg