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.