What Is Going On With Our IMCO Lower
 

What Is Going On With Our IMCO Lower? After we ran a poker run I noticed on our lower IMCO. Any ideas what it is or what could be causing it? Or is this normal?

Thanks in advance.

Cavitation burn?

Someone said that today as well.

What would cause that?

are you talking about the circle spot on the side of the gear case? I ran mine for a bit the first time and it has the exact same thing

I am looking at the top and forward portion of the IMCO lower.

couldn't tell if that was flash reflection or not. Mine certainly isn't doing that. does your drive sit that low in the water when on plane?

The X dimension 15.5" and there is also an IMCO extension box with this set up.

I know the X dimension isn't the best I know but would it really lead to this? Is there a cavitation plate add on or alteration that could me made?

Mine has done the same thing, I suspect eletrolysis as the issue, was it left in the water for a couple days? I also have a pretty deep X without an extension box, I've seen this on others too

I would agree with electrolysis. When it's down in the normal position that would put it pretty close to your mercathode on the bottom of the drive housing, perhaps that is what is causing it.

The circles on the side are normal and they are welds that are made after it is cast. They all have them. As to the damage at the front; IDK. Is it corroding? Maybe it was welded up there for some reason.

I spoke with IMCO yesterday and they said it was cavitation burn after they looked at these pictures. Somehow the water coming off the hull and/or extension box is causing this.

So is there a fix?

I also thought it had something to do with electrolysis since it was close to that plate. But when it was taken out the water after three days there was no material or white flakes in the area like on the cathodes.

So is there a fix?

Here is a picture of everything with the drive down.

Any suggestions on what could be causing this or how to fix it? Or, is this really common and I shouldn't worry about it?

Thanks

I disagree with the cavitation burn, I've had cavitation burn on both outboard lowers and my B1. Cavitation burn is caused by extreme low pressure, so you wouldn't see it on a leading edge. It's much more common at the rear of the drive close to the prop and on B1's it's more common on the skeg. How fast do you run?

Strange place for cavitation burn. You could try sharpening the leading edge like this:
http://i763.photobucket.com/albums/x...s796Medium.jpg

Thank you for the reply guys. I am not running all that fast, 68-69. The first time I noticed it was after a poker run on Lake of the Ozarks.

What could have or is causing this? Could it be electrical?

Also does anyone have a picture of cavitation burn?

What would be the best way to sharpen the leading edge (tools, cutting)? The last thing I want to do is F it and cause a bigger problem.

Were you running a lot of trim? Could the trim angle cause the low pressure area?

I do have to run a lot of trim to get the bow up. The prop is out getting re-worked hopefully this will help with the bow lift.

Your running that drive too deep - I would raise the drive. It will improve your top end and the boat will handle better at speed because you won't have to trim as high.

This is cav burn on an Imco shorty. I worked mine with sanding discs and scotchbrite discs with an air grinder. Then I went over the whole thing with sandpaper. I also welded up 3/4 inch of the rear of the water inlet and opened it up to the front. I added a slight bevel at the rear of the inlet to lower water pressure and stop the cav burn. I'll try to attach a pic of a Wilson blueprinted lower also.
http://i763.photobucket.com/albums/x...s750Medium.jpg
http://i763.photobucket.com/albums/x...oblueprint.jpg

Thanks for the picture. I do not want that to start happening anytime soon!

Is there a speed that this becomes more prevalent or can it happen anytime?

What grit sandpaper did you use?

Does blueprinting the lower get rid of the potential of cavitation burn?

Thank you for the help!

That pic is what usually happens when you run positive trim with an Imco shorty. I've looked at a lot of them and most had the burn there to some degree. Wilson says that they can get you 2-3 mph with their blueprinting. I tried to copy their work but it is not truly blueprinted so speed gain was minimal. I was more out to fix the cav burn issue so I had the burn welded up and the rear of the water inlet welded up. I streamlined the angle of the bullet quite a bit too. The scotchbrite pads will clean up the entire surface pretty nicely. I followed up with 320 grit wet sandpaper. IDK what is going on with yours but it doesn't look like cav burn to me and your drive doesn't look too low to me. A close up high resolution pic of your problem spot might tell more.

As far as the speed, IDK. I do know that the damage in my picture happens when I trim up to get maximum speed which on my boat is 83-85 mph.

I wonder if you are trimmed in such a way, at a certain speed (not necessarily top speed) that water is hitting the top of the lower and running over it... that would cause the low pressure area.

Is there anything in the way to cause a strange water disturbance going to the drive? water pickup, tranducer, etc.

Thanks for the additional information. I will try to get a high res close up and post it.

In the meantime I will look into the Wilson blue printing, maybe this will be a good, do I dare say, winter project.



I was told to install a flush drain plug in the box. The person who suggested this is on a race team. I wonder if that is causing a slight disturbance? Other than that there is nothing else down there.

I talked to a guy at Wilson and they are the real deal. They are set up to give a quick turnaround and won't take your drive unless they are ready to do the work immediately. A single is $2,000 I believe. I wanted to do it but with business down this summer I decided to give mine a go myself. http://www.wilsoncustommarine.com/Page_2_6OGO.html

Thanks for the link.

Update....

I unplugged the Mercathode and it stopped. The boat hasn't been in the water for more than 3 days straight for a long weekend. Still kind of confused but at least I isolated the cause for now.

Interesting, mine looks the same, as I've stated no extension box and don't have to run excessive trim, I might try unplugging mine as well

Good to hear! Have you any ideas on how to squeeze a few more MPH out of it?

Short of putting a Whipple on her, no. :angry-smiley-038::lolhit: