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I have a 1987 Merc 150 and the anti cavitation plate is cracked. I found the crack along the left side. aft end of the horizontal wings just above the propellor, but it traverses closely along the left side of the tower. The crack ends by turning away from the tower (main vertical body) and ends about an inch from the body. the entire crack is about 6 inches long. It is tight and aligned with the base material.
How would you all suggest I deal with it? Weld? Doubler riveted?

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Please post a photo showing the crack in detail.

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Clearly we'd be able to give better advice with a picture, but my inclination would be to:

- drill a 1/8" hole at the very end of the crack to keep it from growing
- fabricate a reinforcing plate from 316 stainless to "bridge" the crack - may need more than one depending on the crack dimensions and shape
- mount the bridge (or bridges) on the top side of the plate, using 316 SS machine screws with the screw heads on the underside (and washers there too)
- try to mount the bridges so they pull the crack closed, if it's displaced (do this as a dry mount first)
- fill the crack and the drilled hole with JBweld, then do the final mount on the bridges to pull it all together
- sand and paint if you care
- go boating

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There are a number of ways to fix the problem but on a 25 year old two stroke, I would not bother. Leave it be. Even if it breaks off, it is not going to affect performance significantly.

If you are set on doing anything, the easiest and cheapest would probably to buy a used gearcase and swap it out in its entirety. A good used gearcase for the 25" shaft Mercury 150 can be had for $600 or less. It would cost that much to pull the existing gearcase, pull the gears set out, weld the gearcase, reinstall the gears, shim, seal and reinstall.

It's just not worth it on a motor that *might* be worth $1500.

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If you do decide to leave it, I'd at least drill the stop holes at each end.
Is there enough material left to just cut it off and attach an aftermarket hydrofoil cavitation plate? I'm looking at my West Marine catalog and some versions don't require drilling any holes. Even better, they're cheap. I confess, I've never used one, but I see them all the time, so somebody must like them.

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grind or file it out a little and use j b weld. sand smooth and paint

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I had the anti cavitation plate of my old Suzuki 200 welded. I used it for several years after with no ill effects. If I remember correctly, I did change the gear case lube after the weld job.

Doug

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Cav plates break all the time and are easily repaired. Bring it to a prop shop and they should be able to weld to where you cannot notice where is was broken.

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My 1985 70hp yamaha had a similar split in the hub surrounding the prop. The aluminum was starting to peal away and I wanted to keep it from getting worse. No marine shop would touch it, so I went to my auto mechanic, who is also a good welder. He broke the piece off where it wanted to break and heli-arced sp? it back in place. We removed the prop but nothing else [there is rubber at the hub/prop mount and the gears nearby]. I ran the motor for at least 5 more years and sold running fine when I re-powered last year.
So; I say weld it.

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I got a crack in my old 1991 Mercury 115 back around 1998. Fortunately I have some welding experience and the equipment so I TIG welded it myself. I ground down the repair, primed and painted. Until the day I re-powered with an E-Tec a couple years ago, the repair held and could not be detected with the casual eye.

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Are you insured? If so, and the shop deems it a strike crack, it is covered.

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Tracking threads is not easy. I lost this one for sometime it seems. Thanks for all the advise. I haven't had it repaired yet but I am in the middle of a refit of electronics and a battery location move.
But I am going to have a boat shop have a go at it soon.

Thanks again everyone.