Plate broke off along with fin on outdrive. What this cause? Boat runs fine till 3000rpm then just revs and won't go anywhere. What do you guys think?

ON plane it just starts to rev above 3000 RPM? If so you got something solid stuck between the prop and the cav plate and NOW you have a blown engine coupler.

And it didnt wax the prop???

I have had this happen before....

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And the prop did not like it....

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That looks like your skag cracked at the front and bent back into your prop and then the prop threw it into the cav plate. OUCH.

My skeg had a chunk out of it, I bought a merc replacement skeg. paid a recommended local guy $100 to weld it on....within 15 minutes it pealed straight back on the weld into the prop. completely trashed the lower, cracked the bullet, bent the prop shaft, twisted the down shaft etc..... My 150 dollar attempt to fix a small mostly cosmetic problem ended up costing me a whole lot more.....

You should have called me

So now you tell me... lol