I've got the 19" K-Planes on my boat and the fork lift operator hit it such that a chunk broke off. I'm going to have it welded back together and I was wondering if it was possible to extend them at the same time by welding on an extension--maybe 6'" to 10" range?

I know there are replacement planes available but a little welding of correct material and braces would be a lot cheaper.

Has anyone tried this? Is there too much force on them for this to be practical?

shouldn't be a problem at all as long as you don't go too big, otherwise you will need larger clyinders otherwise the planes won't move at high speeds and you will wind up taking the internal seals in it. The only other problem is that cast aluminum is tough to weld compared to billet, it can be done just make sure you take it to someone that knows what they are doing.

I think you would be better off to bolt a full plate to the bottom of what you have. I believe a weld will crack sooner or later