RE: seamaster engine mount from plans?
I just looked at mine, which is an untested $50 swap meet item with a ST .51 and a warped wing. It has no subfin at all, and the servo in the fuse with a regular mount. I think a subfin is not really needed in the Seamaster at least. It is only needed for taxiing in the wind and once it is up to speed it doesn't matter. It would be in the water until the speed is up. The best subfins I saw were long brass L shaped affairs that were loosely fit to bounce up when hitting obstructions. I would use 1/8" alum. for the motor mount and maybe longish countersunk screws. I like that idea but would check to make sure the motor isn't loose every flight, and use the blue loctite if it is a problem. It is really no harder to mount a motor this way than a plastic one. 8 holes to mark out and drill, and four of them can be through drilled easier than with a plastic mount.