Re: seacast for aluminum transom?
I did a 17 ft aluminum style flat bottom (I think it might have been a seanymph, if not, very similar style early to mid 80's I think, etc...) sporting a 35 horse with seacast some years ago. The transom was literally riveted in and would have been a bear to get replaced. I cut two rectangular holes in the top, say 4 or 5 inches wide, and using a combination of a long auger drill bit and a vacuum cleaner with a piece of pvc pipe for an extension, I removed all the rotted wood. I poured the seacast into the cavity in two pours after making sure to tape over the few places where there might have been a leak.
That was something like 10 years ago and the guy is still running the boat - he mentioned some time ago that the transom will probably outlast the rest of the boat! It was a little heavier, but not enough to make a real difference, and it was far stronger than the original wood ever was.
Oh, and where I cut the holes to get acces, i made sure to slightly overfill at the end and then just shaped it to the contours of the aluminum. after touching it up and painting, you couldn't even tell where it was done. It wasn't cracked or anything when I looked at it last time I saw the boat a cuple of years ago. (it went to a friend).