Dan,
Welcome.
This is no more than an observational thought, but at 10 foot, is the entire "back" of the boat wooden, with two cones poking out where the toobs stop, or does the inflatable tube go right round the boat and the engine clamps onto a small wooden bracket held onto the toobs by a bit of bent metal tube in 4 ruber mounts?
This is important - if it's 10' long I think you may have a Redcrest, which will only take something like 3Hp - hanging a 15 on one of them if you are lucky it will stay afloat long enough to pull the starter cord - more likely rip the entire bracket off just by the weight of the engine!
Whatever it is, you've still found the best place to talk about it though!