the transom is rated at 20kw so i assume that a 30hp would be the max i could put on it.
there's some optimistic rounding in there - and 25HP might be the "safer" approach!
my question is that if i put a 30 on the back instead of the 15 how much extra power / speed would i get? [/quote]
Its difficult to predict these things accurately but the "theory" goes something like this:
(Speed)^2 = (HP / weight) x (hull constant)
The constant depends on the hull type etc. Since its the same boat we are talking about here then the hull constant shouldn't change. So ignoring the fact that a bigger engine will be heavier - doubling the power will make it about 1.4x faster. i.e. 22 mph becomes 31 mph. Slightly less because your going to add something like 10% to the total weight.