If the engine is still in the boat you could get it prop shaft dyno'd. I have one and do it all the time. However there are very few shops that have a prop shaft dyno. I can do up to 800 hp at the prop. If you are anywhere near central Texas give me a call.

Bob

Bob did mine. I would need to look back but, I believe he and Dustin picked up abot 55hp on mine. I would have never known without the dyno. Bobs shop also dynos with full wet exhaust and your complete fuel system. That was also a bonus to me to know every part works.