Re: How much is too much?
Safety-wise, it's not black and white so close to the line. My dad repowered our 1972 Galaxie 15 runabout with a Merc 70 in the mid-eighties, replacing the factory delivered 40 hp. The boat's max rating was 65...but we couldn't find a new 65 hp at the time and went with the next closest...70. Needless to say we were just fine from a safety point of view for the 15 additional years we ran the boat. From a liability perspective, though, exceeding is exceeding and I'm sure a lawyer would have made the most of it had we been in an accident during that time even one completely unrelated to the powerplant.
Don't get me wrong, limits are there for a reason, but I also know enough about engineering to know that, for instance 56 hp isn't going to cause a hull failure or become dangerous on a hull rated for a 55...Where the line truly exists is harder to ascertain and most of the boating public can't be trusted to find out for themselves. Hence the healthy safety margin in the design to compensate. And of course, a hull degraded by age or neglect could be hard pressed to hold together with a motor well under the max.
A semi-related anecdote -- when I was scouring North America for my dream Scout, I had some discussions with a gentleman who had a Dorado 170 with a 115 hp Yammy on it. Curious to find one that wasn't powered by the usual 90, I called Scout boats with the hull ID and asked them if they were familiar with the boat (ah...try THAT at SeaRay...
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The great folks there looked it up and indeed remembered building that boat with the special request for more power. Although right at the top of the max hp rating they assured me it was "well within" the hull's capability to handle. I didn't buy that boat for other reasons but it was reassuring to know max hp on that boat does not put it on the edge of safe.)
I know I'm rambling but it's a nice evening, I've got nowhere to be and my wine and canaloni is going down nice.
