HP ratings for small craft

I am wondering if anybody might knowhow the max horsepower ratings are calculated for inflatable boats. I have a small roll up with a 15hp merc on it, but the 20hp merc weighs only 20lbs more...is it a weight thing with regard to the transom? or is there another concern which determines the max hp rating?

Thanks, Brandon

 

There should be a manufacturers CE-signboard (while in Europe) where the maximum power is told. Don't exceed it.

 

I know that, my question was regarding the engineering side of the equation and not the consumer side..how from an engineering standpoint is the max HP figured?

 

The engine thrust will produce a moment that must be countered by the structure of the boat. This is the design load that the boat is designed to withstand with some reasonable margin of safety. The actual weight of the engine affects the buoyancy calculations and generally determines how far the tubes need to extend aft of the transom. If you exceed the rated horse power you are reducing the margin of safety and may risking hull failure. Over powering may not cause an immediate failure, but it could cause damage in the fibers that would have a cumulative effect over time.

Regards;
Mike Schooley