Andy

The calculator takes care of sailboats and different types of power
boats, namely planing and semi-displacement.
So yes you can work out speed with a given HP, just select planing
hull type and follow the steps.

http://www.psychosnail.com/BoatSpeedCalculator.aspx

I'll be updating the calculator in a month or so with two additional
sections, engine selection, and propeller sizing.
Calculating the correct propeller isn't quite as easy as it sounds,
but the first step is to work out your boat speed (or resistance) and
the power required for a desired speed of your particular hull type.

Hope this helps and let me know if you have any questions comments.