It only seems logical to divide the full-size speed by the scale. Think of this: at 25 knots, the full-size boat will travel her own length, 26.25 feet, in 0.622 seconds. Ideally, the 1/15 boat should travel her own length, 21", in the same amount of time to be at the scale speed of the 25 knot full-size boat. This comes out to 1.92 actual statute miles per hour for the 1/15 boat. Roughly, 2 mph.

Using this scale speed calculator jibes with my reasoning:
http://www.mcr5.org/articals/speed.php