20 feet would be close enough for calculations. 1.34 x sq root of LWL gives the theoretical hull speed in knots: However, this does not take into account the design of the hull, and some are more efficient than others. Beam, chine form, keel shape, etc are all issues in this equation.
For efficiency the slower the better; for example 4.47 is the sq root of 20, and 1.34 gives you about 6 knots. Lets say that you run at 4.5 knots, you will get better mileage than if at 6 knots. I have fuel burn curves for my 25: at 4 knots I am getting about 7 miles per gallon. At 6 knots I am getting closer to 4 miles per gallon.
To plane one has to "climb" over the bow wave, and a full displacement boat cannot do that. A semi displacement boat will exceed this speed. The C Dory is very easily planed--and because of the flat bottom will be planing at a speed where the Rossie 246 will be dragging a huge wake because it is a semi displacement speed. But..don't rely on calculations to give the "max speed" you can run in "no wake" zones.
