One simple rule of thumb is "One installed HP for every 500 pounds of displacement will give hull speed." That's for average boats with B/L around 3 and D/L's of 260-400. And hull speed in this case will be a speed/length ratio of 1.34.
A long time ago Jay Benford designed a 35' double-ender for Ernie Gann, a boat called Strumpet. It wasn't too fat (beam 12'4") or heavy (disp 24,600 lbs - D/L = 335). They installed a huge engine, a 98 HP (at 2500 RPM) six-cylinder Ford with 2.9:1 reduction. They thought they had something special when they found she would happily run up to 8.57 knots, a S/L of 1.52. It turns out they just installed about twice the required HP. She would run at S/L 1.33, 7.5 knots, at 1800 RPM producing something less than 50HP.
The little TimberCoast Troller at 3500 pounds will run up to a S/L of 1.7, a double-ended displacement hull. This is accomplished by installing 3-4 times the required HP.