If a boat has a waterline length of 25 feet on her feet and you heel that boat such that there is now 27 feet in the water, you still have waterline length. The formula doesn''t specify how you get waterline length, just that you have it. When the boat heels, you change the waterline length. It is now 1.34 times the square root of 27 feet instead of 25 feet. Sail boats heel. The increase in waterline length with heeling is one thing that accounts for some people finding that they get more hull speed underway than they calculate sitting on the dock on the basis of the manufacturers stated designed waterline length. Of course there are factors that cause variation between boats such that the 1.34 may not produce exact hull speed of a given boat. But it is close. The boat is in the trough between the bow and stern waves and is trapped by it unless it can plane, no matter what that wave length is. The only way the D/L could influence hull speed is if you increase the D without increasing the sail area. It is assumed, no matter what the hull form, that you have enough power to reach the theoretical hull speed. Light boats are more easily driven, but they still make waves when the go through the water.