The maximum theoretical velocity of a given boat through the water, which is 1.5 times the square root of its waterline length in feet, divided by the distance to port in miles, minus the time in hours to sunset cubed.

The maximum theoretical speed that a given hull length and displacement will travel through the water in a displacement (non-planing) boat.

The maximum speed that a boat can travel through the water without planing (skimming across the surface). Hull speed is fixed by a boat's waterline length, the longer the length, the greater the hull speed. Easy Wind's hull speed is about 7 knots.

the fastest a keelboat will go, usually dependent on length of the hull at the waterline.

The theoretical speed a boat can travel without planing, based on the shape of its hull. This speed is about 1.34 times the square root of the length of a boat at its waterline. Since most monohull sailboats cannot exceed their hull speed, longer boats are faster.

Hull speed is a common rule of thumb based on the speed/length ratio of a displacement hull, used to provide the approximate speed potential of the hull. Hull speed is typically not a term used by naval architects (they will use a specific speed/length ratio for the hull in question) but is most often found used by amateur builders of displacement hulls, such as small sailboats and rowboats.