In marine hydrodynamic applications, the Froude number is usually referenced with the notation  and is defined as:
where  is the relative flow velocity between the sea and ship,  is in particular the acceleration due to gravity, and  is the length of the ship at the water line level, or  in some notations. It is an important parameter with respect to the ship's drag, or resistance, especially in terms of wave making resistance.
In the case of planing craft, where the waterline length is too speed-dependent to be meaningful, the Froude number is best defined as displacement Froude number and the reference length is taken as the cubic root of the volumetric displacement of the hull: