Eigenfrequency (hull girder)

The hull-girder eigenfrequency is a natural frequency of the ship’s primary vertical or horizontal bending vibration, the lowest (two-node) vertical mode usually being the most important for fatigue and comfort. It is found from a free-free beam model with distributed mass, including added mass of water, and bending stiffness EI: the two-node vertical frequency scales as a constant times the square root of EI over the mass per unit length divided by L cubed. Schlick’s and Todd’s empirical formulas give first estimates, with full eigenvalue analysis on a finite element model. Excitation near this frequency by propeller, machinery, springing, or whipping produces resonant hull vibration.