A hull (11) has mounted thereon a number of accelerometers (20) arranged to provide a data signal corresponding to movement or vibration of the hull (11) to which each is fixed. The hull (11) and/or a thrust block (18) has mounted thereon a number of mass dampers (21) that are controlled by a detector (23) arranged between the accelerometers (20) and mass dampers (21). The detector (23) calculates from the data signal generated by the accelerometers (20) different modes of resonance so as to determine the onset of the excitation of a particular resonance mode and controls the mass dampers (21) so as to generate forces to selectively dampen the resonance mode.