is the imaginary unit). It is common to only consider the absolute value of the RAO if the phase between the excitation and the ship motions are irrelevant. It is common to add a linearised viscous damping term, to account for the strong non-linearity of the damping force, especially in the roll motion. This term is for simplicity often taken as a fraction of the critical damping, . The expression is then: where: A good simplification is to use the infinite frequency added mass, in the above expression to find a frequency independent critical damping value. See also Ship motions Ship design Naval architecture References Ultramarine Inc. web page illustrating RAO curves and describing their uses (note: contains content aimed at ship design professionals) Shipbuilding Naval architecture