Linear seakeeping predictions are attractive for design space exploration and preliminary or simple case motion estimates due to calculation speed and relatively simple input requirements. Linear seakeeping theory is an established prediction method with well-known assumptions. One of these assumptions is the assumption that the motions are small. The validity of this assumption is investigated by comparisons with a body exact nonlinear seakeeping code over a range of significant wave heights. A modern naval destroyer and a generic tumblehome ship are examined over a range of speeds, wave headings, and sea significant wave heights. A comparison between linear and nonlinear seakeeping results for the two hull forms show range of linear behavior for different geometries. A general metric based on relative motion is proposed to quantify the validity of the assumption and indicate up to what point linear seakeeping is appropriate including effect of hull form, speed, and relative wave heading.

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