This section outlines the method used to describe a vessel's response in a seaway.

For most purposes, it is sufficient to model the vessel as a set of coupled spring, mass, damper systems undergoing simple harmonic motion. This is assumed by Seakeeper and most other seakeeping prediction methods. This method may be successfully applied to the analysis of the vessel's motions provided that these motions are linear and that the principle of superposition holds. These assumptions are valid provided that the vessel is not experiencing extremely severe conditions.

The Response Amplitude Operator (RAO), also referred to as a transfer function (this is similar to the response curve of an electronic filter), describes how the response of the vessel varies with frequency. These are normally non-dimensionalised with wave height or wave slope. Typical heave and pitch RAOs are shown below:

It may be seen that the RAOs tend to unity at low frequency, this is where the vessel simply moves up and down with the wave and acts like a cork. At high frequency, the response tends to zero since the effect of many very short waves cancel out over the length of the vessel. Typically the vessel will also have a peak of greater than unity; this occurs close to the vessels natural period. The peak is due to resonance. An RAO value of greater than unity indicates that the vessel's response is greater than the wave amplitude (or slope).