Several numerical methods are available for estimating a vessel's response. One of the most widely used methods is Strip Theory. Seakeeper uses this method to predict the vessels heave and pitch response. Roll response is estimated assuming that the vessel behaves as a simple, damped, spring/mass system, and that the added inertia and damping are constant with frequency.

This method, originally developed in the 1970s (Salvesen et al. 1970) is useful for many applications; unlike more advanced methods, it is relatively simple to use and not too computationally intense. Other, more advanced methods, require computational resources beyond those available to the average naval architect.

Strip theory is a frequency-domain method. This means that the problem is formulated as a function of frequency. This has many advantages, the main one being that computations are sped up considerably. However, the method, generally becomes limited to computing the linear vessel response.

The vessel is split into a number of transverse sections. Each of these sections is then treated as a two-dimensional section in order to compute its hydrodynamic characteristics. The coefficients for the sections are then integrated along the length of the hull to obtain the global coefficients of the equations of motion of the whole vessel.

Finally the coupled equations of motion are solved.

Further details of Seakeeper's implementation of the Strip Theory method is given in the Appendix A Strip Theory Formulation on page 80.

Fundamental to strip theory is the calculation of the sections' hydrodynamic properties. There are two commonly used methods: conformal mapping and the Frank Close Fit method. The former is used by Seakeeper and will be discussed at greater length.

Conformal mappings are transformations which map arbitrary shapes in one plane to circles in another plane. One of the most useful is the Lewis mapping. This maps a surprising wide range of ship-like sections to the unit circle, and it is this method that is used by Seakeeper. Lewis mappings use three parameters in the conformal mapping equation, but by adding more parameters, it is possible to map an even wider range of sections. See: Calculation of Mapped Sections on page 23 for more information.