The built-in yaw wave drift coefficients for semisubmersibles appear to be identically zero at all wave periods. Why is this?
Because of the high degree of azimuthal symmetry of semisubmersibles, wave drift yaw moments are very small and highly dependent on vessel specifics; it is therefore impossible to produce a “typical” or “average” set of yaw coefficients that will meaningfully apply to all semisubmersibles. This is in contrast to the surge and sway drift forces, which depend principally on the net column waterline lengths. We have therefore set the built-in yaw coefficients exactly to zero. If you need more complicated characteristics, with non-zero yaw coefficients, you will need to create your own and provide them in a DRFTCOFS.txt file. Note that it is entirely possible to “mix and match” built-in and user-specified coefficients, so that you could use built-in coefficients for surge and sway and supply your own for yaw.
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