I am confused by the differing treatment of current and vessel motion by SeaSoft. Shouldn a vessel moving at 5 knots into a stationary fluid be equivalent to a stationary vessel in a 5 knot current?
This is a very subtle and important point. The quick answer to your question is yes. However, for computational convenience and for historical reasons, vessel speed in the SeaSoft simulations effects *only* the wave-frequency response of the vessel and current effects *only* static and low-frequency forces and motions. One important point: Any irregular wave specification (peak period and spectral shape in particular) is referred to a vessel-fixed frame (that is, the internally produced spectrum replicates that which would be measured by an imaginary wave probe stationary relative to the vessel); this is a consequence of wave-basin testing practice in which the wave spectrum is usually measured by just such a probe. This situation persists in the presence of a mean current, or vessel speed, or both. In wave-frequency-only simulations (Shipsim, Semisim, etc.), one cannot specify a separate current. In order to obtain RAOs that would be realized in a wave-basin test with an underlying cu
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