This paper presents two independent methodologies for defining the seakeeping performance of vessels in various sea conditions. The results of a unique series of sea trials are compared to a new time domain computer program (FREDYN) that predicts ship performance and dangerous operating conditions in extreme seas. Both methods independently define the seakeeping performance during various ship operations such as transiting and weather deck operations.

Sea trials were conducted during various operations for two classes of small ships (less than 220 feet long) in a wide range of sea conditions. The use of improved trial procedures at specific geographical locations enabled these trials to establish operational limitations and provide challenging environmental conditions for this size ships.

The computer prediction (FREDYN) was developed by the Cooperative Research Navies (CRNAV) Dynamic Stability Working Group to provide a unique method of predicting seakeeping performance in extreme sea conditions. Dangerous events such as broaching and capsizing are able to be predicted, along with more accurate motion predictions than in the past. This method will be especially useful in the future for the designs of small ships.

The operational performance of ships A and B begins to decline in sea state 5 conditions as indicated by higher motion levels as well as coursekeeping and broaching problems. Capsizing events are predicted for ship A in sea state 6 and 7 conditions; and for ship B in sea state 7 conditions. The good agreement between the sea trial results and FREDYN predictions adds greatly to the credibility of the predictions of these events.

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