Practical seakeeping performance measures for high speed displacement vessels

The seakeeping performance of high-speed displacement vessels is generally assessed, in a probabilistic manner, by using criteria-based measures of merit. Given a set of seakeeping criteria, these measures, such as the Percentage Time of Operation (PTO) and the Seakeeping Performance Index (SPI), could provide an assessment of the operability of the vessel in a specified sea area. The criteria-based measures are based upon the probability of exceeding specified ship motions in a sea environment particular to the vessel's mission. Given the operational area of the vessel, the percentage of time the vessel operates in a particular sea state can be determined by comparing the predicted motions to the motion limiting criteria. However, because the seakeeping criteria are used in a pass/fail manner, the criteria-based procedures give no credit for reducing motions that do not exceed criteria. This may result in misleading conclusions when the relative seakeeping performance of alternative design concepts is compared. It is shown that criteria-free measures, based upon a normalized summation of critical ship responses for a range of ship speeds and wave headings in a specified sea area, may produce more reliable results when the seakeeping performance of alternative designs are compared. This approach also has the clear advantage of not requiring a set of seakeeping criteria, which is subjective in nature. To demonstrate the effect of typical measures of merit on seakeeping performance assessment, six typical high-speed displacement vessel forms are considered in a comparative analysis. Both the criteria-free and criteria-based measures of merit are employed to compare the seakeeping performance of selected vessels. The results indicate that the choice of seakeeping performance measure of merit depends on the nature of seakeeping analysis and criteria-free measures offer a simple, practical, and realistic assessment in comparative seakeeeping studies.