A ship responds to wave conditions depending on the ship heading angles to the 

 encountered waves . A wave train coming directly toward the how (head seas ) will 

 cause a ship to pitch with little roll motion, whereas, a wave train approaching 

 the team of a ship (beam seas) will cause a ship to roll with little pitch motion. 

 Since ship response is a function of encountered wave angle and wave energy 

 distribution, it is clear that applying just the wave height spectrum to the ship 

 response amplitude operators (RAO's) will not accurately predict or estimate the 

 ship motions. It thus becomes necessary to determine the distribution of wave 

 energy for each direction and frequency. The product of the wave energy and RAO 

 for each direction and frequency will yield a ship motion for each given wave 

 encounter angle and its associated frequency. Linear superposition of the motions 

 at each direction and frequency will yield a total ship response for a specific 

 motion. The assumption of linear superposition of ship response is valid provided 

 the seaway and motions are not extreme. 



The measurement of directional waves also has been used to validate the Navy's 

 hindcast/forecast ocean wave model. The Spectral Ocean Wave Model (SOWM) , based on 

 work by Pierson, et al., has been operational at the Fleet Numerical Oceanographic 

 Center (FNOC).- 5 This model hindcasts directional wave spectra based on the input 

 wind velocities derived from local and global pressure fields. The model also 

 generates directional wave forecasts based on current meteorological forecasts. 

 Some of the analyzed results have been applied in ship design and operations and 

 have been reported by Bales, Cummins and associates. '•-' ' ' ' 



Until the last decade, U.S. Navy ships rarely were designed to consider the 



Q 



natural environment. It became necessary to account for not only the man-made 

 threat and calm water characteristics, but also the environmental conditions 

 affecting the ship, primarily ocean waves. Since ships seldom operate in calm 

 water, it is important to design ships to operate well in the natural environment. 

 This provides a greater percentage of operability and a more habitable condition 

 for the crew. 



The data produced by the SOWM are being implemented in the Navy ship design 

 process. However, prior to utilizing the data, some degree of confidence should be 

 developed for it. The energy spectrum has been validated using various techniques, 

 such as radar altimeters on satellites, wave buoy measurements, and observations on 



*A complete listing of references in given on page 25 



