From the speed reduction curves (Figures 8, 9, and 10) the percentage speed losses 

 were evaluated and are shown in Figures 14 through 20. Here the percentage speed loss is 

 shown as a function of wave height (ship scale) for various wave lengths. Figures 14 through 

 16 compare the three types of ships where the thrust used for each would produce design speed 

 in calm water, Figures 17 through 19 show the same comparison for a thrust corresponding to 

 approximately two-thirds design speed and finally, Figure 20 shows the percentage speed loss 

 for the destroyer escort with thrust for one-third design speed in still water. Tests were not 

 made at this thrust for the Series 60 and the SAN FRANCISCO. The speed in waves of these 

 models at this thrust was so low that the accuracy and reliability of the results obtained in the 

 facility used were in question. The difficulty of measuring low speeds arises from the fact 

 that the motion of the model produces waves which disturb the prescribed wave pattern which 

 in turn results in unsteady model motion. 



A more useful presentation of the information contained in these figures is given in 

 Figures 21 through 26. These figures show the combination of wave heights and wave lengths 

 which result in 1, 5, 10, and 20 percent speed loss for each ship. Figures 21 and 22 are appro- 

 priate for the destroyer escort with thrust for design and two-thirds design still water speed. 

 Figures 23 and 24 pertain to the Series 60 and Figures 25 and 26 to the SAN FRANCISCO for 

 the above thrust conditions. Figures 21 through 26 may prove useful when determining the 

 limiting sea condition in which standardization trials may be conducted. The acceptability 

 of the sea condition depends on the type of ship and applied thrust as well as on the wave 

 dimensions. If a maximum speed loss of 1 percent is permissible. Figure 21 shows that stand- 

 ardization trials for the destroyer escort (with thrust for design speed in still water) can be 

 conducted effectively in waves whose length is 75 percent of the ship length if the wave height 

 is no greater than 3 ft. In longer waves the trial results will still be useful if the wave 

 heights are smaller; at the critical wave length (\ = 1.141), the maximum acceptable wave 

 height is 2 ft. At the lower thrust (Figure 22) the 1-percent speed loss is obtained for \/L = 

 0.75 when the wave height is approximately 1 ft. Figures 23 and 25 show that acceptable sea 

 conditions for trial purposes at design speed are more limited for the cargo and merchant ships. 

 For the ships which these represent, trials should not be run in waves higher than approxi- 

 mately 2.5 ft if the wave length is as large as 0.75L, and at the critical wave lengths the wave 

 height should not be greater than 0.5 ft. A comparison of Figures 23 and 24, and 25 and 26, 

 show the more limited extent of acceptable sea conditions for conducting the trials when the thrust 

 is reduced. 



Figures 27 through 31 compare the performance of the three ships in waves of constant 

 height. Figures 27 and 28 show the speed reduction for thrust for design and two-thirds 

 design speed in still water, respectively, in waves of h/L = 0.017. Figures 29 and 30 show 

 the same comparison in waves of h/L = 0.033. The dashed line represents the estimated speed 

 for the SAN FRANCISCO obtained by extrapolation from the original data. Figure 31 compares 

 the speed reduction for the three models in waves of h/L = 0.017 for a thrust which would 

 produce 14 knots (ship scale) in still water. In Figures 27 through 30 the s^eds have been 



