242 



THEORY OF SEAKEEPING 



25 0.50 0.75 1.00 !.25 1,50 '. 



X/L 



Fig. 5 Model speed in waves under various operating conditions (from Abkowitz, 3-1957(?) 



2.00 



100 per cent at the lower jsower cdrre.sponding to the 

 speed of (>.7 kiint.'i in wa\'es. 



2.33 Ship-model correlation. Three aspects are to 

 be considered under the heading of ship-model correla- 

 tion : 



(a) Scale effect. 



(b) Effect of tank walls. 



(c) Similarity law for ship anil model speeds. 



The scale effect and the effect of tank walls usuallj- 

 occur simultaneously and are difficult to separate. 

 Szebehely, Bledsoe and Stefun (3-1956) compared test 

 results under three conditions; namely, a 5 ft model in a 

 140 ft towing tank, and 10-ft and 20-ft models in the 

 large 1800-ft-long tank. The comparison of only 5 and 

 10-ft models was presented by numerous graphs and the 

 data on the 20-ft model were not yet available. The 

 models used in this series of tests were small in relation 

 to the sizes of towing tanks and it may be assumed that 

 there was no tank wall effect. The loss of speed in waves 

 was found to be identical for two models in waves of 

 constant height equal to I48 of the model length. The 

 loiss of .speed of the 5-ft model was, however, greater 

 than that of the 10-ft model when tested in waves of 

 constant (wave length)/(wave height) ratio of 30. 



Gerritsma (3-1957 XSMB Symp.) tested three series 

 60, 0.60 block coefficient models in a tank 14 ft wide and 

 317 ft long. The models were 6.15, 8 and 10 ft long. 

 In this ca.se, therefore, both the tank-wall effect and the 



scale effects were potentially present. Neither of these 

 has affected motions in waves and excellent agreement 

 was found among three models. However, important 

 differences in resistance of three models in wa^-es were 

 found. These appear to be caused by the tank-wall 

 interference and primarily are manifested by the occur- 

 ence of humps in the resistance curves at waves of X/L 

 = 1. The size of this hump increased with the size of 

 the model. 



Taken together, the results of the two investigations 

 just outlined are reassuring as to the motions, but leave 

 the prol)lem open as to the resistance in waves. Further 

 investigations are necessary. In particular, tests for 

 resistance in waves should be made as soon as the large 

 rectangular tanks (maneuvering tanks) will he available. 

 The tank-wall effect evidently will not lie present in these. 



The similarity law (for lack of a better term) for 

 model's speed loss in waves was discussed bj' Abkowitz 

 (3-1957a and 3-1957c, NSMB Symp.). Abkowitz made 

 a theoretical analysis of the speed loss of models and 

 ships under several assumptions as to the ship's and 

 model's condition. The.se assumptions are: 



1 A constant towing force applied to the model. 



2 A constant resistance of the ship (approximately 

 constant propeller thru.st). 



3 Constant effective horsepower for the ship. 



4 Constant RPM for the model. 



5 Constant RP.AI for the ship. 



