262 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 52.20 



wake from a vertical turbulence-stimulating strut 

 having a diameter of say 0.01 the model beam and 

 a submergence equal to the model draft, when 

 towed a short distance ahead of a small model, 

 causes a measurable change in its resistance. It is 

 often necessary to wait from 10 to 20 minutes 

 between runs in the basin, when towing a large, 

 heavy model, to insure that the residual currents 

 left in its wake have diminished to the order of 

 0.01 kit or less. On the basis of a 20-ft model 

 running at 4 kt, this means that if the model kept 

 on going it would be 200 lengths away from the 

 finishing point of the run in the course of 10 min. 

 For a 500-ft ship traveling at 20 kt in a channel of 

 comparable relative size, this is the equivalent of 

 over 3.3 nautical miles. It is one reason why a 

 ship has to make a long approach run before 

 entering a measured mile [SNAME "Standardiza- 

 tion Trials Code," 1949, p. 7; van Lammeren, 

 W. P. A., RPSS, 1948, p. 352] to insure that it 

 does not meet, as a counter-current, its own wake 

 from the preceding run. 



Model-basin experience indicates that the 

 presence of the walls and bottom, not too far 

 from the model track, damps out the residual 

 currents due to towing and self-propelling. Since 

 the basin depth is of the order of 20 times the 

 model draft and its width the order of 20 times 

 the beam, the corresponding dimensions for a 

 ship like the ABC design of Part 4 would be 520 

 ft depth and 1,460 ft width — hardly a restricted 

 channel! 



It is reported, from not-too-precise observa- 

 tions, that at 8 to 10 lengths abaft the stern of a 

 high-speed towing vessel of form similar to a 

 destroyer, at a T^ of about 1.4 or 1.5, the effect 

 of the toAving-vessel disturbance on a towed 

 vessel of similar type is practically neghgible. 



52.20 Bibliography on Wake. Considering 

 how little was known or understood about wake 

 conditions abaft a ship a century ago (about 



1855), the literature which has accumulated on 

 the subject since that time is Uttle short of 

 tremendous. S. A. Harvald has recently (1950) 

 made a valiant endeavor to systematize the 

 available data, and to arrive at a logical and 

 reliable method of predicting wake conditions 

 from a design, but he has found the answer most 

 elusive, as indicated in Sec. 60.8. It appears 

 certain that the problem will have to be studied 

 analytically, beginning at its fundamentals. 

 Fortunately for the profession at large, this work 

 is now (1955) in progress in the United States. 



In view of the inclusion in Harvald's study of 

 a practically complete bibliography on wake, 

 there are mentioned here only a few special papers 

 and others which have appeared since 1950: 



(1) Dahlmann, W., Hoppe, H., and Schafer, O., "Messung 



der Wassergeschwindigkeiten neben der Schiffswand 

 (Measurement of Water Speeds near a Ship Hull)," 

 WRH, 7 Sep 1926, pp. 415-419 



(2) Baker, G. S., "Ship Wake and the Frictional Belt," 



NECI, 1929-1930, Vol. XLVI, pp. 83-106 and Pis. 

 Ill, IV; discussion on pp. 141-146. Describes 

 results of tests on planks and ship models, and on 

 the fast channel steamer Snaefell and the single- 

 screw merchant ship Ashworth. 



(3) Baker, G. S., "Wake," NECI, 1934-1935, Vol. LI, 



pp. 303-320 and D137-D146. Describes results of 

 tests on two models, and full-scale observations on 

 the Ashworth and on the Pacific Trader. 



(4) Igonet, C., "Note on Wake," ATMA, 1938, Vol. 42, 



pp. 543-569. This paper has apparently not been 

 translated into English. 



(5) Sohoenherr, K. E., and Aquino, A. Q., "Interaction 



Between Propeller and Hull," TMB Rep. 470, Mar 

 1940 



(6) Harvald, S. A., "Wake of Merchant Ships," Danish 



Tech. Press, Copenhagen, 1950; copy in TMB 

 library 



(7) Harvald, S. A., "Three-Dimensional Potential Flow 



and Potential Wake," Trans. Danish Acad. Sci., 1954 



(8) Korvin-Kroukovsky, B. V., "On the Numerical 



Calculation of Wake Fraction and Thrust Deduction 

 in a Propeller and Hull Interaction," Int. Shipbldg. 

 Prog., 1954, Vol. 1, No. 4, pp. 170-178. 



