346 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 59.15 



Ship Speed, kt 



.Fig. 59. J Variation op Thrust-Load Coefficient 



WITH Ship Speed for U. S. Navy Tug YTB 500, 



When Running Free and Under Two Towing 



Conditions 



Figs. 59.H, 59.1, 59.J, and 59.K give plots of 

 these values for a few vessels on which thrusts 

 have been accurately measured during standardi- 

 zation and other trials, and for some selected 

 models. The ship-trial data were taken from the 

 following sources: 



Fig. 59.H U. S. S. Hamilton. Trial of 9 May 1933; 



Tables 20 and 21 of pp. 274-277, SNAME, 1933. The 



speeds corrected by trial analysis were used for spacing 



the ordinates in this figure. 

 Fig. 59.J U. S. S. YTB 500. Trials of 1, 5, 13, 14, and 15 



April 1948, using data in the Bureau of Ships (U. S. 



Navy Department) archives. The trials involved running 



free and with light and heavy tows. 



Corrected Ship Speed Throuqh the Water, kt 



Fig. 59. K Variation of Thrust-Load Coefficient 

 With Ship Speed for the S. S. Clairlon 



Fig. 59.K S. S. Clairlon. Trials of 8 and 9 October 1931; 

 SNAME, 1932, pp. 17-44. Corrected thrusts T are 

 from Sheet I of Appendix 2 on p. 32; corrected ship 

 speeds V through the water are from Part II of Appen- 

 dix 1 on p. 31. The speeds of advance Va were calculated 

 from the corrected ship speeds V through the water by 

 using wake fractions derived from a plot of those 

 tabulated in Part I of Appendix 1 on p. 31. 



There are included also data derived from the 

 tests of three self-propelled models, as follows: 



Fig. 59.1, diagram 1. Projected merchant ship design; 



data from TMB archives. 

 Fig. 59.1, diagram 2. Canadian lake freighter, Lpp = 647.25 



ft, Ottawa model 95; National Research Council, 



Ottawa, Report MB-137 of 3 Jul 1951. 

 Fig. 59.1, diagram 3. ABC ship design, with transom stern, 



Fig. 78.Nb of Part 4. 



Similar data can easily be plotted for any ship 

 model from the results of the self-propelled tests. 



When plotted on a basis of ship speed, or of 

 speed-length quotient, the data reveal no definite 

 pattern, except that the thrust-load coefficient 

 may diminish with increase of speed in the ranges 

 below the designed speed. For fine, fast hulls it 

 may increase with speed, up to a T^ value of about 

 1.5 or 1.6. At this point the effect of the first 

 trough of the ship's Velox-wave system is making 

 itself felt, with a diminishing wake fraction w 

 and an increasing ratio of speed of advance V a to 

 ship speed V. On a high-speed vessel, like the 

 destroyer Hamilton, the wake fraction decreases 

 toward zero, or may become actually negative, 

 with an appreciable increase in the Va/V ratio. 

 With a given thrust the thrust-load coefficient 

 decreases at a greater rate, because Va is squared 

 in the denominator of the expression for Ctl ■ 

 However, in the case of the Hamilton, cavitation 

 was encountered on the propellers at speeds in 

 excess of 30 kt, so that other factors entered the 

 picture. 



In the case of the model of the transom-stern 

 ABC ship, for which the Ctl data are plotted in 

 diagram 3 of Fig. 59.1, there are rather sudden and 

 unexplained changes in the range of 15-21 kt. 

 These undoubtedly bear some relation to the 

 changes in the values oi Wt and t, indicated on 

 Fig. 78. No, but the relationship is by no means 

 clear. More data and a much more thorough 

 analysis of this problem are required before the 

 designer can attempt a prediction of the Ctl 

 variation with speed for any given case. 



59. 1 5 Approximation of Screw-Propeller Thrust 

 from Insufficient Data. It often becomes neces- 

 sary, in the early stages of a ship design or in the 



