SIO 



HYDRODYNAMICS IN SHIP DESIGN 



Src. 56.11 



0.3 0.4 0.5 0.6 0.8 1.0 1.5 E 3 4 



Fig. 56. N Variation of 72r/A With Speed-Length 

 Quotient for Two Vessels 



To determine whether the variations in Rt/^ 

 Jor any one ship follow the tentative meanline of 

 Fig. 56.M for variations in its own speed range, 

 a second plot in Fig. 56.N indicates the variation 

 of Rt/^, on a base of T„, for the destroyer 

 Hamilton and for the transom-stern ABC ship 

 designed in. Part 4 of the book. At and not too 

 far below the designed speeds of each vessel, at 

 T^ values of roughly 1.9 for the Hamilton and 

 0.9 for the ABC ship, the agreement is fair for 

 the former and remarkably good for the latter. 

 This agreement covers both the slope and the 

 position of the curves with respect to the tentative 

 meanline of Fig. 56.M. Barkla also found, for the 

 eleven boats mentioned by him in the reference 

 cited earUer, that the total resistance-weight 

 ratios for all of them plotted along S-shaped 

 curves that lay remarkably close together. 



It appears from the foregoing that a single 

 meanline gives an acceptable quick approximation 

 to the value of /2r/A for a wide range of T^ , 

 regardless of the type of ship involved. Also that 

 its slope affords a means of approximating the 

 change in i?r/A to be expected for a given 

 change in '1\ at or near the designed speed. 



It is manifest that additional data are needed 

 for defining the meanline, either as a line or as a 

 lane, in the region of Taylor quotient above 1.5. 



56.11 Changes in Resistance with Changes of 

 Trim and Displacement. It is customary, when 

 towing a ship model, to observe the resistances at 

 weight displacements other than the designed 

 value W, say at O.IOTF light and O.IOIF heavy. 

 Often a third set of resistances is measured at 

 0.20T'F' light, all with zero trim. A fourth set of 

 runs may be made in a so-called ballast condition, 

 in which the ship is carrying a light load or cargo 

 but is trimmed well by the stern to keep the .stern 

 propeller submerged. Despite the wealth of data 

 on this subject known to be available at the 

 David Taylor Model Basin, there has been no 

 opportunity to analyze it and place it in readily 

 usable form. 



The designer is interested primarily in the 

 effective and propeller powers at these light and 

 heavy weights and trims. Further discussion of 

 the corresponding prediction procedures is there- 

 fore found in Sec. 60.3, under the subject of ship- 

 powering. 



56.12 Measured Thrusts and Towing Pulls on 

 Ships. Although thrustmeters were used on me- 

 chanically driven ships as long ago as 1845 

 [SNAME, 1934, p. 153], it is only within the past 

 few decades that reliable and consistent thrust 

 measurements on ships have become available. 

 The measured net thrust, corrected for the axial 

 component of the weight of all the rotating parts 

 of the shaft systems, as described in the reference 

 quoted and as discussed in Sec. 59.16, can be used 

 directly for calculating the TD/Q ratio or the 

 thrust coefficient Kt of the ship propeller. To 

 obtain a measure of the actual ship resistance, 

 these measured full-scale thrusts must be cor- 

 rected by application of the thrust-deduction 

 fraction t. Despite the possible existence of some 

 scale effect in determining the value of t from 

 self-propelled model tests the derived ship- 

 resistance values may be taken as reasonably 

 correct. 



Ship thrust data measured in this way are 

 available for a number of ships, as Hsted: 



(a) U. S. S. Pruill, destroyer; SNAME, 1921, pp. 14-32 



(b) S. S. Claiiion (1931 trials), cargo vessel; SNAME, 



1932, pp. 17-44 



(c) U. S. S. Hamilton, destroyer; SNAME, 1933, pp. 



270-277 



(d) M. S. San Francisco, combination passenger and cargo 



vessel; SNAME, 1936, pp. 195-227 



(e) U. S. S. YTB 500 and YTB 602, 100-ft harbor tugs; 



SNAME, 1948, p. 278 ff 



(f) S. S. Lucy Ashlon, e.x-paddle steamer, driven by gas- 



jet engines, with the ship's own propulsion devices 



