Sec. 60.3 



SHIP-POWERING DATA 



355 



ship on a base of displacement_weight W for 

 various values of T, = V/'VL, where V is 

 (apparently) the trial speed in kt. H. Volpich, in 

 a discussion of this paper on pages 236-239 of 

 the reference [also SBSR, 20 May 19.54, pp. 634, 

 636], gives a nomogram (Fig. 12) for the power 

 approximation of single-screw diesel-driven coas- 

 ters embodying deadweight carrying capacity, 

 ship length, brake power, and ship speed. It is 

 useful for a quick and ready approximation to the 

 power of a small ship. 



J. L. Bates has published contours of constant 

 effective power P e for fast yachts having lengths 

 in the range of 100 to 500 ft, speeds in the range 

 of 10 to 20 kt, and the following ranges of form 

 coefEcient: 



(a) Prismatic coefficient Cp from 0.62 to 0.66 



(b) Displacement-length quotient A/(0.010L)^ 

 from 40 to 45 



(c) Maximum-section coefficient Cx from 0.75 

 to 0.83. These curves are to be found in MESA, 

 September 1921, pages 678-680. Similar contours 

 of constant effective power, for speeds in excess 

 of 22 or 23 kt, are to be found in the 1920 edition 

 of the Shipbuilding Cyclopedia [Simmons-Board- 

 man, New York]. 



Contours of constant effective power for vessels 

 of fine underbody, comprising yachts intended 

 primarily for ocean cruising, coastal passenger 

 vessels, gunboats, and certain seagoing tugs, are 

 given by Bates in Figs. 3-5 on pages 681-683 of 

 the MESA reference. The curves cover a Cp of 

 0.56, a range of displacement-length quotient of 

 from 100 to 150, and a range of Cx of from 0.87 

 to 0.93. Representative vessels in the selected 

 groups have, according to Bates, the charac- 

 teristics listed in Table 60. a. 



Here again it is noted that while the craft 



selected vary as to type they vary only little in 

 those proportions affecting hydrodynamic resist- 

 ance. A great many additional sets of contours 

 would be needed to cover the whole ship-design 

 field. 



Bates' effective-power data are based upon test 

 results from the Taylor Standard Series of models 

 and from miscellaneous EMB models. While ship 

 forms have changed somewhat since this analysis 

 was made, the data should still serve for quick 

 estimates of effective power for vessels of the 

 proportions listed. 



Since friction resistance for a ship is found by a 

 direct calculation in any case, the friction power 

 is derived invariably by the formula Pp = RpV. 

 The difficulties in determining the effects of 

 roughness, described in Chap. 45, are of course 

 reflected in a determination of the friction power. 

 Numerical values of friction power are rarely 

 employed in ship design but they are useful in 

 illustrating the effects of changing the wetted 

 area and of surface roughness. 



60.3 Effect of Displacement and Trim Changes 

 on Effective Power. Knowing the effective power 

 P E for the designed displacement and trim of a 

 given vessel, usually as the result of a model test, 

 it is often required to estimate the Pe for a 

 somewhat different displacement of that vessel; 

 possibly also for a different trim. Designer's and 

 operator's requirements, besides calling for the 

 effective-power variations corresponding to the 

 usual 10 per cent light and heavy displacement, 

 often extend to the so-called ballast condition, 

 especially for cargo vessels. Here the weight 

 displacement for a cargo-vessel design may 

 approach half the designed value, and the trim 

 by the stern of that vessel may be 0.3 or more of 

 its designed draft. 



As an aid in estimating these effective-power 



TABLE 60. a — Characteristics of Selected Vessels in the Powering Groups of J. L. Bates, 1920-1921 

 The references from which these data were taken are listed in the accompanying text. 



Type 



Length, ft 



Displacement, 

 long tons 



Longitudinal 



Prismatic 

 Coefficient, Cp 



(1^ 

 \100> 



V 



VI 



6,940 



17,470 



568 



950 



1,371 



1,575 



950 



0.552 

 0.576 

 0.59 

 0.59 



0.584 

 0.565 

 0.578 



100 



124.3 



138.5 



102.8 



127.1 



138.5 



163 



1.085 



0.832 



0.91 



1.31 



1.075 



0.8 



1.05 



