Sec. 60.12 



SHIP POWERING DATA 



377 



stern model of the ABC ship, as self-propelled, 

 Fig. 78. Nc indicates a maximum r}p of about 0.78 

 at 15 kt for the ship, a value of 0.76 at the de- 

 signed speed of 20.5 kt, and a diminished value 

 of only 0.70 at about 22.4 kt, assuming that there 

 is enough reserve of power to drive the ship that 

 fast. 



The designer is again reminded that the pro- 

 pulsive coefficient is to be regarded solely as a 

 means of predicting a shaft power from an esti- 

 mated or known effective power. It is not to be 

 taken as a measure of merit in itself. A high 

 value of yjp may be associated not only with a 

 high value of effective power Pe but also with a 

 high shaft power Ps ■ Thus a model test of 

 design A may predict for speed V an effective 

 power Pe of 7,200 horses, a shaft power Pg of 

 9,000 horses, and an vp = Pe/Ps of 0.80. For 

 the same speed V, a test of design B, to meet 

 exactly the same performance specifications, 

 may predict an effective power Pe of 8,100 

 horses, a shaft power Ps of 10,000 horses, but 

 an 7]p of 0.81. Thus, a ship built to design B, 

 having a greater tj^ , would actually require a 

 heavier and more expensive propelling plant, 

 and more fuel to drive it, than a ship built to 

 design A, with a lower r)p . This is the reason for 

 stressing the use of a merit factor — and an estimat- 

 ing or predicting factor as well — which takes 

 account of shaft power directly. 



60.12 Data from Self -Propulsion Tests of 

 Model Ships and Propellers. For the designer 

 who is laying out a vessel not unlike many which 

 have been run self-propelled in model scale in 

 the past, there are available in the technical 

 literature a considerable number of graphs which 

 give model test data in the form used for many 

 years by the Experimental Model Basin and the 

 David Taylor Model Basin [Bu C and R Bull. 7, 

 1933, Fig. 8, p. 31]. Several of these graphs are 

 reproduced as Figs. 60.Q through 60.T, of which 

 Fig. 60. Q gives data for a U. S. Maritime Com- 

 mission C-2 design, and Fig. 60.R for a Great 

 Lakes bulk ore carrier, the Philip R. Clarke. 

 Others are listed hereunder, with the type or 

 name of ship, or both, and with enough source 

 information to locate them in the Uterature. 



In many cases, including the figures hsted, the 

 graphs are not accompanied by the necessary 

 information to understand, to analyze, or to 

 make use of them fully. This pertinent informa- 

 tion should include a body plan and enough of 

 the adjacent part of the ship to show the pro- 



peller position(s) with respect to the hull and 

 appendages, a model propeller drawing, a set of 

 characteristic open-water test curves of the 

 propeller (s), and perhaps a wake-survey diagram 

 as well for each propeller position. This is one of 

 the reasons for the rather comprehensive and 

 elaborate form adopted for the SNAME Pro- 

 peller Data and Self-Propulsion Data sheets, 

 samples of which are reproduced in Figs. 78. Ma 

 through 78. Nc. 



I. Single-Screw Vessels 



(a) High-speed cruisers of the U. S. S. Wampanoag and 



Ammonoosuc classes of 1867. Self-propulsion data 

 derived from tests of EMB model 2569, with EMB 

 model propeller 685, were published by James Swan 

 [SNAME, 1927, PL .36]. This plate gives the principal 

 dimensions only. The text of the paper is on pp. 

 43-54. 



(b) Cargo ship, U. S. Mar. Comm. Cl-S-Dl design, with 



a reinforced-concrete hull of straight-element form. 

 350 ft by 54 ft by 26.25-ft draft; displacement 

 10,590 tons. Body plan shown in Fig. 76.C. Repre- 

 sented by TMB model 3754M. Prediction data 

 from self-propulsion test 2, in ballast condition, at a 

 displacement of 6,200 tons and a trim of 6 ft by 

 the stern, are given in Fig. 60.S. 



10 M 12 13 14 15 16 17 18" 

 Ship Speed, kt 



Fig. 60.Q Self-Propellbd Model Test Curves 

 FOR U. S. Maritime Commission C-3 Design 



