CHAPTER 60 



Ship-Powering Data for Steady Ahead Motion 



60.1 General 354 



60.2 Estimation or Calculation of Effective and 



Friction Power 354 



60 . 3 Effect of Displacement and Trim Changes on 



Effective Power 355 



60 . 4 Methods and Factors Involved in Predicting 



Shaft Power 358 



60.5 Axial-Component Wake-Fraction Diagrams 



at Propulsion-Device Positions 358 



60.6 Three-Dimensional Wake-Survey Diagrams 360 



60.7 Interpretation and Analysis of the TMB 



Three-Dimensional Wake Diagram . . . 362 



60.8 Estimating the Ship- Wake Fraction .... 368 



60.9 Prediction of the Thrust-Deduction Fraction 370 



60.10 Finding the Relative Rotative Efficiency . . 374 



60 . 1 1 Determination of the Propulsive Coefficient . 375 



60.12 Data from Self-Propulsion Tests of Model 



Ships and Propellers 377 



60 . 13 Merit Factors for Predicting Shaft Power . . 380 



60.14 Shaft-Power Estimates by the Ideal-Effi- 



ciency Method 383 



60.15 Estimating Shaft Power for a Fouled- or 



Rough-Hull Condition 385 



60 . 16 Increasing the Power and Speed of an Exist- 

 ing Ship 387 



60.17 Powering for Two or More Distinct Operat- 



ing Conditions 388 



60.18 Backing Power from Self-Propelled Model 



Tests 388 



60.1 General. Although not always expressed 

 in so many words, one aim of naval architects and 

 marine engineers for the past century or more 

 has been to find an adequate method of calculating 

 directly the power necessary to drive a given ship 

 at a given speed. By direct calculation is meant a 

 determination of the ship power in the early 

 stages of the design, directly from the data on 

 paper, using whatever handbook or reference data 

 that may be necessary, but without the building 

 or testing of a model. The discussion in this chapter 

 is limited generally to methods of direct estimate 

 or calculation. 



Chap. 57 describes methods of estimating the 

 total hull resistance Rt oi ship forms, making 

 use of several different methods and various 

 sources of test data. Certain of the powers used 

 in ship design are derived readily from this 

 resistance. Others, like the shaft power, can not 

 be calculated directly nor can they be estimated 

 easily. 



It is emphasized here, as elsewhere in Parts 3 

 and 4 of the book, that the ship designer needs 

 several different methods to give the required 

 engineering answers. Choice as to the method 

 selected, or as to which of several successive 

 approximations is to be used, depends upon the 

 time available for finding the answer, and the 

 precision required in it. A rule-of-thumb procedure 

 may be most appropriate for one situation yet 

 highly unsuitable for another. 



One caution against all methods of estimating 

 and predicting ship power is that they shall be 

 based on test and other data that are as compre- 

 hensive as possible. Limiting one's basic data to 

 those for one kind and size of ship may be mis- 

 leading or result in downright inaccuracies for 

 borderline cases. 



60.2 Estimation or Calculation of Effective 

 and Friction Power. The derivation of towrope 

 or effective power for a ship, when its resistance 

 is found by the procedures described in Chaps. 

 56 and 57, involves only one simple step in 

 multiplication, since Pe = RtV. When the 

 resistance is not known, either by estimate, 

 calculation, or test, its value is by-passed, so to 

 speak, by having the marine architect determine, 

 in a single step, the probable effective power for 

 a given ship form. 



Many graphs and tables for finding the effective 

 power for ships have been prepared and published 

 over the years. It is difficult to assess their 

 validity and usefulness because of uncertainty as 

 to what basic data were used and how reliable 

 these data were in the first place. In most cases, 

 the graphs and tables cover vessels of one type 

 only; possibly even of a small range of size or 

 shape. For example, J. C. Robertson and H. H. 

 Hagan, in their paper entitled "A Century of 

 Coaster Design and Operation" [lESS, 1953-1954, 

 Vol. 97, pp. 204-256, esp. Fig. 3 on pp. 212-213], 

 give curves of brake power Pb for this type of 



354 



