502 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 66.35 



liminary-design stage or that alternative designs 

 be carried along far enough to indicate their 

 relative merit in meeting the established require- 

 ments and specifications. In a sense, this is only 

 a part of good planning, which pays handsome 

 dividends in any kind of endeavor. The phenom- 

 enal success of the transatlantic Cunarders 

 Lusitania and Mauretania (old), designed in the 

 early 1900's, was due directly to the extraordinary 

 amount of study and preparation carried out 

 along many different lines before their plans 

 were completed. 



The preparation of multiple design studies 

 permits an excellent degree of bracketing for the 

 final design by extending these studies deliberately 

 into regions beyond those contemplated for the 

 actual ship. Rather surprising results, exceeding 

 those possible by following conventional lines, 

 are often unearthed or revealed in this manner. 



There are facilities available in practically all 

 maritime countries for exhaustive testing of ship 

 models under a wide range of conditions, at a 

 cost that is small in proportion to the ship cost. 

 There is little excuse for embarking on a major 

 shipbuilding project without comparative tests, 

 on model scale, of several different hull forms. 

 For the proposed 4-day American superliner of 

 the early 1930's, T. E. Ferris built 22 models and 

 tested no less than 14 of them [SNAME, 1931, pp. 

 314-315]. For the transatlantic liner America of 

 the late 1930's, the Newport News shipyard alone 

 tested approximately 50 models, although these 

 were small ones and some of them involved 

 changes in the principal dimensions [SNAME, 

 1940, p. 10]. 



66.35 Laying Out Other Types of Hulls. It 

 ■is intended that the discussion and the design dia- 

 grams in the preceding sections of this chapter 

 cover the preliminary hydrodynamic design of 

 ships within a rather wide range of proportions. 

 This range extends as far as the limits of the co- 

 efficients and parameters of the various graphs. 

 Since most of the plots are based upon 0-diml 

 variables, the range of size extends all the way 

 from boats to liners. The design of the round- 

 bottom motor tender for the ABC ship, carried 

 through in Chap. 77, reveals much the same 

 procedure as for the larger vessel. 



For special-service vessels, certain proportions 

 and functions are exaggerated at the expense of 

 resistance, propulsion, and other characteristics 

 normally considered important. Chap. 76 dis- 



cusses these variations for a rather wide variety 

 of hull forms. 



66.36 Effect of Unrelated Factors Upon the 

 Hydrodynamic Design. The requirements for 

 the ABC design in Chap. 64 were deliberately 

 set up, as should be the case for every boat or 

 ship, to give the designer as much freedom as 

 possible to shape and proportion the hull. This 

 applies to the parts both above and below water, 

 in an effort to produce the maximum of perform- 

 ance so far as all phases of water flow and ship 

 motion are concerned. At the least, he should 

 have latitude in establishing the one feature 

 which may be found most critical when developing 

 the design. If, for example, rather severe limits 

 are imposed on the layout of a high-speed ship 

 in everything except the length, the designer still 

 can do a great deal by adjusting the length and 

 by fixing the shape and proportions of his vessel 

 to meet the exacting requirements imposed upon 

 him. 



More often than not, however, the designer is 

 forced to employ his strongest arguments to 

 obtain the latitude he needs in such a principal 

 feature. All too frequently he is stymied and must 

 make the best of a situation which he realizes 

 from the beginning is crowding him against the 

 wall. Faced with a reluctance on the part of the 

 ship owner or operator to make the ship longer 

 than a set figure, confronted with the forces of 

 nature in shortening the roll of a ship which is 

 too wide, or recognizing the limitations of channels 

 which the ship must traverse, he is driven to 

 fuller forms than he would otherwise select or 

 to the incorporation of features which his better 

 judgment tells him to avoid. 



Often, too, through no fault of anyone in 

 particular, factors not even distantly related to 

 hydrodynamic features are given priority over 

 them. Whether the water flow is of paramount 

 importance or not it is still governed by certain 

 physical laws. The designer must get this knowl- 

 edge, then use it to minimize the harmful effects 

 of unrelated factors, and to assess these effects 

 when they can be minimized no further. 



It goes without saying that many considerations 

 other than the ones discussed in this chapter 

 enter into a determination of the weight and 

 volume displacements, the principal dimensions, 

 the proportions, the shape, and the general 

 arrangement which mark the end of a preliminary 

 ship design. One of these, and a most important 



