Src. 65 J 



GENERAL PROBLEMS OF DESIGNER 



457 



However, design to the 20.5-kt requirement means 

 that this can be made the trial speed, under ideal 

 trial conditions. Both the design and the ship 

 can be proved on trial, leaving the owner-operator 

 with the assurance that the ship has an adequate 

 margin of both power and speed. The general 

 method followed here for the ABC ship has been 

 used for the design of merchant-type naval 

 auxiliaries for the U.S. Navy for twenty years 

 or more. It has been found eminently successful, 

 for services of varied nature, in most of the oceans 

 of the world. 



65.4 Basis for the Selection of Ship Dimen- 

 sions. At a very early stage in the design of a 

 ship, or perhaps before that design is really begun, 

 it is necessary to determine the basis for the 

 selection of the principal ship dimensions. These 

 include, not only the customary linear length, 

 beam, depth, and draft but the volume, dis- 

 placement, and weight; possibly even the general 

 shape of the ship. Almost invariably it must be 

 decided whether: 



(a) The ship is to be designed on a weight-carrying 

 basis 



(b) The design is to be on a volumetric basis, to 

 provide space 



(c) Inflexible Umits are to be imposed on certain 

 dimensions, such as for a ship which must fit 

 inside certain piers at a terminal or which must 

 pass through locks in a canal. 



The displacement of ships carrying cargoes of 

 high weight densities or specific gravities is as a 

 rule fixed by the weight of the cargo or load to 

 be carried plus the weight of hull and machinery, 

 fuel, consumable stores, and margin. The under- 

 water hull must possess sufficient volume to 

 support the total weight in water of the specified 

 density. When ships are to carry bulky but not 

 particularly heavy cargoes such as railway cars, 

 trucks, automobiles, and other vehicles it is 

 generally possible to carry much of this cargo 

 volume above the designed waterUne. Due 

 regard is of course given to metacentric stability 

 and other requirements. In designs of this kind, 

 a large if not the greater part of the useful enclosed 

 volume of the ship is above water, leaving only 

 enough volume in the underwater hull to carry 

 the total weight. 



In the case of submarines it is necessary to 

 crowd within the pressure hull everything which 

 can not be entirely or partly surrounded by water 

 when the vessel is submerged or running on the 



surface. The volume of the pressure hull, plus the 

 volume of all structure, fittings, and equipment 

 lying in the water when the submarine is sub- 

 merged, determines the displacement or total 

 weight of the vessel for water of the specified 

 density. This volume displacement is substantially 

 the same for the vessel under any running con- 

 dition, in water of a given specific gravity, because 

 the scale weight of the ship remains substantially 

 the same. If fuel is consumed it is replaced by an 

 equivalent weight of water, and so on. It is 

 possible, however, to vary the amount and 

 percentage of reserve buoyancy in a submarine 

 design by varying the shape and volume of the 

 outer hull, since the main-ballast tanks between 

 the two are empty in surface condition and filled 

 with water when submerged. For submerged 

 propulsion all this volume, plus all the water 

 volume in the free-flooding spaces, has to be 

 taken into account, just as if it were frozen 

 into ice and carried along with the ship. This is 

 the bulk volume of the submarine. 



Because of available space around and depth 

 under the ship when docking and maneuvering, 

 of limited first cost, of adequate metacentric 

 stability, or of some factor not remotely related to 

 hydrodynamic design it often becomes necessary 

 to impose some definite or arbitrary limits upon 

 the principal hnear dimensions for a given weight 

 or volumetric capacity. This is where the de- 

 signer's troubles really begin. 



65.5 Determination of the General Hull 

 Features. The general hull features of a new 

 ship design can be determined in either of two 

 ways. One can start figuratively in the air — or 

 better, in the water — with only the operating 

 requirements, and fashion the ship out of the blue. 

 Alternatively, one can expand or contract the 

 hull "of a known vessel of good performance" 

 [EUis, J. J., Froude, R. E., INA, 1892, p. 211] and 

 thus obtain a first approximation to a new vessel 

 which will fulfill those requirements. 



It is theoretically possible for an experienced 

 ship designer, working to a given set of specifica- 

 tions, to select a type of hull, to determine its 

 general shape, to define its proportions, and to 

 make a tentative decision to embody in it some 

 special or unusual feature by working only from 

 available reference libraries, including his own. 

 It has often been done, and most successfully, 

 despite the many indefinite and unpredictable 

 elements which come into the picture. Indeed, it 

 is often much better to start with a clean sheet, 



