HYDRODYNAMICS IN SHIP DESfGN 



Sec. 76.22 



carriers, in which the various mstallations and 

 the weights can not be balanced transversely 

 (g) Ships for special operations which have 

 elaborate and expensive apparatus installed on 

 one side only. 



The degree of asymmetry designed into a hull 

 is measured conveniently by the follo^ving: 



(i) The port and starboard partial beams, meas- 

 ured in the same way as for the half-beam on a 

 normal symmetrical vessel 



(ii) The percentage of the maximum beam by 

 which the center of buoyancy CB is shifted from 

 its normal centerplane position. This is measured 

 by the transverse distance between the CB and a 

 vertical longitudinal plane through the construc- 

 tion centerhne, as compared to the maximum 

 beam. 



(ni) The percentage of the total displacement 

 volume which is shifted from one side of the hull 

 to the other, relative to the vertical longitudinal 

 plane through the construction centerline. For 

 example, if 53.7 per cent of the underwater volume 

 lies on one side of that plane and 46.3 per cent 

 on the other side, the percentage of asymmetry 

 in volume is (53.7 — 46.3)/2 or 3.7 per cent. 



The numerous form coefficients based upon 

 ratios of various areas and volumes to the areas 

 or volumes of the circumscribing rectangles or 

 parallelepipeds are calculated in the same manner 

 for the asymmetric as for the symmetric ship. 



A few notes may be set down for the design of 

 asjrmmetric hulls which are to be built that way, 

 based upon the usual demand for reasonable if 

 not minimum power, maximum speed, and 

 acceptable maneuverability of the asymmetric 

 vessel, correspondmg to those for one that is 

 symmetrical: 



(1) The CG is to be found in the same vertical 

 plane as the CB, offset from the construction 

 centerplane toward the wide side, with the vessel 

 upright and carrying the designed load 



(2) The CB and the CG should, whenever prac- 

 ticable, have essentially the same offset for load 

 conditions fighter than the designed, to insure 

 that the ship remains upright at all drafts and 

 trims 



(3) Based upon a construction centerplane that 

 passes through the hull terminations at the bow 

 and stern, the propulsion devices are usually, but 

 not necessarily mounted symmetrical to that plane 



(4) On the assumption that both the friction and 



the pressure resistance are exerted in a vertical 

 longitudinal plane through the center of buoyancy, 

 which appears reasonable, but that the resultant 

 propelling thrust is exerted along the construction 

 centerhne, there is a constant moment due to 

 these forces alone which acts to swing the ship 

 toward the wide side. Unless the length-beam 

 ratio is less than 5 or 6, this constant turning 

 moment is fikely to be of small consequence 

 although it will always be of the same sign. It is 

 wise to augment the steering control for a ship 

 of this type over that provided for a normal 

 design. The augment may equal but need not 

 exceed the percentage by which the center of 

 buoyancy is offset, described in (u) preceding. 



76.22 Design Problems in Multiple-Hulled 

 Craft. It sometimes happens that a large beam 

 can be accepted for the purpose of carrying objects 

 which are bulky and awkward to handle but 

 relatively light in weight. It may be possible to 

 improve the metacentric stabfiity or load distri- 

 bution in a craft by utilizing two or more widely 

 spaced but narrow hulls instead of one wide hull. 

 The term catamaran is in this book restricted to 

 craft which have two hulls of approximately 

 equal size. An outrigger canoe is considered to 

 have one main hull only; the outrigger is a form 

 of auxfiiary-displacement device corresponding 

 somewhat to the wing-tip float of a flying boat or 

 seaplane. A craft having one main hull and one 

 supplementary hull abreast on either side is known 

 as a trimaran. The side hulls may be of approxi- 

 mately the same .size or they may be smaller 

 than the main hull. 



In view of the many possible uses for towed or 

 self-propelled craft mth multiple hulls no attempt 

 is made to set down their requirements here. 



The crux of a suitable design of water craft in 

 which two separate hulls must move along 

 easily, side by side, is the shaping of the region 

 between the two hulls. Only rarely can the hulls be 

 made sufficiently short and narrow, compared to 

 theii' spread, that they may be considered as 

 independent bodies, hydrodynamically speaking. 

 If they are slim enough to produce relatively 

 narrow velocity and pressure fields, they may 

 still be long enough to cause interferences between 

 the surface-wave patterns between the hulls, as 

 shown in Fig. 76.0. The diverging crests of the 

 inside Velox wave systems will meet each other 

 and be reflected on or about the construction 

 centerplane, just as if there were a thin plate 

 mounted vertically between the hulls in that plane. 



