812 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.30 



quantities of stationary water which can enter 

 there. When these are picked up and accelerated 

 by transverse structural members and other 

 parts housed mthin the superstructure a useless 

 loss of energy is involved. 



The volume of water occupying certain small 

 free-flooding spaces below the surface waterplane, 

 rarely to be found on surface craft, is considered 

 as a part of the ship weight which must be 

 carried along. Outside water must be displaced 

 around these spaces the same as around the other 

 parts of the hull. In one respect these several 

 volumes of water are treated as though they were 

 blocks of ice, frozen in place. If the free-flooding 

 spaces have external openings in regions where 

 there is a pressure gradient on the outside of the 

 hull, a flow of water through the spaces takes 

 place because of this pressure gradient. Energy 

 expended in setting up and maintaining this flow, 

 generally of an eddying character and invariably 

 undesirable, is energy lost so far as the propulsion 

 of the submarine is concerned. 



III. Equilibrium of Static and Dynamic Forces- 

 In a surface vessel, nature takes care of the 

 balance between the hydrostatic weight and 

 buoyancy forces by adjusting the latter so as to 

 equal the weight force imposed by the crew. If 

 the weight is increased by loading something 

 aboard, the vessel sinks to provide the additional 

 buoyancy. In a submerged submarine the buoy- 

 ancy force is usually fixed by the total volume 

 of the water-excluding structure and external 

 parts, combined with the density of the sur- 

 rounding water. The crew then has to make the 

 necessary internal adjustments in the weight 

 force, by admitting or expelling or shifting 

 variable-ballast water, if equilibrium is to be 

 maintained. 



While it is possible for the primary static 

 forces of weight W and buoyancy B to be bal- 

 anced by taking in or pushing out variable- 

 ballast water, this is the exception rather than 

 the rule in submerged opei-ation. It is rarely 

 possible to achieve an exact balance of primary 

 hydrostatic forces, and moment as well, when 

 underway beneath the surface. In addition there 

 are vertical dynamic forces and moments gen- 

 erated when the vessel is underway by the lack 

 of symmetry of the outer hull about any longi- 

 tudinal horizontal plane, by the presence of deck 

 erections and appendages, and by small inclina- 

 tions of the fore-and-aft axis to the direction of 



motion. Combined, these render the equalization 

 of weight W and buoyancy B a matter of hydro- 

 dynamics as well as hydrostatics. 



Admitting and expelling variable-ballast water 

 in suitable quantities and at the proper locations 

 as the submarine is moving can take care of the 

 preponderant hydrostatic inequalities between W 

 and B. However, the hydrodynamic inequalities 

 usually vary in magnitude with ship speed. 

 They, as well as the undetermined {W-B) values, 

 are normally compensated by vertical forces 

 derived from hydrofoil action of the diving 

 planes. 



It is found that an amazing variety of submarine 

 forms are afforded adequate control in rising and 

 diving by the usual arrangement of bow and 

 stern planes. 



IV. Metacentric and Pendulum Stability. The 

 problem of adequate metacentric stability when 

 in the so-called "awash" condition, during either 

 diving or surfacing, undoubtedly involves a time 

 element to a small degree, and hence partakes of 

 the nature of hydrodynamics. There are free 

 surfaces in most if not all of the main-ballast 

 tanks at some time or other during flooding or 

 blo\\'ing. Each tank is in communication with the 

 sea and only indirectly with its companion tank 

 on the opposite side of the vessel. For this reason 

 it is necessary to reckon a loss of BM correspond- 

 ing only to the square moment of area of the free 

 surface in each tank about its own fore-and-aft 

 axis, and not about the longitudinal axis of the 

 vessel. As a practical matter the time element 

 enters particularly in the blowing-down operation 

 while surfacing. This requires some minutes, 

 dining which time the loose water in each main- 

 ballast tank is in direct communication with 

 the sea through the flood valves at the bottom 

 of the tank. There must be sufficient control over 

 the compressed air delivered to the main-ballast 

 tanks on the two sides to hold the vessel in a sort 

 of average upright position, even when acted upon 

 by a heavy beam sea, with water surging up and 

 down in the tanks. 



For a consideration of the forces and moments 

 involved in maneuvering submerged it is neces- 

 sary to realize that the submarine in that con- 

 dition has no effective or intact sur face -waterline 

 area (and no loose water), so that BM = and 

 the metacenter M coincides with the center of 

 buoyancy CB. What holds the submerged sub- 

 marine upright is the fact that the center of 



