Sec. 76.30 



DESIGN OF SPECIAL-PURPOSE CRAFT 



813 



"Surfoi 



BuoyancN; Force B - 



Stern Up^^ -->_J^uM Axis 

 Restoring Arm 



Submarine Acts Qs a 

 Pendulum with Weight 

 at CG and Support at CB 



Fig. 76.W Sketch Illustrating Pendulum Stability 

 OF Submerged Submarine in Plane of Symmetry 



gravity CG lies always below the combined 

 center of buoyancy and metacenter. This gives 

 the vessel what is known as pendulum stability, 

 with G always below M, illustrated in Fig. 76.W. 

 Any moment of weight forward or aft of the 

 submerged CB causes the vessel to trim in that 

 direction until the center of gravity is directly 

 underneath the center of buoyancy. A small 

 offset may result in a large trim angle unless the 

 necessary correction is made to the CG position. 

 When the vessel is inclined submerged by some 

 external moment, either in heel or in trim, there 

 is always a pendulum-type restoring moment 

 acting to level it off at an equilibrium attitude. 

 When underway, this moment is superposed on 

 the hydrodynamic moments. 



V. Hull Shape and Propulsion. The question is 

 repeatedly raised, when submarine shape and 

 propulsion is discussed, why the submarine de- 

 signer should not take advantage of the develop- 

 ments of nature, involving a process of evolution 

 extending through untold millenniums. He is 

 questioned as to why he does not copy some sort 

 of aquatic fish or mammal, such as the shark or 

 the porpoise, or even the whale. These creatures 

 are known to be, for their size, capable of prodi- 

 gious speeds, exceeding 20 kt in spurts for the 

 porpoise and the whale. They make these speeds, 

 at least in the case of the porpoise, with what 

 seems to be effortless ease. 



The achievements of fish and aquatic mammals, 

 in the matter of propulsion and maneuvering and 

 related operations, have been the subject of 

 scientific, engineering, and physiological study for 

 three-quarters of a century or more. Research 

 along these lines is continuing, steadily if slowly. 



A few features already kiarned from them are 

 discussed in Sec. 15.0 and illustrated in Fig. 15. C 

 under flexible-fin propulsion. 



However, in an effort to copy features that will 

 be helpful in submarine design, two obstacles 

 present themselves. They have not yet been sur- 

 mounted. 



The first is that, while the general mechanism 

 of fish propulsion is known, man has not dis- 

 covered the exact method by which the fi.sh or 

 the sea mammal achieves its highest speeds or 

 s-wims rapidly for long periods of time. Mani- 

 festly, these methods must be known and under- 

 stood before they can be copied or utilized. For 

 example, it is asserted that, in order to maintain 

 a speed of 20 or 25 kt while converting food 

 energy into propulsion energy and motion at the 

 highest rate known to man, a porpoise would have 

 to consume his entire weight in small fish every 

 half hour. It seems certain that the animal does 

 not do this, at least while traveling continually at 

 such high speed. It is also asserted that a pigeon, 

 if as inefficient as some man-made airplanes, 

 would have to carry along an internal-combustion 

 engine with a power of several horses, in order to 

 make its known speed through the air. 



The second is that for the great majority of 

 creatures in this category, propulsion is based 

 inherently upon flexure of the body as a whole, 

 in addition to motion of the fins, tail, and flukes. 

 Certainly, swimming at the high relative speeds 

 desired always involves body undulations. Man 

 has not yet invented a submarine structure 

 which can withstand great hydrostatic pressures 

 and house a powerful propelling plant, while at 

 the same time possessing great flexibility and 

 capable of generating S-shaped undulations which 

 travel along its length. 



Actually, because of their varying environment 

 and their different needs, fish and mammals are 

 often rather poorly streamlined by modern 

 hydrodynamic and aeronautic standards. If it 

 becomes necessary, on a submarine, to depart 

 from a good streamline shape, that of some fish 

 or mammal could be accepted. 



VI. Maneuvering Submerged. The term maneuver- 

 ing, as used here, involves: 



(1) Transient and steady motions in a straight 

 line parallel to the submarine axis 



(2) Steering and turning in a plane that is hori- 

 zontal or nearly so 



