814 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76 J 1 



(3) Rising, diving, and depth-keeping in a 

 vertical plane. 



The bulk shape and bulk volume of the vessel 

 are involved in each case, as contrasted to a 

 smaller underwater volume for the surface ship. 

 If submerged to a depth h of the order of several 

 times its hull height h, the effect of the free 

 surface above it is negligible. This situation is 

 contrasted with that of the surface ship, sur- 

 rounded by an air-water interface upon which 

 gravity waves can be formed. 



Heel when turning, usually only an inconven- 

 ience on a surface vessel, can introduce vertical 

 forces and moments on a submarine which 

 affect its trim attitude and perhaps its depth of 

 submergence. Unless the submarine carries a 

 topside rudder of the type described in Sec. 37.14 

 and illustrated in diagram 2 of Fig. 37. C, the same 

 steering rudder that "handles" the normal 

 lateral area on the surface must, for horizontal 

 steering and turning submerged, "handle" the 

 greater lateral area of the entire bulk of the 

 submarine. 



One feature related to both speed and pro- 

 pulsion and to maneuvering requires mention. 

 Unlike a surface vessel the speed and propulsion 

 requirements for a submerged submarine can not 

 be dissociated from those of trim and attitude. 

 In other words, it is useless for a submarine to 

 be able to travel at a certain speed submerged 

 unless the vessel can be held at the desired 

 attitude and depth at that speed. 



76.31 Lightships or Light Vessels. The 

 design of a lightship or a light vessel, whether 

 manned or unattended when on station, repre- 

 sents some extremely interesting problems in 

 hydrodynamics, which have benefited from only 

 sporadic scientific study in the past. A few of 

 these are discussed here, in rather limited scope. 



Stated briefly, the requirements for a lightship 

 are for a vessel that shall: 



(1) Display the necessary Ughts or other identifi- 

 cation, send out both abovewater and underwater 

 sonic signals, transmit radio signals, and perform 

 similar functions. These are best accomplished 

 when the vessel is upright and at zero trim 

 although they must be carried out under all 

 ship-motion conditions. 



(2) Remain afloat and operable and on station, 

 invariably an exposed one, regardless of the state 

 of the sea and of the wind and weather. In fact, 

 the worse the conditions, the more necessary it 



is that the vessel maintain its charted position. 



(3) Exert the minimum pull, consistent with 

 other characteristics, on its mooring cable or 

 line, when subjected to wind, or current, or both 



(4) Be free of violent or undesirable yawing 

 under the influence of wind or waves or both 



(5) Reduce the motions of rolling, pitching, and 

 heaving to a minimum consistent with other 

 requirements and the wavegoing conditions to be 

 encountered. This is to reduce fatigue of personnel 

 and wear and tear on equipment, whether the 

 vessel is manned or unmanned. 



(6) Be able to propel itself at reasonable speed 

 and to maneuver, if the vessel is of this type. 



The lightship is in a class with the life-saving 

 boat discussed in Sec. 76.32 with respect to its 

 abiUty to remain on station, completely operative 

 except for its propulsion plant, when all other 

 vessels of its size (and larger) are required to 

 seek shelter in port. 



Its freeboard coefficient or freeboard-to-length 

 ratio is much greater than that of any other type 

 of craft except a life-saving boat. Similarly, the 

 reserve-buoyancy ratio is very large, perhaps 

 even larger than that of the latter craft. 



Lightships may have much more than the usual 

 amount of sheer, especially forward, unless the 

 vessel has one more deck height, for its whole 

 length, than would be customary in other vessels 

 of its size. 



While low resistance in a lightship when 

 moored in a seaway is one of the major require- 

 ments, some of it may be sacrificed to permit the 

 use of special features furthering the vessel's mis- 

 sion. One such feature is a set of extremely wide 

 roll-resisting keels, possibly stayed by auxUiary 

 struts extending from near the outer edges of 

 the keel to the adjacent hull [SBSR, 17 Jan 1935, 

 p. 66]. Another would be an effective pitch- 

 damping device, provided a successful one could 

 be devised for this type of vessel. Indeed, any- 

 thing which assists in damping any part of the 

 wavegoing motion is a useful feature, whether 

 the craft is manned or unattended. 



Self-propulsion, if provided, is almost never a 

 primary feature, but the drag exerted by the 

 stationary propeller in a current should be held 

 to the minimum practicable amount. 



In only a very few cases does the technical 

 literature contain reasonably complete principal 

 dimensions, hull parameters, form coefficients, 

 and other characteristics of lightships. Table 76.g 



