Sec. 7 6.30 



DESIGN OF SPECIAL-PURPOSE CRAFT 



809 



Fig. 76.U Landing Craft with Inverted V-Bottom 



AT Stern 



Official U. S. Navy photograph. The small auxiliary 



rudder forward of the strut is for control when backing. 



The hole in the rudder permits withdrawing of the 



propeller shaft without unshipping the rudder. 



if driven hard, the W-sections forward might 

 produce large accelerations and decelerations, as 

 does the sea sled. The under side of the stern of a 

 landing craft with a single inverted vee of small 

 slope is illustrated in Fig. 76. U. 



Although it has to date (1955) been used on 

 small vessels only, there is a great advantage in 

 having hydraulic jet propulsion available under 

 the bottom when getting off a bank or beach. 

 By directing the jet forward, toward the region 

 where the bow is aground, it is extremely useful 

 for washing away the sand or soil and freeing 

 the vessel easily. 



General arrangement drawings and principal 

 dimensions of landing and beaching craft of 

 intermediate size (LCF and LOT) are given by 

 G. de Rooij ["Practical Shipbuilding," 1953, Figs. 

 791 and 792 on pp. 368-369]. Model test data for 

 four models of landing craft are found on SNAME 

 RD sheets 38, 40, 43, and 44. 



76.30 Some Hydrodynamic Design Problems 

 Common to All Submarines. There are no books, 

 and there is little technical literature, which 

 discuss the design of submarine vessels [Hay, 

 M. F., "The Design of Submarines," SNAME, 

 1909, pp. 233-255]. The hydrodynamic design 

 alone involves matters of diving and surfacing, 

 dynamic equilibrium, speed and propulsion, 

 .maneuvering, and wavegoing, both surface and 

 submerged. Diving and surfacing involve in turn 

 the flooding, venting, and blowing of the main 

 ballast tanks which provide the reserve buoyancy 

 of the submarine when it is on the surface. 



It is impossible, within the space allotted in 

 this book, to do more than describe briefly certain 

 special problems encountered in this hydro- 

 dynamic design, having to do primarily with 

 operation submerged. One seldom finds these 

 problems discussed scientifically in any kind of 

 literature. A statement of them, with their 

 present solutions (or lack of solutions), should 

 not only broaden the outlook of the marine 

 architect engaged in the design of surface vessels 

 but also give him a more penetrating insight into 

 the influence of hydrodynamics on the design of 

 all kinds and sizes of water craft. 



I. Requirements. It might be thought strange, 

 were it not for so many other missing ship- 

 operation requirements, that no basic require- 

 ments for submarine vessels have ever been 

 formulated and published. Those which follow 

 are sketchy but they may at least serve as the 

 groundwork for development in the future. They 

 are based upon a possible, even though seemingly 

 remote utilization of the submarine vessel for 

 peaceful purposes. No attempt is made to go into 

 detail or to insert numbers in these requirements: 



(a) Submerge and emerge, while stationary or 

 under way, when initially on the surface or 

 submerged. This may or may not have to be 

 accomplished within a given interval from the 

 "execute" signal, starting from a given set of 

 conditions as regards ballast water carried and 

 percentage of reserve buoyancy. 



(b) Run submerged at a given nominal depth, 

 throughout the complete speed range, without 

 varying up or down more than a given amount 

 from that depth 



(c) Run submerged, throughout a given speed 

 range, without exceeding specified trim angles 

 (by the bow or stern) for the series of speeds 



(d) Maintain a given depth, within specified 

 hmits, when the excess of static weight or buoy- 

 ancy reaches a certain limiting amount, generally 

 a percentage of the total weight or buoyancy, 

 at all submerged speeds above a certain minimum 



(e) Hold a given depth and maintain a level 

 fore-and-aft attitude, within not-too-close limits, 

 when underway submerged at a very slow speed, 

 less than a certain maxmium. This is the operation 

 known as hovering. The speed is so low that 

 dynamic control is rather weak. 



(f) Change depth, either up or down, within 

 a given elapsed time from level running at the 

 original depth to level running at the new depth. 



