808 



HYDRODYNAMICS IN SHIP DESIGN 



Ser. 76.29 



and because it has a prow which resembles that 

 of a scow more than that of a boat, the provision 

 of adequate freeboard at the running attitude 

 is particularly important. This is almost im- 

 possible to calculate and there is little background 

 of empirical data for reference. Towing and 

 simple self-propulsion tests of small-scale models 

 are definitely indicated. Fortunately, when the 

 drag is almost wholly due to pressure and when 

 the wavemaking aspects are to be studied, tests 

 can be accomplished in any of the small model 

 basins. 



76.29 Vessels Designed for Beaching. It is 

 reported that the Chmese junk became, in the 

 course of its long development, a rather more than 

 passable landmg craft because of the almost 

 total lack of piers and wharves along the traffic 

 routes. In fact, it is supposed that the ancient 

 use of a non-watertight forepeak and a watertight 

 collision bulkhead at its after end stemmed from 

 the hopelessness of keeping tight the hull seams 

 around the stem with constant beaching. While 

 the shallow-draft sternwheel river steamers of 

 America were not designed expressly for beaching 

 they were eminently adapted for tying up along 

 the river banks and handling cargo whenever 

 the occasion demanded and wherever there 

 was a bank suitable for the purpose. Like the 

 amphibian, therefore, the first craft designed for 

 transferiing cargo directly to a bank or beach 

 were engaged in peaceful pursuits. The need for 

 peacetime landing craft may be expected to 

 continue as long as shore facilities lag behind 

 human needs. 



Requu-ements for vessels to carry heavy, bulky, 

 and expensive cargo for landing directly on the 

 beach must state the: 



(a) Minimum slope of beach wherever a landing 

 needs to be made 



(b) Minmium depth for keeping one end of the 

 craft waterborne or, conversely, the distance 

 within which the ship shall approach the water's 

 edge at the beach 



(c) Range of tide or water level to be expected 

 at and during the landing 



(d) Strength and direction of tidal and other 

 currents at the bank or beach 



(e) Wind waves and surf to be encountered 



(f) Percentage (approximate) of the useful load 

 which can be devoted to changing the trim of the 

 craft to accommodate the depth and slope of the 

 beach 



(g) Relative importance of landing on a beach 

 as compared to maintaining speed and wavegoing 

 in the open sea. 



The foregoing requirements are in addition to 

 those for a ship of normal design which lands at 

 a pier or quay. 



The conflicting requirements of (1) acceptable 

 wavegoing behavior for ocean voyages and (2) 

 shallow draft with trim by the stern for landing 

 may be met by the same procedure as for ships in 

 ballast and for tankers traveling light, that is, 

 by the use of liquid (water) ballast. This can be 

 pumped out just before landing at the beach or 

 it can be shifted aft to give the desired trim. 



A much worse problem is not so easily solved 

 by the compromises often encountered in ship 

 design. This involves the almost inevitable use 

 of a broad, flat, shallow bow, adapted to landing 

 on broad, sand beaches but wholly unsuited for 

 wavegoing. The only reasonable solution to this 

 impasse appears to be the use of liquid ballast, 

 liquid fuel, or liquid cargo in the forward part of 

 the vessel. 



The bow is, by this means, pushed down as far 

 as possible, in an effort to keep the flat portion 

 always under water, so that slamming does not 

 occur there. The hquid is shifted aft when ap- 

 proaching the beach, to hghten the vessel forward 

 and to accommodate the slope of the beach. 



Proposals are made from time to time for a 

 landing craft to be fitted Avith bow propellers 

 and to run in waves with the deep end forward. 

 This sounds attractive but has the disadvantage 

 of a high thrust-deduction fraction for screw 

 propellers positioned ahead of the hull, as well 

 as inadequate submergence and racing because 

 of the large ampfitude of pitch at the bow when 

 traveling in waves. 



A form well adapted to operation in reasonably 

 rough water and to running in toward shore 

 through heavy swells and surf is one having a 

 flattened W-section, similar to a pair of inverted- 

 vee hulls, like two sea sleds placed side by side. 

 Three projecting keels may be fitted under it to 

 act somewhat as longitudinal stabilizing fins and to 

 resist slewing, yawing, and broaching. In addition, 

 the three keels provide great lateral stability 

 when beached, to say nothmg of an excellent dis- 

 tribution of the beaching load. Under certain 

 circumstances the absence of flat, horizontal 

 surfaces under the bow might defer or ehminate 

 slamming. Under other sea conditions, especially 



