790 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.23 



which two canoe-shaped hulls were combined 

 with a shallow scow type of hull. A small body 

 plan of this craft, accompanied by a photograph 

 of it under sail, is published in Yachting [Dec 

 1946, p. 72]. In this case, however, the designer 

 did not have to concern hmiself too much with 

 air flow between the hulls because at its higher 

 speeds the craft sailed at a large angle of heel, 

 with only the leeward "canoe" in the water. 



Sec. 25.23 contains a number of references to 

 catamarans and trimarans in the modern (1955) 

 technical hterature. Older Uterature goes back at 

 least as far as Dixon Kemp's "A Manual of Yacht 

 and Boat Sailing" of 1882 [Cox, London, 3rd ed.]. 

 In Chap. XXVI of this book, pages 348-356, 

 entitled "Double Boats," Kemp describes and 

 illustrates a number of catamarans, including 

 several built by N. G. Herreshoff in 1876 and the 

 years following. C. Grasemann and G. W. P. 

 McLachlan mention two ships designed for 

 English Channel service, each having twin hulls. 

 The Castalia, built by the Thames Ironworks 

 Company m 1874, had two half hulls with the 

 inboard sides vertical. The Express, a more suc- 

 cessful venture, buUt by Messrs. Andrew Leshe 

 and Company at Hebburn-on-Tyne in 1878, had 

 two complete hulls ["Enghsh Channel Packet 

 Boats," Syren and Shipping Ltd., London, 1939]. 



C. J. Wickwire describes the 35-ft catamaran 

 designed and built by D. and A. Locke of Detroit, 

 based upon careful design studies supplemented 

 by model tests [Lakeland Yachting, Jul 1953, pp. 

 28, 40-41]. The extreme beam is 12 ft; the indi- 

 vidual hulls, flat on their outboard sides and 

 cambered on their inboard sides, each have a 

 beam of 4.5 ft. Certain notes relating to modern 

 practice which may be found useful in the design 

 of catamarans are embodied in a paper by R. F. 

 Turner entitled "Catamarans, Past, Present and 

 Future" [SNAME, Pearl Harbor Sect., 13 Sep 

 1955; abstracted in SNAME Bull., Oct 1955, pp. 

 31-32]. 



76.23 Requirements for and References on 

 Ferryboats. While most double-ended vessels, 

 whether self-propeUed or not, are intended for 

 the short-haul transportation of people, creatures, 

 and objects as ferryboats, their use is not neces- 

 sarily restricted to this type of service. They are 

 therefore considered here primarily as vessels 

 which must operate equally well in either direc- 

 tion. There are, to be sure, many one-direction 

 car and train ferries, running on longer routes, 

 which load over the bow or stern and which in 



most cases have to back for relatively long 

 distances out of or into their slips. The latter are 

 definitely special-service vessels. 



Because they are intended to fill such a great 

 variety of needs it is difficult to formulate general 

 requirements for the hydrodynamic design of 

 ferryboats. A few of these, covering their special 

 features, follow. 



For double-ended vessels: 



(1) Limiting length, maximum beam, and draft, 

 for existing or contemplated shps 



(2) Abihty to start and stop promptly, at large 

 values of acceleration and deceleration, for cutting 

 down the running time, for maneuvering, and for 

 emergency stopping 



(3) Excellent steering and maneuvering charac- 

 teristics. Most ferries are required to cross tidal 

 currents at large angles. Furthermore, most of 

 them cross the normal routes of water traffic 

 nearly at right angles. 



(4) Great metacentric stability because all the 

 useful load is above the main deck and it is well 

 to hmit the fist when the vehicle loads are not 

 symmetric or when the passengers all rush to 

 one side. This happens usually when the vessel is 

 carrying only part of its full load [SNAME, 1926, 

 pp. 228-229]. 



(5) Large flare in the abovewater body for in- 

 creasing the metacentric stabiUty as the load 

 displacement and the draft increase [MESR, 

 Mar 1939, p. 109] 



(6) An unnecessarily large transverse metacentric 

 height is to be avoided because it does not permit 

 the vessel to yield and to roll readily when it 

 strikes the "racks" on either side when entering 

 its slip [Stevens, E. A., SNAME, 1896, p. 100] 



(7) Large longitudinal metacentric stabihty, to 

 prevent the ends from being depressed unduly 

 when the weights are concentrated there, in load- 

 ing or unloading [DuBosque, F. L., SNAME, 

 1896, pp. 95-96] 



(8) Large deck overhangs, sponsons, and the 

 like, because the useful load is one of volume 

 rather than of weight. At the same time the 

 overhangs must be kept clear of wind waves and 

 the ship's own waves. 



For single-ended vessels: 



(9) No excessive flare under the car-deck or 

 vehicle-deck level, to cause pounding and slam- 

 ming when wavegoing 



(10) If required to back into, or out of long 



