792 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.24 



(v) San Diego-Coronado ferrj' Crown City, Mar. Eng'g., 

 Apr 1954, pp. 58-59; Dec 1954, p. 79 



LoA = 242. 13 ft B of hull = 46 ft 



Lpp = 230 ft D = 17.25 ft 



B over guards = 65.13 ft H approx. = 11.5 ft 

 A = 995 t 



(w) Lengthened ferry Princess Anne, Mar. Eng'g., Jul 

 1954, pp. 66-67, 81. After conversion, 



Lqa = 350 ft B over guards = 59 ft 



Lpp = 340 ft D = 19.1ft 



H = 10.5 ft. 



(x) Ferry Cameron, Mar. Eng'g., Aug 1954, p. 63 

 (y) Automobile and passenger ferry Evergreen State; 

 see Mar. Eng'g., Jan 19.55, p. 70; Diesel Progr., 

 Mar 1955, pp. 40-41; Diesel Times, Nov 1955, p. 7. 

 Said to be one of the largest ferries of its tjrpe in 

 the world, with a length of 310.17 ft, a beam of 

 73.17 ft over the guards, a beam of 53.5 ft at the 

 waterline, and a depth of 23.25 ft. At a draft of 

 15.0 ft it displaces 2,022 tons. There is a diesel- 

 electric drive to separate shafts and single 10.5-ft 

 diameter propellers at each end of the vessel. The 

 power which can be applied to each propeller is 

 about 3,000 horses; 10 per cent of this is delivered 

 to that propeller which is at the bow on any one 

 run and 90 per cent to that at the stern. The trial 

 speed is 15 kt at 171 rpm. 

 (z) De Rooij, "Practical Shipbuilding," 1953, Figs. 801 

 and 802 on pp. 374-375. 



76.24 Characteristics of Propelling Plant and 

 Propulsion Devices for Double-Ended Vessels. 



When simplicity of propelling plant is a primary 

 requirement, as usually occurs on ferryboats 

 designed for short runs, both end screw propellers 

 are coupled firmly to the same shaft so that they 

 run at identical rates of rotation. This means, as 

 described in Sec. 33.8, that neither propeller 

 runs at an efficient advance coefficient, and the 

 propulsive coefficient is hkewise low. This situa- 

 tion was described admirably by F. L. DuBosque, 

 well over a half-century ago, and it has improved 

 little, if any, since that time: 



"The usual speed of this boat in ferry service is 11 

 miles per hour, and at this speed it requires 20 per cent 

 more power to propel the boat with two screws than with 

 one screw pushing, and 69 per cent more power to propel 

 the boat with the screw at the bow than at the stern. 

 If the same power could be put into one screw at the 

 stern as is used by the two screws, the speed would be 

 increased from 11 miles to 11.53 miles per hour. It is 

 clear, therefore, that the bow screw is inefficient. When 

 under way, it thrust a column of water against the bow 

 of the boat at a velocity equal to the shp ratio of the 

 screw, and considerable power is absorbed through friction 

 of the blade surface; but a ferryboat's bow becomes its 

 stern at each succeeding trip, and it is therefore im- 

 possible to dispense with the forward screw" [SNAME, 

 1896, pp. 94-95]. 



If both propellers must exert thrust simultan- 

 eously to accelerate the craft at the high rates 

 required for short runs, or to drive it at the re- 

 quired speed, the type of propelUng machinery 

 is preferably such as to permit varying the rates 

 of rotation and delivering the maximum power 

 to each propeller. This is done by: 



(1) Providing a separate prime mover for each 

 propeller, connected by separate shafts 



(2) Utihzing an electric, hydrauhc, or other type 

 of drive in which individual motors on each 

 propeller shaft are driven from a central generating 

 plant. This is not too difficult even though 

 separate dynamos (generators) and pumps are 

 not installed for each motor. 



(3) Uncoupluig the bow propeller and permitting 

 it to free-wheel, while the vessel is driven entirely 

 by the stern propeller. This is possible only if 

 either propeller can deUver the necessary power. 

 It is not as uneconomical as it seems because of 

 the higher wake fraction and greater propulsive 

 efficiency for the stern propeller. To be sure, it 

 requires the fitting of some kind of clutch, fluid 

 couphng, or free-wheehng device between the 

 prime mover and each propeller. 



Model tests with ferryboats having all three 

 methods of propulsion, carried out in the Swedish 

 State Model Basm at Goteborg, are described in: 



(a) Nordstrom, H. F., and Freimanis, E., "ModeUforsok 



med en Farja (Model Experiments with a Ferr}')," 

 SSPA Rep. 7, 1947. Summary in EngUsh. 



(b) Nordstrom, H. F., and Edstrand, H., "Propulsion 



Problems Connected with Ferries," SSPA Rep. 17, 

 1951. Entirely in English. 



For a ferryboat which travels bow first on its 

 runs, which enters its shps either bow first or 

 stern first, and which is handicapped by narrow 

 slip clearance, cross winds, loose ice, and the hke, 

 an admirable solution is to employ an under-the- 

 bottom rotating-blade propeller at the bow. This 

 may supplement one or two rotating-blade pro- 

 pellers or screw propellers at the stern. The bow 

 propeller augments propulsive power when de- 

 sired, provides powerful lateral forces and steering 

 effects at the bow, and creates a backward flow 

 of water at the head of the ship when entering a 

 shp, so as to clear out debris and ice [Virginia 

 ferry Northampton, Motorship, New York, Aug 

 1950, pp. 26-27, 43]. The principal drawback to 

 this arrangement is that, if the rotating-blade 

 propellers are not retractable, and are not used to 

 help propel the vessel, they must always be idled 

 when underway. 



