Sec. 76.17 



DESIGN OF SPliCIAL PURPOSE CRAFT 



781 



open docking recess and a tail gate, or as a single- 

 section floating drydock with a ship bow and the 

 same tail gate. 



The question of whether any floating drydock 

 should be self-propelled is not at issue here. 

 These notes are included for the benefit of a 

 designer who may be called upon to lay out a 

 self-propelled craft of this type, whatever its 

 service or purpose may be. As a rule, he will be 

 called upon to work on a craft which is not 

 suitable for self-propulsion but must be made so 

 by his resourcefulness or ingenuity. 



An early form of self-propelled floating drydock 

 with Ruthven hydraulic propulsion was illus- 

 trated by Vice-Admiral Sir E. Belcher many 

 years ago [INA, 1870, pp. 197-211 and Fig. 2, 

 PI. I]. It seemed logical at that time to use 

 hydraulic jet propulsion since the dock had to be 

 equipped with pumps in any case to handle the 

 water in its various compartments. A similar 

 type of drydock, proposed by G. B. Rennie [INA, 

 1883, Vol. XXIV, p. 225ff] was intended to be 

 propelled by six hydrauhc jets on each side, 

 indicated on Plate XV of the reference. 



A similar dock with one screw propeller out- 

 board at each after corner was proposed by L. 

 Clark in a somewhat later paper, supplemented by 

 some comments of W. Froude [INA, 1877, Vol. 

 XVIII, PL XIV, Figs. 2 and 3; also discussion 

 on p. 196]. Froude reminded the author (Clark) 

 that: 



"If you put a screw close to a ship, cut off without any 

 run at all, the propulsive effect of the screw becomes 

 absolutely nil." 



A ship-shaped, floating drydock (ARD 1), cap- 

 able of handUng a destroyer of the middle 1930's, 

 was designed by the Bureau of Yards and Docks 

 of the Navy Department and built for the U. S. 

 Navy in 1934. This dock was designed to accom- 

 modate propelling machinery and propellers for 

 self-propulsion but was never so fitted. It had an 

 overall length of 393 ft, a beam of 60 ft, and a 

 depth of 33 ft, with a maximum draft when 

 flooded of 30 ft. Bow and stern views of this dock 

 were published at that time and later [SBSR, 23 

 Aug 1934, p. 200; Motorship, New York, Apr 

 1950, p. 27; Mar. Eng'g., Apr 1954, p. 110, shows 

 a stern view of the larger ARD 8, Lqa = 486 ft, 

 B = 72 ft]. 



A bow aerial view of an ARD floating dock 

 appears on the front cover of Bureau of Ships 

 Journal for April 1953. A stern aerial view is 



reproduced in Fig. 76.. I. The positions originally 

 intended for the propellers of ARD 1 were under 

 the after outer corners of the stern, slightly 

 forward of the tail gate. 



A not-too-blunt bow, illustrated in the Bureau 

 of Ships Journal photograph, is a necessity if a 

 drydock or any craft of this general shape is to 

 be self-propelled at a reasonable speed without 

 an inordinately large propelling power. Assuming 

 a 420-ft waterline length a nd a n upper limit of 

 speed of 10 kt, T, is 10/V420 = 0.488, F„ = 

 about 0.145. From Fig. 66.1 the optimum water- 

 line entrance slope is 32 deg but since the graph 

 is relatively steep in this region the slope could 

 be raised to 40 deg or more if other more important 

 requirements forced this change. 



A floating drydock requires considerably more 

 bed clearance, or water under it, than the ships 

 being docked. Since the dock is often moored close 

 inshore, generally in areas not useful for operating 

 ships, the overall draft must be kept to a minimum. 

 This complicates the problem of where to put the 

 propulsion devices while retaining a reasonable 

 degree of efficiency in their operation. 



A multiple-arch type of stern, similar to the 

 single-arch stern described in Sec. 67.16, appears 

 to offer the best promise of good propulsive 

 efficiency while maintaining the stiffness necessary 

 in the floor structure of any floating drydock. 

 Such a stern would have a profile corresponding 

 approximately to that for the single-arch ABC 

 stern shown in Figs. 67. M and 67.0. The pro- 

 pellers could be large compared to the fight draft, 

 as their upper tips could extend above the WL. 

 They would be protected excellently all around, 

 and they could even be made accessible for in- 

 spection and repairs by flooding the forward 

 dock tanks. The multiple skegs should give 

 excellent stability of route, and the multiple 

 rudders abaft them equally good maneuvering 

 characteristics. 



The design of a ship-shaped floating drydock 

 for towing only, embodying skegs or rudders or 

 both at the stern, is discussed in Part 5 of Volume 

 III, under the design of towed craft. 



76.17 Design of Temporary Bows for 

 Emergency Running and Towing. A design 

 problem akin to that of shaping a bow for a 

 self-propelled floating drydock is laying out a 

 temporary bow on a ship which has lost its own 

 bow. Towing or pushing a vessel terminated at 

 its forward end only by a flat watertight bulkhead 

 is an extremely precarious operation. A danger- 



