816 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.32 



embodies the rather meager information of this 

 type which could be collected for six lightship 

 designs dating from 1881 through about 1914. 

 Additional information and data, on old as well 

 as modern vessels, are to be found in the brief 

 list of references which follows: 



(a) Body plan of Elbe lightship, with a B/H ratio of 



25.25/12.46 = 2.03, and a rise of floor of 24 deg, is 

 shown in Schiffbau, 26 Jun 1912, pp. 715-722, PI. .3 



(b) Idle, G., "The EfTect of Bilge Keels on the Rolling of 



Lightships," INA, 1912, pp. 103-123 and Pis. 

 IX-XI. Figs. 10-13 on PI. X illustrate by flowlines 

 the assumed motion of the water around the 

 ship hull and around the bilge keels when rolling. 



(c) Cook, G. C., "The Evolution of the Lightship," 



SNAME, 1913, pp. 97-118 and Pis. 52-63. Gives 

 considerable historical data. Shows arrangement 

 plans, body plan, and lines of self-propelled U.S. 

 Lightship 94. Principal dimensions and hull co- 

 efficients are listed in Table 76. g. The wedges of 

 immersion and emersion are intended to be nearly 

 equal. The bilges are very slack. The full-load dis- 

 placement at 18.58-ft draft is about 1,072 t. Static 

 stability is a maximum at about 60 deg, and very 

 large at 100 deg. The roll-resisting keels are of 

 triangular section, 1.5 ft wide. See also The Ship- 

 builder (now SBMEB), Jan-Jun 1914, Vol. X, pp. 

 215-220. 



(d) Cooper, F. E., Liverpool Eng. Soc, Mar 1914; also 



The Shipbuilder (now SBMEB), Jan-Jun 1914, 

 Vol. X, pp. 295-296. Describes the non-self-propelled 

 light vessel Alarm, whose dimensions are listed in 

 Table 76. g. The bilge keels extend for 0.6L and are 

 about 1.2 ft wide. 



(e) New Lurcher No. 2 Lightship, Diesel Prog., Mar 



192, p. 59 



(f) New Overfalls Lightship, Mar. Eng'g., Apr 1953, p. 45 



(g) New Ambrose Lightship, Mar. Eng'g., Sep 1953, p. 85 

 (h) Lightship Kish Bank, illustrated in SBSR, 27 Jan 



1955, p. 36, has a centerline hawsepipe just above 

 the DWL for mooring on station plus a regular 

 hawsepipe on each bow with port and starboard 

 bower anchors and separate chains. The Motor 

 Boat and Yachting, Aug 1954, p. 357, shows a 

 photograph indicating extreme local sheer at the 

 bow. 



(i) Light vessel Osprey, SBMEB, Jun 1955, pp. 417-418; 

 SBSR, 30 Jun 1955, p. 841. This non-self-propelled 

 ship has an overall length of 136.42 ft and a beam 

 of 25 ft, with "exceptionally large bilge keels." The 

 mooring is by 1.75-in chain, attached to a 4-ton 

 mushroom anchor, with port and starboard 1.5-ton 

 anchors in reserve. 



(j) De Rooij, G., "Practical Shipbuilding," 1953, Figs. 

 793 and 794 and Sec. 206 on pp. 369, 373. 



76.32 Life-Saving or Rescue Boats. The 



hfe-saving or rescue boats discussed in this section 

 are the self-propelled craft which are based on 

 shore .stations, or on large station ships afloat. 

 They e.xclude the lifeboats carried by passenger 



and other vessels, intended only to remain afloat 

 and stay together on the scene until the rescued 

 personnel can be taken aboard another vessel. 

 Despite improved methods of removing personnel 

 from vessels in distress it appears that life-saving 

 boats operating from home or shore stations 

 will be required for many years to come. 



Presumably general design requirements exist 

 for these craft but none have been located in the 

 technical literature, especially none that relate 

 directly to hydrodynamics. The following specifi- 

 cations appear to meet these needs: 



(1) The craft .shall be able to operate in the open 

 sea or to stand by in practically any weather, no 

 matter how severe 



(2) The boat shall be self-righting without crew, 

 when rolled to any angle of heel up to 180 deg 



(3) It shall be virtually unsinkable; in other 

 words, it shall remain afloat, even when moder- 

 ately damaged 



(4) The reserve-buoyancy ratio, with crew only, 

 shall be exceptionally large, preferably of the 

 order of 2 or 3. The wind resistance inherent in 

 this large abovewater volume is accepted. With 

 all the rescued personnel which can crowd aboard, 

 the reserve-buoyancy ratio shall be not less than 

 1.0. 



(5) There shall be a covered deck at both ends, 

 of arched or turbleback form, designed to reUeve 

 itself of water and spray which comes aboard well 

 before the craft is subjected to a succeeding load 

 of water 



(6) All cockpits and depressions in the weather 

 deck shall be self-baihng, with the boat carrying 

 its crew and the maximum number of rescued 

 personnel 



(7) The free-running speed, without passengers 

 but with crew and full fuel and stores, shall be 

 sufficient to reach a disaster scene expeditiously. 

 It shall be not less than 10 kt, in smooth water 

 and no wind, and preferably 12 kt or higher. 



(8) The propulsion devices shall be housed, pref- 

 erably, within the overall fair surface of the hull. 

 In any case it shall be possible to operate them 

 in shallow water, with the boat jiust aground, or 

 when directly alongside a larger vessel. 



(9) The fore-and-aft position of the propulsion 

 devices shall be such that they continue to 

 produce thrust when running under heavy 

 pitching conditions 



(10) The draft shall be the minimum compatible 

 with other recjuirements listed 



