Sec. 7632 



DESIGN OF SPECIAL-PURPOSE CRAFT 



817 



(11) The type and position of the rudder(s) 

 shall be such that steering or maneuvering control 

 of the boat is maintained at all times, regardless 

 of the state of the sea. 



It is clear from item (1) preceding that these 

 boats represent very nearly the ultimate in the 

 wavegoing performance required of any vessel 

 at sea. It might almost be said that life-saving 

 boats should float and remain upright when all 

 other craft sink or turn bottom side up. To achieve 

 requirements (1) and (4) they are distinguished 

 by very large freeboard at the bow and stern, 

 usually very nearly equal at both ends, by a 

 very large sheer, and by a very large volume of 

 abovewater reserve buoyancy, often in the form of 

 closed compartments intended for buoyancy only. 



The self-righting requirement (2) means that 

 the boat should have positive transverse meta- 

 centric or positive pendulum stability when 

 inclined to amj 'position in roll. It should not only 

 right itself from heel angles between 90 and 180 

 deg but should be capable of rolling completely 

 over (360 deg), without serious damage, if caught 

 by a breaking beam sea. 



Cockpits and other depressions exposed to the 

 weather are rendered self-bailing by making them 

 watertight, with freeing slots passing out through 

 the sides or down through the bottom. Some 

 water enters through these slots when the boat 

 pitches and rolls but the amount is small, usually 

 only enough to cause slippery footing or to freeze 

 into objectionable ice in cold weather. 



On certain life-saving boats of older design, 

 which were not required to be self-righting, it was 

 customary to fit one or two projecting appendages, 

 resembhng roll-resisting keels, along the bulge on 

 each side. These were slotted so that, in the event 

 the boat turned bottom side up, the survivors 

 could use them as hand rails. In fact these 

 appendages were wide enough to serve rather 

 well as roll-resisting keels, despite the hand holes 

 cut through them. There is no reason why a self- 

 righting rescue boat could not be fitted with one 

 or two pairs of roll-resisting keels, to improve the 

 wavegoing performance and to serve as fenders 

 or guards for the lower portions of the hull. 



To meet specification (7), propulsion may be by 

 ducted propellers, approaching pure hydraulic-jet 

 propulsion, or by screw propellers working in 

 tunnels of suitable shape. Hotchkiss propellers, 

 with impellers entirely inside the hull, are often 

 employed. Any type of propeller or impeller 



taking suction from the bottom of the boat must 

 be able to work in water-mud or water-sand 

 mixtures. 



A Gill screw propeller, fitted inside an internal 

 duct, taking suction through a grating and dis- 

 charging through a rotable "deflecting nozzle," 

 is sometimes used for a lifeboat installation 

 [SBSR, 27 Jul 1939, pp. HI, 113]. Figs. 59.Da 

 and 59. Db show the general arrangement and 

 method of operation of the Hotchkiss and Gill 

 propellers, respectively. 



It is pointed out in reference (o) of the attached 

 list that rescue-boat speeds of the past, usually 

 not exceeding 8.5 kt, are no longer considered 

 adequate. Speeds up to 20 kt, to be incorporated 

 in German life-saving boats under design at the 

 time of writing (1955), will call for a rather 

 drastic modification in the double-ended form 

 which has been standard since the days when all 

 these craft were propelled by oars. Transom 

 sterns of moderate width are indicated for speed- 

 length quotients exceeding about 1.2 or 1.3. It 

 is possible that the high-speed, round-bottom, 

 transom-stern patrol boat of small size, already 

 developed into a craft capable of withstanding 

 extremely heavy weather, may be found suitable 

 for life-saving boats meeting the requirements 

 listed at the beginning of this section. 



Appended is a brief list of references relating 

 to life-saving or rescue boats. Although by no 

 means complete it will give the reader some idea 

 of the design problems involved: 



(a) Reynolds, O., "On Methods of Investigating the 



Qualities of Lifeboats," Manchester Literary and 

 Philosophical Society, 14 Dec 1886, in which Pro- 

 fessor Reynolds advocated the use of models 



(b) Corbett, J., "Experiments with Lifeboat Models," 



INA, 1890, pp. 263-283 and Pis. XIV and XV. 

 Reserve-buoyancy ratio of craft in the period 

 1862-1890 varied from about 0.75 to 2.1. 



(c) Bamett, J. R., "Motor Lifeboats of the Royal 



National Lifeboat Institution," INA, 1910, pp. 

 112-119 and 129-139; also PI. X. This paper con- 

 tains the principal lifeboat requirements, expressed 

 in somewhat general terms. 



(d) Everett, H. A., "Stabihty of Lifeboats," SNAME, 



1913, pp. 133-143 and Pis. 73-82. While this paper 

 applies to lifeboats as carried on larger vessels, it 

 contain interesting information for the designer 

 making a study of life-saving craft. 



(e) Bamett, J. R., "Recent Developments in Motor 



Life-boats," INA, 1922, pp. 283-290 and Pis. 

 XIX-XXII 



(f) Bamett, J. R., "Motor Life-boats of the Royal 



National Life-boat Institution," INA, 1929, pp. 

 225-236 and Pis. XXIII and XXIV 



