Sec. 76.2 DESIGN OF SPECIAL-PURPOSE CRAFT 



TABLE 76.C — Data on Fast Launches and Torpedoboats Prior to 1905 



753 



the well-known racing scull or shell. This is 

 designed and built for rowing by one, two, four, 

 or eight persons. Of no strictly utilitarian purpose, 

 except to provide exercise in the open air and 

 athletic competition, the racing shell apparently 

 has achieved the ultimate in hydrodynamic 

 development entirely by cut-and-try methods. 

 Principal dimensions and form data for an 8-oared 

 shell of some decades back, published by F. H. 

 Alexander ["The Propulsive Efficiency of Row- 

 ing," INA, 1927, pp. 228-244 and PI. XXV; 

 abstracted in SBSR, 21 Jul 1927, pp. 76-77], are 

 set down in Table 76.b, to be found on page 229 

 of the reference. Of interest are the comparative 

 data on a 10-oared whaleboat and a cross-channel 

 steamer. The high value of Sr/A of 95 lb per ton 

 for the racing shell is rather remarkable, although 

 not too far out of line with the data in Fig. 56. M. 

 Also to be noted is the fact that the friction re- 

 sistance for the racing shell may be as high as 

 95 per cent of the total. 



Attempts to reduce the friction resistance by 

 reducing the wetted surface, using a shorter 

 length and a greater beam, gave no reduction in 

 resistance [EMB Rep. 117 of Aug 1925 and EMB 

 Rep. 283 of Feb 1931]. The fact that the smaller 

 i?„ accompanying the shorter length increases 

 the value of Cf was possibly overlooked in this 

 investigation. A transom stern was tried on model 

 scale, hkewise with no improvement. With their 



extremely fine bows, and with clean, pohshed hull 

 surfaces, it is possible that the racing shell 

 benefits from some laminar flow at and abaft the 

 bow. 



However, a shape of these extreme proportions, 

 having L/B ratios of 25, 30, or more, in which 

 the human propelling machinery is not part of 

 the craft proper, is obviously of limited applica- 

 tion. In fact, the craft, with its crew, does not even 

 have positive metacentric stability. On any 

 narrow hull of this kind the oars must be kept out 

 and the craft balanced by holding them in the 

 water. 



There is a possibihty that concentrated loading 

 in long, flexible craft of this type may, because of 

 sagging, change the form and the hydrodynamic 

 characteristics in the operating condition. 



A comprehensive treatment of the hydro- 

 dynamic aspects of racing-shell design is given in 

 EMB Report 117, published in August 1925. 

 Informative in this respect is an article of about 

 that time in the Scientific American [Jun 1925, 

 pp. 369-370]. 



Considering fast launches and torpedoboats as 

 two other types of craft with fine, slender hulls, 

 some interesting data on vessels of the vintage of 

 1905 and earlier are given by C. H. Crane 

 [SNAME, 1905, p. 369]. The technical informa- 

 tion in Table 76. c, adapted from the Crane 

 reference, contains calculated values of T^ which 



