Sec. 69.5 



GENERAL DESIGN OF PROPULSION DEVICES 



571 



reliable so as never to require any reserve pro- 

 pelling unit(s) to bring the vessel home in the 

 event of casualty. It is, however, vastly more 

 reliable than was the machinery of a former age, 

 which saw a shift to twin-screw machinery 

 largely to provide this measure of safety. 



The position of the propelling machinery in 

 the vessel is of interest in the hydrodynamic 

 design principally because it: 



(d) Affects the declivity and the horizontal 

 angle (s) of the propeller shaft(s), as well as the 

 shape and positions of the shaft struts or bossings 

 external to the main hull 



(e) Affects the size and shape of the hull in way 

 of large motors, gears, condensers, or other 

 machinery parts which need certain clearances 

 from the hull structure 



(f) Concerns the readiness with which the screw 

 propellers are kepi submerged in all operating 

 conditions. This item is considered most important 

 [Rupp, L. A., and Jasper, N. H., SNAME, 1952, 

 pp. 352, 354]. 



(g) Affects the disposition of the products of 

 combustion and the air resistance of stacks, 

 standpipes, or other deck erections 



(h) Controls the possibilities and the methods of 

 underwater gas exhaust. 



Assuming that the ABC ship is to be driven by 

 screw propellers at the stern, the following hne of 

 reasoning was employed when determining the 

 proper number. With the tentative beam-draft 

 proportions of the first combinations of Table 

 66.e of Sec. 66.11, varying from 78.25:26 to 

 74:26, it appears easy to fit twin screws. However, 

 the propellers are there to drive the ship, not 

 necessarily to make it easy to design or construct. 

 On the basis of requirement (5) of Table 64.a, for 

 "Performance of the required transportation as 

 efficiently and economically as the present state 

 of the art permits," a single screw is definitely 

 indicated. The resultant saving in fuel and 

 corresponding increase in other useful items 

 should be of the order of 3 per cent. It is agreed, 

 however, between the designer and the future 

 owner and operator, that a prehminary design 

 with twin screws is to be worked up if time and 

 opportunity permit. 



The layout selected for the ABC design calls 

 for the machinery units to be placed as far aft as 

 practicable, following in some measure the 

 designs of the combination passenger and cargo 



vessels San Francisco (old) and Maui of the 

 early and middle 1910's, running between San 

 Francisco and Honolulu. Two later Matson 

 hners, designed primarily for carrying cargo, and 

 having the single-screw machinery installed way 

 aft, were the Manulani and the Manukai [MESA, 

 Sep 1921, pp. 707-708]. This arrangement, 

 incidentally, was adopted as far back as the 

 period 1843-1845 in the American auxiliary 

 sailing ships Commodore Preble and Bangor 

 [Bradlee, F. B. C, "Steam Navigation in New 

 England," Salem, 1920] and in the Edith and the 

 Massachusetts, built for R. B. Forbes [American 

 Neptune, Jan 1941, Pis. 2, 5]. A much later 

 version is the Shaw-Savill finer Southern Cross, 

 a pure passenger ship, with a single mast and 

 single stack, far aft [111. London News, 19 Dec 

 1953, p. 1020; MENA, Dec 1953, p. 569; SBSR, 

 Int. Des. and Equip. No., 1954, pp. 3-4]. In 

 Europe an after machinery position was first 

 used on the EngUsh coastal colher John Bowes 

 in 1852 [Bowen, F. C, SBSR, 30 Sep 1937, pp. 

 421-422]. It is no longer necessary, as on the 

 latter vessel, to put the machinery and the smoke 

 stack "right aft," out of the way of the fore-and- 

 aft sails on three masts. Nevertheless, the other 

 reasons for placing the machinery in the stern 

 are as valid today as they were in 1845, 1852, 

 and again in 1892, when the steamer Turret was 

 built in this fashion. These reasons, stated at the 

 time by F. C. Goodall [INA, 1892, pp. 194-195] 

 and later by G. C. V. Holmes ["Ancient and 

 Modern Ships," 1906, Part II, p. 120] are sum- 

 marized here from those authors: 



(1) When the ship is without cargo it helps her 

 to trim by the stern, and thus gives good immer- 

 sion to the propeller 



(2) Water or fuel tanks may be fitted in the after 

 part of the vessel, to help submerge the propeller, 

 as shown on Plate XXIII of the Goodall paper 

 of 1892. This is now standard on all Great Lakes 

 vessels, and is incorporated in the ABC design. 



(3) The main shaft is shorter and lighter, with 

 fewer bearings to watch and lubricate 



(4) The hold space occupied by the shaft tunnel 

 is saved 



(5) Using the most valuable part of the hull, the 

 rectangular section amidships, greatly facilitates 

 the stowage of cargo [SBMEB, Jan 1953, p. 4] 



(6) Less useful volume is lost, around the machin- 

 ery components, if they are placed in the after 

 part of the vessel. 



