568 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 69.3 



are to be used, such as those on a self-propelled 

 floating crane. 



Concerning the Jiumbcr of propulsion devices, 

 the following considerations govern: 



(a) Available depth of water in operating areas, 

 limiting extreme draft, probable draft of hull of 

 vessel, and approximate beam-draft ratio of hull. 

 This item involves factors such as adequate tip 

 submergence to prevent air leakage to submerged 

 devices, adequate hydrostatic head to prevent 

 cavitation, most efficient blade lengths, and per- 

 missible extension, if any, below the baseplane. 

 It may be necessary, for some of these reasons, to 

 fit a larger number of small-diameter or small-size 

 propulsion devices, or a fewer number of large- 

 diameter ones. 



(b) Special operating requirements such as those 

 for maneuvering in restricted waters and emer- 

 gency stops. Wing propellers provide appreciable 

 swinging moments by going ahead on one and 

 backing on the other. 



(c) Probability of large exciting forces and exces- 

 sive hull vibration if large powers are concentrated 

 in too few propulsion devices. As of 1955, the 

 largest power applied to the propeller of a single- 

 screw vessel, the tanker W. Alton Jones, is about 

 22,000 horses [Mar. Eng'g., Nov 1955, p. 119]. 

 However, powers of the order of 50,000 horses 

 are being developed by the individual propellers 

 of quadruple-screw installations, and these may 

 eventually be reached by the propellers of fast, 

 single-screw ships. K. E. Schoenherr pointed out 

 some years ago that it was then "possible to 

 design propellers to transmit 50,000 horsepower 

 on one shaft and to obtain propulsion efficiency 

 of as high as 80 per cent on single-screw ships" 

 [SNAME, Phila. Sect., 21 Feb 1947]. 



(d) Interference likely to be caused with internal 

 arrangements in the vessel by the presence of one, 

 two, or more shafts, including their shaft alleys 

 or tunnels 



(e) Efficiency of propulsion. One large propulsion 

 device, if there is room for it and if good inflow is 

 indicated, almost invariably proves more efficient 

 than two or more small ones, reckoning efficiency 

 here on the basis of the least propeller or shaft 

 power for a given weight displacement and length 

 of the ship. 



(f) Reliability and safety aspects of the entire 

 engine-shaft-propulsion-device combination 



(g) A good general rule is to use no more pro- 

 pulsion devices than the special requirements of 



the design demand, and no more than the number 

 for which good working positions can be found. 



69.3 Positions and Limiting Dimensions. The 



matter of how far to keep the thrust-producing 

 areas away from the hull and its boundary layer, 

 or how close to bring them to the hull, is discussed 

 for screw propellers in Sees. 67.23 through 67.25 

 and for paddlewheels and other devices in Chap. 

 71. 



The selection of the particular position of the 

 propulsion device(s) in any ship takes into 

 account: 



(1) The nature of the flow into the possible 

 positions, considering wake fraction, uniformity 

 of inflow velocity, non-axiahty, and absence of 

 excessive turbulence 



(2) The possibility of fitting contra-flow or 

 contra-guide devices adjacent to the propulsion 

 device (s) 



(3) Freedom from entrained air coming along 

 the hull from forward, leakage air coming from 

 the free surface, and eddies in or trailing abaft 

 separation zones 



(4) The tip submergence considered necessary in 

 view of the specified loading and anticipated 

 wavegoing conditions 



(5) Permissible depression of the blade tips 

 below the baseplane. Usually this can not exceed 

 about 4 or 5 ft, the height of the keel blocks in 

 the average drydock.' 



(6) Available propelling-machinery positions 

 within the hull. 



Each design case is considered on its own 

 merits, taking account of all the items listed in 

 this section and in Sec. 69.2. It is difficult to set 

 down rules which apply even in the majority of 

 cases, especially when the relative importance of 

 the several controlling items often is not known 

 in advance. N. J. Brazell has given some good 

 ideas in a paper entitled "The Positioning of 

 Propellers and Shafts" [ASNE, Feb 1948, pp. 

 32-48]. On the motor vessels Brunshausen and 

 Brunsbiittel the single propeller is mounted about 

 9 ft abaft a clear-water stern post, in an effort 

 to improve propulsive efficiency and to reduce 

 vibration [SBSR, 12 Jan 1956, p. 38; MENA, 

 Jan 1956, p. 28; both references embody photo- 

 graphs of the stern]. 



The result of an analysis by H. Dickmann 

 [Ingenieur-Archiv, 1938, p. 452], set down briefly 

 by K. E. Schoenherr [PNA, 1939, Vol. II, p. 145], 

 is that: 



