652 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 71.13 



producing blades are hinged so that their axes 

 are swung into positions generally parallel to 

 the direction of motion. In a 2-bladed folding 

 screw propeller each blade folds aft so that its 

 blade axis is sensibly parallel to the shaft axis, 

 with a frontal area considerably smaller than 

 that of a feathered propeller. Both feathering and 

 folding propellers are illustrated by J. Baader on 

 page 210 of his book "Cruceros y Lanchas 

 Veloces (Cruisers and Fast Launches)" [Buenos 

 Aires, 1951]. 



A 2-bIaded screw propeller with folding blades, 

 for sailing vessels with auxiliary power, arranged 

 to swing aft and form a continuation of the 

 propeller hub, was proposed and illustrated by 

 Henry Claughton in the early 1870's [INA, 1873, 

 pp. 52-55 and PI. IV]. The folding was accom- 

 plished by a rod which slid lenghwise within the 

 hollow propeller shaft. The inventor claimed, 

 and rightly so, that the folding propeller was 

 superior to the feathering one because the blades 

 of the latter always have some objectionable 

 twist, no matter what the angle at which they 

 are feathered. At the time of writing (1955) 

 folding screw propellers are available in diameters 

 of the order of 1 or 2 ft. They are reported to 

 have a drag only 0.1 that of a solid propeller 

 when it is not rotating [Ship and Boat Builder 

 and Naval Architect, London, May 1953, p. 378]. 



An adaptation of the "shadowing" 2-bladed 

 propeller, lying in the eddies behind a wide 

 sternpost, is what may be called the housing 

 propeller. This device, pulled in axially with its 

 shaft and held snugly against the hull, was 

 proposed a half-century ago for large vessels 

 [Hamilton, J., INA, 1903, pp. 233-235 and Pis. 

 XXX, XXXI; De Eusett, E. W., INA, 1903, 

 p. 237]. In the 2-bladed form, and with the feather- 

 ing feature developed in recent years, it is possible 

 to swing the blades nearly fore and aft. They 

 can then, when the propeller and shaft are pulled 

 inward, be drawn into a relatively narrow groove 

 left for this purpose in the end of a skeg sup- 

 porting the shaft. Provided the mechanical 

 problems can be worked out, this affords one 

 solution for a 2-bladed bow propeller on an ice- 

 breaker, mentioned later in Sec. 76.26. Here 

 the propeller is either very useful if it can be run 

 or it is a nuisance if left extended, depending 

 entirely upon the ice conditions. 



71.13 Auxiliary Propulsion for Sailing Yachts. 

 There is only one answer to the question often 

 asked by yachtsmen: Where is a good place to 



put a permanent propeller, to serve as auxiliary 

 propulsion on a sailing craft? There is no good 

 place. In almost any position the propeller is a 

 nuisance for sailing and a misfit for propulsion. 

 To be honest, there is not even a least objection- 

 able place. All locations have disadvantages, from 

 the point of view of both saiUng and propulsive 

 efficiency. Even the temporary outboard pro- 

 peller, hung over the stern or the side, has its 

 drawbacks. 



The usual position and arrangement, embodying 

 a centerline propeller working in an aperture cut 

 partly from the sternpost or the after end of the 

 keel and partly from the rudder, is depicted in 

 diagram 2 of Fig. 71.D. To be sure, the propeller 

 outflow jet impinges on the rudder but a sailing 

 craft has a rudder adequate for controllability at 

 low speeds, regardless of the method of propul- 

 sion. The propeller aperture, small compared to 

 the boat profile, almost necessarily has thick 

 boundaries. If the craft is of wood, the edges of 

 the opening are liable to be very thick compared 

 to the propeller diameter. As much fairing as 



FISH-EYE VIEW 



Schematic Lowut of As-ymmetnc Screw Propeller with 

 Propeller ond All Appendoqes 



on One Side 



Rudder Stock 



^^--Jesioned Woterline 



Rudder 

 Stock 



Moment 

 Tronsmilted 

 Through Here to 



Lower Portion 



Propeller Shoft Line 



Leodinij Edi^e of Rudder is Recessed 

 nto Keel, so that Gap is Smoll 



9 



Deep Keel '- 



Rudder Stock 



..^^Jesiqned Woterline 



Propeller [/* \s 



is S\\c^ht\^/\ Rudder\\ , Hinge Gop i s Smoll for Its 



Offset so \ \\ Entire Lenoth 



that Shaft \^ ^^ 



ond Tube are ^^~^---__3^ 3 



Clear of Rudder Stock ^^ 



Fig. 71. D Auxiliart Propulsion Arrangements 

 FOB Yachts 



