Sec. 71.14 



DESIGN OF MISCELLANEOUS PROPULSION DEVICES 



653 



practicable on the forward and after edges is 

 indicated, to prevent separation behind the 

 forward side of the aperture, to keep the wake 

 velocities from being irregular, and to minimize 

 the augmentation of resistance on the after side 

 of the aperture. A 2-bladed propeller lies generally 

 within the aperture when mechanical propulsion 

 is not required, 'provided there is a blade-position 

 marking inside the vessel, and there is some way 

 to place and hold the shaft in the proper angular 

 position. The aperture detracts from the steering 

 and maneuvering characteristics of the vessel but 

 as the rudder is generally large enough for control 

 at speeds much less than that at which the 

 propeller drives the craft, it remains adequate 

 for the purpose. 



A propeller may be mounted above the top of the 

 rudder with a short exposed drive shaft and a 

 short single-arm strut [Rudder, Mar 1954, p. 37; 

 May 1954, p. 78]. The propeller shaft must be 

 offset slightly from the centerline to clear the 

 rudder stock. 



A somewhat more ship-shape installation, based 

 on both the mechanical and the hydrodynamic 

 features involved, is to drop the upper end of 

 the rudder and leave, in its stead, a fixed portion 

 of the after end of the fin keel under the hull. The 

 propeller shaft passes through this fixed portion 

 and the propeller is mounted abaft it. The 

 arrangement permits a reasonably sharp fin-keel 

 ending ahead of the propeller, with fillets between 

 keel and hull which taper to zero at the extreme 

 after end, about as indicated at 3 in Fig. 71. D 

 [Yachting, May 1951, p. 62; Rudder, Apr 1954, 

 p. 37]. Again, however, the propeller and its 

 shaft must be offset slightly from the centerline, 

 to permit the shaft tube and the rudder-stock 

 casing to clear each other. The propeller bossing 

 in the fixed fin above the top of the rudder then 

 has an offset termination similar to those of cer- 

 tain bossings and multiple skegs in large ships 

 but with the disadvantage that in the smaller 

 craft the flow may cross the swelling for the 

 shaft at a slightly greater angle. 



A yacht in which the mechanical propulsion 

 is of the same order of importance as the sail 

 is that known as the motor sailer. A most useful 

 discussion of the propulsion and design problems 

 of this class is given by D. Phillips-Birt [Rudder, 

 May 1955, pp. 12-15, 46-55]. 



J. Baader devotes a whole chapter to the 

 auxiliary propulsion of the sailing yacht ["Cruceros 

 y Lanchas Veloces", Buenos Aires, 1951, pp. 



249-256 (in Spanish)]. In this chapter he gives 

 three design graphs for the auxiliary powering of 

 these yachts. 



71.14 Vertical Drive for Screw Propellers; 

 Under-the-Bottom Propellers. The combination 

 of a screw propeller mounted on a short horizontal 

 shaft and driven by bevel gears from a vertical 

 shaft is knowri everywhere in the form of the 

 familiar outboard-motor propulsion unit. How- 

 ever, design features which can be accepted for 

 units in which the power in horses rarely exceeds 

 one or two hundred are not always workable in 

 larger units, such as are hkely to be utilized in 

 the future. On all the modern portable installa- 

 tions there is a horizontal barrier or "anti- 

 cavitation" plate to prevent air in the separation 

 zone abaft the casing which carries the vertical 

 drive shaft from working down the after edge of 

 the casing into the propeller disc. This plate must 

 be' considerably larger and must extend farther 

 aft when a more powerful propeller is used. 

 Indeed, the vertical casing sections will them- 

 selves require lengthening and fining. Efficient 

 and reliable water-excluding and lubrication 

 devices and systems are called for if the vertical 

 drives are to transmit powers in thousands of 

 horses instead of hundreds, and if they are to 

 run continually for days on end. 



Besides being retractable by hinging or swing- 

 ing, as in the customary outboard-motor assembly, 

 vertical- drive propeller systems lend themselves 

 to packaging in self-contained units. These may 

 be installed in vertical recesses or wells and 

 removed when desired, similar to the Sea Otter 

 installations of World War II. The propellers at 

 the lower ends of the assemblies may project 

 below the baseplane, or below a flat, cut-up 

 portion of the vessel at the stern, similar to that 

 for the usual type of rotating-blade propeller 

 installation. Large-diameter 2-bladed propellers 

 may be passed through small-diameter wells or 

 small openings by keeping the blades vertical. In 

 the early days of steam it was possible to lift 

 the 2-bladed propeller up on deck through a 

 well in the hull at the stern. A modern installation, 

 proposed for an oceanographic research vessel, 

 is shown in Fig. 33. J. 



A partly sectioned isometric view of a modern 

 outboard-propulsion unit in package form, de- 

 signed to swing the propeller upward by mechan- 

 ical means, is shomi by A. C. Hardy ["Modern 

 Marine Engineering," London, 1955, Vol. II, 

 p. 154]. 



