Sec. 71.12 



DESIGN OF MISCELLANEOUS PROPULSION DEVICES 



651 



which is expected to be immersed when the 

 propeller is operating, indicated by diagram 7 

 of Fig. 15.G. 



The unbalanced blade-disc forces from the 

 immersed and working blades, depicted in dia- 

 gram 1 of Fig. 33. K, are usually balanced by 

 introducing equal and opposite forces from a 

 second propeller rotating in the opposite direction. 

 If these forces are desired or needed to give the 

 craft angular acceleration when turning, and if 

 the turning is always in the same direction, as 

 in a motorboat which always makes left-hand 

 turns during a race, they are balanced by a 

 lateral force produced by a constant rudder 

 angle when straight running is desired. 



Since there are certain to be air holes instead 

 of cavitation pockets on the forward or reduced- 

 pressure sides of the partly immersed blades, the 

 available thrust is low, as is the maximum pro- 

 peller efficiency. Conservative design calls for 

 the use of an efficiency not greater than half or 

 two-thirds that of the same propeller working 

 fully submerged. 



At the average draft and running condition 

 to be expected in service, the hub of a surface 

 propeller should be just clear of or just touching 

 the water on its under side. This may eliminate 

 the necessity of a watertight stuffing box for the 

 shaft. It will certainly free the propeller of friction 

 resistance on the hub surface. 



A cover or guard is almost a necessity over the 

 upper blades of a surface propeller, partly for 

 safety and partly to keep down the showers of 

 spray and geysers of water which would otherwise 

 be thrown up at the stern. 



71.11 Asymmetric Propulsion. It is often 

 convenient to offset the propulsion device from 

 the centerplane of a vessel, especially in auxiliary 

 yachts where an aperture for a centerline pro- 

 peller adds to the sailing resistance and detracts 

 from the efficiency of the steering rudder [Baader, 

 J., "Cruceros y Lanchas Veloces (Cruisers and 

 Fast Launches)," Buenos Aires, 1951, Fig. 203, 

 p. 255 and Fig. 205, p. 256. The situation is similar 

 to that of a crippled multiple-screw ship driven 

 by one wing propeller, with all other propellers 

 missing. 



Moment calculations, supported by service 

 experience with damaged vessels, indicate that 

 if the rudder area is sufficiently large and if the 

 offset of the thrust line from the center of gravity 

 does not exceed 15 per cent of the maximum 

 beam, the asymmetric moment of thrust is only 



of secondary importance in steering and possibly 

 also in turning. For a saiHng yacht, this may 

 require carrying some small compensating rudder 

 angle. As the mechanical propulsion is an auxiliary 

 drive at the best, the helm handicap can be 

 accepted for larger benefits which may be derived 

 when sailing. 



A rather comprehensive article entitled "A 

 Propeller for the Auxiliary," with several illus- 

 trations of asymmetric drives for sailing yachts, 

 is pubhshed by M. E. Williams [Yachting, Feb 

 1955, pp. 54-55, 114]. This article reveals that 

 asymmetric drives and auxiliary drives, the 

 latter discussed in Sec. 71.13, both require the 

 same serious consideration of flow in their 

 vicinity as is given to the propeller position on a 

 much larger vessel. 



71.12 Feathering and Folding Propellers. 

 The proper design of all vessels with two or more 

 means of propulsion, such as the sailing yacht 

 with auxiliary power, requires that the propulsion 

 devices of one system be not a hindrance when 

 those of the other system are being used. The 

 problem of the effect of screw-propeller resistance 

 on the sailing speed of a yacht is discussed by 

 K. S. M. Davidson and D. S. Connelly ["If We 

 Hadn't Been Dragging That Propeller," Yachting, 

 May 1940, p. 68]. Permitting one propulsion 

 device to free-wheel or to coast, if it will, is one 

 answer but not always the best one. 



In the early days of steam as an auxiliary 

 power in saiUng ships this problem was usually 

 solved by fitting a 2-bladed propeller and placing 

 its blade axes vertical when it was stopped. The 

 sternposts on those ships were so wide that the 

 two blades, if not the hub, could lie neatly in the 

 "shadow" of the post, within the eddies of the 

 separation zone directly abaft the post. Other 

 solutions of this problem are to feather the pro- 

 peller blades, as is done on modern aircraft, to 

 fold them, or to house the propeller in some 

 suitable manner. J. Scott Russell mentioned 

 feathering propeller blades nearly a century ago 

 [MSNA, 1865, p. 474]. 



A feathering propeller is defined as one whose 

 thrust-producing blades can be turned on their 

 own axes so that the blade sections are generally 

 parallel to the direction of motion. An under- 

 the-bottom rotating-blade propeller would be 

 feathered, for example, by rotating each blade 

 on its spindle axis so that all the leading edges 

 would face directly forward. 



A folding propeller is one in which the thrust- 



