Sec. 69.13 



GENERAL DESIGN OF PROPULSION DEVICES 



579 



combustion engines can run at the proper rate of 

 rotation to develop their maximum power. It may 

 also be found in service that fouling rates, with 

 consequent increased friction powers, are higher 

 than those contemplated during the design. 



The foregoing advantages, including the very- 

 practical one of replacing a damaged blade, are 

 balanced against the slightly diminished efficiency 

 resulting from the larger hub and from unfairness 

 around the blade attachments. This matter is 

 discussed at length in Sec. 70.43. 



Reversible propellers, in which the blades 

 swivel and the shaft continues to rotate in the 

 same direction, eliminate reverse gears and 

 reversing mechanism in the propelhng plant. This 

 is only done at the expense of mechanical com- 

 pUcation in the shaft and the propeller, increased 

 diameter and bulk of the propeller hub, and 

 certain hydrodynamic disadvantages described in 

 Sec. 32.19. However, if reversibihty of thrust is 

 the primary object, the latter are not too import- 

 ant. The problems are then primarily ones of 

 engineering rather than hydrodynamics. 



So many extraneous problems enter into a 

 decision to use controllable propellers that no 

 attempt is made to give them here. The designer 

 may with benefit study the references Usted in 

 Sec. 32.19 and repeated here for convenience: 



(a) McEntee, W., SNAME, 1927, pp. 87-91, and Pis. 



43-47 



(b) Gutsche, F., Zeit. d. Ver. Deutsch. Ing., 15 Sep 1934, 



p. 1073 



(c) Ackeret, J., Escher-Wyss Bull., May-Jun 1935, p. 63 



(d) Fea, L., Ann. Rep. Rome Model Basin, 1938, Vol. VII, 



pp. 74-89 



(e) Rupp, L. A., "Controllable-Pitch Propellers," 



SNAME, 1948, pp. 272-358 



(f) Burrill, L. C, "Latest Developments in Reversible 



Propellers," IME, 1949, Vol. LXI, pp. 1-11; INA, 

 1949, Vol. 91, pp. J3-J32 



(g) Nichols, H. J., "An Hydraulically Controlled C-P 



Propeller System," Motorship, New York, May 

 1949, pp. 22-23, 44-46 



(h) Baader, J., "Cruceros y Lanchas Veloces (Cruisers 

 and Fast Launches)," Buenos Aires, 1951, p. 209 



(i) Doell, H. A., "What is the Controllable-Pitch Pro- 

 peller?" Mar. Eng'g., Aug 1953, pp. 71-76 



(i) Van Aken, J. A., and Tasseron, K., "Comparison 

 Between the Open- Water Efficiency and Thrust of 

 the Lips-Schelde Controllable-Pitch Propeller and 

 those of Troost-Series Propellers," Int. Shipbldg. 

 Prog., 1955, Vol. 2, No. 5, pp. 30-40. 



A word of caution is included: Do not expect 

 that it will be found worth while to install a con- 

 trollable propeller solely to enable the propeller 

 to run at the proper pitch for any given loading 



condition [Rupp, L. A., SNAME, 1948, p. 273]. 



The problems involved in the detailed design 

 of controllable propellers are so speciaUzed and so 

 complex that no attempt is made to present any 

 of them in this chapter. It may be assumed that 

 organizations which have produced successful 

 service installations are in much better positions 

 to design controllable propellers than is the ship 

 designer. 



The fairing caps of the hubs of many controllable 

 propellers contain essential parts of the blade- 

 shifting mechanism and are as long if not longer 

 than the propeller hub proper. It may be necessary 

 to cut a notch in the leading edge of an all- 

 movable rudder blade to clear the hub cap 

 [Mar. Eng'g., Feb 1953, p. 1; Jan 1955, p. 104]. 



69.12 Propulsion Devices to be Used with 

 Contra-Vanes, Centra-Guide Sterns, and Contra- 

 Rudders. In general, as described in Sees. 33.12, 

 36.8, 36.9, and 37.16, it is immaterial whether 

 rotation is introduced in the inflow jet of a screw 

 propeller and taken out by that propeller or 

 whether the rotation produced by the propeller is 

 taken out by a contra-rudder or equivalent device 

 placed in the outflow jet. However, for a given 

 average resultant water velocity at the blade 

 elements, the angular speed of the propeller is 

 usually sUghtly less if some rotation is imparted to 

 the inflow jet ahead of it. Actually, as far as the 

 design of the propeller is concerned, it may be 

 proceeded with on the basis of no prerotation, 

 following which an adjustment may be made to 

 determine the probable angular speed for a given 

 torque and thrust. 



Good propulsion-device design, and good ship 

 design as well, calls for a thrust-producing device 

 which inherently leaves as little systematic dis- 

 turbance as possible in its wake, and which 

 requires the least amount of predeflection or pre- 

 rotation in the inflow jet. 



69.13 Disadvantages of Unbalanced Propul- 

 sion-Device Torque. On semi-planing and plan- 

 ing craft carrying multiple propellers below the 

 hull in such position that the flow to both the 

 port and starboard sides of each propeller is sub- 

 stantially the same, there are numerous practical 

 reasons for selecting and installing main engines 

 and propellers which rotate in the same direction. 

 However, this arrangement has the disadvantage 

 that, because of the rotation in the propeller 

 outflow jets or races abaft the propellers, there 

 may be a resultant asymmetrical lateral force 

 from the rudder (s). It may be difficult to find 



