fifiG 



HYDRODYNAMICS IN SHIP DESIGN 



Ser. 71.17 



place the discs of contra-rotating propellers 

 close to each other, spacing them along the shaft 

 axis at only a small fraction of the propeller 

 diameter, just sufficient for the hubs and the 

 blades to clear each other. The following pro- 

 peller is made slightly smaller in diameter than 

 the leading propeller because of the contraction 

 in the outflow jet from the latter. 



If both propellers of a pair rotate oppositely 

 at the same rate, the pitch of the after wheel is 

 made slightly greater to produce generally the 

 same thrust, torque, and power as the forward 

 one. There is no reason, however, why both 

 propellers need rotate at the same rate if other 

 rates or ratios are preferable. 



For contra-rotating propellers such as those 

 mounted on the afterbody of a torpedo, where 

 the flow is definitely converging toward the 

 shaft axis as it approaches the leading propeller, 

 both propellers of the group can be raked aft 

 to advantage, provided centrifugal-force and 

 other factors are properly taken care of. 



The design of contra-rotating propellers poses 

 a problem discussed in Sec. 67.22 for the contra- 

 guide skeg ending. It is to find the exact direction 

 of the flow in the outflow jet from the leading 

 propeller as it meets the leading edges of the 

 blades of the following propeller. This is a matter 

 partly of determining the direction at which the 

 flow leaves the trailing edges of the forward 

 wheel and partly of the amount of induced 

 velocity imparted to it by that wheel, at a dis- 

 tance astern represented by the forward sweep 

 line of the after wheel. 



There are two published design procedures 

 available: 



(1) Lerbs, H. W., "Contra-Rotating Optimum Propellers 



Operating in a Radially Non-Uniform Wake," 

 TMB Rep. 941, May 1955. On page 22 there is a 

 list of 6 references. In general, this method takes 

 account of the effects arising from the difference of 

 the wakes at the propeller discs and from the 

 contraction of the race between them. Tlie design 

 procedure is outlined by steps but no example is 

 given. 



(2) Van Manen, J. D., and Sentid, A., "Contra-Rotating 



Propellers," INA, Apr 1956; SBSR, 3 May 1956, 

 pp. 302-303; SBMEB, Jul 1956, pp. 462-463; Int. 

 Shipbldg. Prog., Sep 1956, Vol. 3, No. 25, pp. 

 459-473. Eight references are given and a numerical 

 example is included. 



H. W. Lerbs observes that contra-rotating 

 pairs of propellers are efficient only for large 

 P/D ratios, say more than 1.2; that is, when the 



tangential components of the induced velocities 

 are large. Normally, large P/D ratios accompany 

 small thrust-load factors Ctl ■ 



71.17 Design Notes Relative to Rotating- 

 Blade Propellers. The functioning of the rotat- 

 ing-blade propeller is described and illustrated in 

 Sec. 15.13 and Figs. 15.1, 15.J, and 15.K. Its use 

 for steering and maneuvering as well as propul- 

 sion is discussed in Sec. 37.22 and illustrated by 

 the diagrams of Figs. 33.H, 37.0, and 37.P. 

 References to test results on propellers of this 

 type are given in Sec. 59.7. Additional references 

 in the technical literature are: 



(1) Mueller, H., "Schiffsmodellversuche im Stromungs- 



gerinne (Ship Model Tests in a Flo wing- Water 

 Channel)," Schiffbau, Sohiffahrt, und Hafenbau, 

 1936, Vol. 37, pp. 168-173, 206 



(2) Mueller, H., "Einfluss des Hohlsogs auf das Arbeiten 



des Voith-Schneider-Propellers (Influence of Cavi- 

 tation on the Working of the Voith-Schneider 

 Propeller)," Zeit. des Ver. Deutsch. Ing., 1938, 

 Vol. 82, pp. 566-568 



(3) Fuller, W. E., "A Radical Departure in the Conven- 



tional Tugboat Design, and a New Use for Cycloidal 

 Propulsion," ASNE, Aug 1953, pp. 639-645 



(4) "German Craft with Voith-Schneider Propellers," 



SBSR, 23 Sep 1954, pp. 409-413 



(5) "Voith-Schneider Propulsion;" booklet of 23 pages 



prepared and published by the J. M. Voith Com- 

 pany of Heidenheim, Germany (in English). Copy 

 in TMB Ubrary. 



It is desired again to emphasize that this type 

 of propulsion device can be employed to produce 

 thrust in any direction within any selected plane 

 of rotation, whether horizontal, vertical, or 

 inclined. Diagram 2 of Fig. 33. H illustrates 

 schematically a rotating-blade propeller fitted to 

 a submarine with its axis horizontal, or nearly so, 

 and its plane of rotation generally vertical and 

 parallel to the plane of symmetry. 



A suitable thrust-load factor Ctl for a 

 rotating-blade propeller, expressed by Ctl = 

 T/{0.5pAoVa), is based upon an area equivalent 

 to Ac which is rectangular in shape. It represents 

 the maximum transverse section through what is 

 known as the basket or barrel formed by the 

 blades, having a height equal to the blade length 

 and a width equal to the diameter of the pitch 

 circle. This corresponds to the hatched area of 

 diagram 6 of Fig. 15. G. In practice, the propor- 

 tions of this rectangle remain sensibly constant, 

 having a ratio of pitch-circle diameter to blade 

 length of about 1.5 to 1.75. W. Henschke gives a 

 sketch and a table of principal dimensions of 

 Voith-Schneider propellers, taken from "Schiff- 



