Ser. 71.17 



DESIGN OF MISCELLANEOUS PROPULSION DEVICES 



057 



baukalender," 1935 ["Schifl'bautechnisches Iland- 

 buch (Ship Design and Shipbuilding Handbook)," 

 Berlin, 1952, p. 192]. For this series of sizes the 

 ratio of (basket or barrel diameter) /(blade 

 length) has a constant value of 1.67. 



The nominal thrust-producing area of a 

 rotating-blade propeller may, and generally does 

 occupy more area normal to the direction of 

 motion than a screw propeller; sometimes more 

 than twice as much. The thrust-load factor is 

 therefore, like that of the paddlewheel, much 

 less than for a screw propeller to do essentially 

 the same work. Diagram 2 of Fig. 34. M and the 

 graphs of Fig. 34.N show that, below a Ctl 

 value of about 2.2, the efficiency of a Voith- 

 Schneider propeller may be expected to exceed 

 the 0.8-ideal-efEciency value of a screw propeller. 



The thrust-producing area adjoins the hull at 

 its upper or inner end, without the tip clearance 

 associated with a screw propeller. The wake 

 fraction at the propeller position is therefore 

 almost certain to be higher — and more variable 

 as well — than for a screw propeller in the same 

 position. There are no published formulas or 

 systematic data available for a prediction of the 

 wake fraction. 



Similarly, the thrust-deduction fraction is not 

 readily determined from orthodox or routine 

 reference data. The area of the propeller inflow 

 jet is larger, because of the larger equivalent A„ , 



but the —Ap's are usually of smaller intensity. 

 It is probable that, with a stern cut away suffi- 

 ciently to provide easy flow to an under-the- 

 bottom propeller, in a direction generally normal 

 to the blade axes, the thrust-deduction fraction 

 will be lower than for a normal-form stern with a 

 screw propeller. 



An efficient design and installation of a rotating- 

 blade propeller calls for blades that are sufficiently 

 narrow to eliminate interference between them 

 and sufficiently long to provide adequate area 

 for the thrust to be delivered. There must be 

 sufficient submersion of the whole assembly to 

 avoid harmful cavitation in way of the upper 

 ends of the blades. While the presence of the 

 large flat under surface of the hull above the 

 propeller minimizes air leakage to the —Ap 

 regions, it is difficult to prevent detrimental 

 cavitation at high blade loadings. 



The arrangement shown in Fig. 71.G is in 

 general similar to that of the stern of the turbo- 

 electric motorship Helgoland of 1939, at that 

 time "the largest seagoing vessel yet fitted with 

 Voith-Schneider propulsion" [SBSR, 21 Dec 1939, 

 pp. 644-645]. This vessel had a length between 

 perpendiculars of 328 ft, a beam of about 43.5 ft, ' 

 and a speed of 17 kt. The rotating-blade propellers 

 were each designed to absorb a shaft power of 

 2,000 horses, with electric driving motors mounted 

 directly on the propeller casings. 



Fig. 71. G Areangbment of Twin Rotating-Bladb Propellers at the Stern 



