Sec. 11.16 



DESIGN OF MISCELLANEOUS PROPULSION DEVICES 



655 



I Propeller 

 I Shaft is 

 Shown 

 Vertical 

 Here But 

 It Con Be 

 /Placed in 

 /Any Desired 

 Position 

 Generally 

 /Porollel to the 

 enterplane 



Fig. 71. F Schematic Arrangement of Vertical- 

 Drive Screw Propeller for Exerting Transverse 

 Thrust 



entrances are rounded, say to a radius of 0.1 or 

 more of the duct diameter. There should be 

 enough hydrostatic head above the uppermost 

 duct to prevent drawing air from the surface. 



The efficiency of a propeller-type device to 

 produce transverse thrust is measured, not by 

 the usual ratio of output to input but by whether 

 or not the device works. This depends upon the 

 lateral thrust developed. The thrust in turn 

 depends first upon the diameter of the propeller 

 and the rate at which momentum is imparted to 

 the water in a transverse direction. It depends 

 also upon the length and shape of the transverse 

 duct, because of the friction and pumping- 

 pressure losses in the water being forced through 

 it. 



If the two openings of the transverse duct lie 

 in regions where there is much slope to the water- 

 lines, there is liable to be some slight but definite 

 effect of these discontinuities when the vessel is 

 underway. It is generally not possible to close the 

 openings when they are not in use so that when 

 the vessel yaws or turns there may be some 

 transverse flow of water through the duct. 

 This may cause windmilling of the propeller 

 unless it is locked. 



The best positions for propellers producing 

 transverse thrust are in the forefoot and in the 

 aftfoot. These are usually thin, so the ducts 



may be short. They are well removed from the 

 center of gravity of the ship, so the turning 

 moments are large. 



Any large transverse opening, such as for a 

 transverse propeller, cut through the thin skeg 

 of a ship just ahead of an unbalanced rudder at 

 its after end, may or may not be detrimental to 

 turning. The opening permits leakage of +Ap 

 built up by the rudder on the inside of a turn. 

 If the vessel is turning at a large drift angle, 

 there may be cross flow through the duct from 

 the outside to the inside of the turn. Further- 

 more, if the propeller is not locked by non- 

 overhauling drive gears or a brake, it may wind- 

 mill under these conditions. 



71.16 Design Features of Tandem and Contra- 

 Rotating Propellers. Normally, it is not good 

 design to place propulsion devices of any kind 

 in tandem, even when they are as far away as 

 the bow and stern propellers of a double-ended 

 ferryboat. If, under unusual circumstances, it 

 becomes necessary or desirable to absorb a 

 relatively large amount of power by a propulsion- 

 device combination occupying a limited disc or 

 thrust-producing area, it is possible to achieve 

 reasonable efficiency by driving the propulsion 

 devices separately. This enables each to run at a 

 proper advance coefficient J = V a.I(iiiU), de- 

 pending upon the speed of advance at its own 

 position. Still better is a type of drive which 

 automatically adjusts its rate of rotation so 

 that its propulsion device absorbs a pre-deter- 

 mined power. 



For tandem screw propellers, described in 

 Sec. 32.20, a device which has possibilities for 

 certain applications is a sort of contra-propeller 

 interposed between the leading and following 

 units of a pair of tandem propellers [Schmier- 

 schalski, H., WRH, I Sep 1939, pp. 278-279; 

 HSPA, Vol. II, 1940, pp. 79-102, with English 

 summary on pp. 217-218]. This star-shaped set 

 of guide vanes, with or without a fixed shrouding 

 around the outside, is intended to recover the 

 rotational energy in the outflow jet of the leading 

 propeller and to impart enough pre-rotation to 

 the inflow jet of the following propeller so that 

 the rotational energy in the final outflow jet is 

 very nearly zero. The mechanical problems 

 involved in placing a fixed appendage of this 

 kind between two tandem screw propellers on the 

 same shaft have not, so far as known, been 

 tackled except on model scale. 



It is customary, although not necessary, to 



