654 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 71.15 



Flow to under-the-bottom propellers, working 

 below vertical wells which house retractable or 

 removable units, is practically axial but it may 

 suffer from non-uniformity because of the pres- 

 ence of the boundary layer. If the propellers 

 are actually below the baseplane, and in a region 

 where the ship bottom is sensibly flat, parallel to 

 the direction of motion, there should be little 

 augment of resistance due to the pressure fields 

 created by the propellers. Although thrust- 

 deduction forces were observed on the Sea Otter 

 model, this is believed due to the fact that the 

 closures at the bottoms of the vertical wells in 

 the models were not watertight. The — Ap and 

 -f-Ap fields set up by each propeller extended up 

 into the lower portion of each well, where small 

 forces acting aft, opposite to the direction of 

 motion, were developed on both the forward 

 and after walls of the well. 



71.15 Design of Devices to Produce Trans- 

 verse and Vertical Thrust. For docking, mooring, 

 and shifting berth, in areas where the port 

 facilities are not adequate, it frequently becomes 

 necessary for a ship to sidle or to change its 

 heading. This usually happens when it is not 

 possible to shift an appreciable distance either 

 ahead or astern, certainly not far enough to 

 create the offset or make the change in heading 

 by the ordinary operations of maneuvering. This 

 offset or change in heading requires the applica- 

 tion of a more-or-less static force at one or both 

 ends of the vessel, in a direction approximately 

 perpendicular to its centerplane and in line with 

 the shift in position desired. 



Normally these forces are applied by tugs 

 pushing and pulling or by ropes between fixed 

 objects and the ship, connected to capstans or 

 winches. When neither the tugs nor the fixed 

 objects are available, and when the offsets and 

 changes of heading are required to be performed 

 frequently as a routine part of a vessel's operation, 

 this may be accomplished by fitting separate 

 auxiliary propellers which apply thrust in a 

 transverse direction [SBSR, 20 Nov 1952, pp. 

 659-660]. 



The simplest installation of this kind is the 

 straight-through type illustrated schematically in 

 Fig. 7 I.E. A substantial shroud ring encircles the 

 propeller to carry the gear teeth. The increase 

 in diameter — and area — at the propeller position 

 enables a larger and more efficient wheel to be 

 used, and partly compensates for the area 

 occupied by the shaft and its bearings, or by a 



bevel-gear mechanism if a different type of drive 

 is employed. The problem here is keeping the 

 gears small enough so that the area occupied by 

 the gear casing is not excessive. A straight- 

 through installation of this general type is found 

 in the Canadian ferry Princess of Vancouver, 

 except that the water is moved transversely 

 through a duct of rectangular section by a 

 Voith-Schneider propeller [111. London News, 19 

 Mar 1955, p. 516; MENA, Mar 1955, p. 112]. 



If sufficient beam is available at the position 

 selected for the transverse propeller or thrust- 

 producing device it may be advisable to use a 

 vertical-shaft propeller in a Z-shaped passage 

 having offset openings to port and to starboard. 

 Fig. 71.F is a schematic arrangement for such an 

 installation. It removes from the water passage 

 the bevel-gear box which would be required for 

 a straight-through transverse duct, with a drive 

 shaft entering at right angles to its axis. The 

 two sets of corner vanes are placed in the two 

 elliptical intersections of the circular ducts, where 

 the increased area compensates for the flow 

 restrictions imposed by the multiple vanes. 



The propeller shaft, shown vertical in the 

 figure, may lead in any desired direction to the 

 drive motor, permitting the latter to be placed 

 in the most convenient and protected position 

 in the ship. The Z-shaped duct remains in the 

 transverse plane through the drive shaft but 

 this plane may be either vertical or horizontal 

 or may lie at any convenient angle. 



To prevent separation of the inflowing water 

 at points such as E, and Ea in Fig. 71. F the 



Alternative Bevel-Gear Drive 



Not Shown Here Are 

 Method of Attaohin* 

 Means of Preventino 



Thrust Beorinas, Stuffing Boxes, and the like 



Fig. 71. E Schematic Arrangement of .Auxiliary 

 Propeller for Exerting Transverse Thrust 



