536 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.23 



Generally no part of the median line is straight 

 until it merges into the centerplane or the skeg 

 construction plane. It may have a parabolic or 

 other suitable shape. If it is expected that the 

 ship will, in some service conditions, run with a 

 portion of the contra-guide skeg ending exposed 

 to the air, the terminal median-line slope at the 

 free surface should not exceed about 3 deg. In 

 any case, it is wise to work gentle slopes into any 

 twisted skeg ending, consistent with achieving 

 the desired prerotation of the inflow jet. If the 

 owner and builder are to go to the trouble and 

 expense of twisting such a skeg ending, the 

 twisting can at least be done properly. 



For twin-skeg endings, assuming outward- 

 turning propellers, the twisting involves deflection 

 of the lower portions of the skegs in an outward 

 direction. This is inadvisable if it produces 

 markedly expanding tunnel sides, specifically 

 cautioned against in Sec. 67.21. On the other 

 hand, one way to increase the efficiency of twin- 

 skeg propulsion is to slow down the water m the 

 lower after portion of the tunnel. Good design 

 therefore indicates as much expansion in tunnel 

 area in this region as is thought to be consistent 

 with regular flow, to be confirmed by thorough 

 tests in a circulating-water channel with the 

 propellers driving. Any adverse or detrimental 

 flow conditions pertaining to the twisted skeg 

 endings will certainly show up when the flow 

 pattern in this region is determined. This is 

 specially recommended if the skeg endings have 

 full lines, with large waterUne slopes, and if 

 contra-guide features are incorporated in them. 



Indeed, a necessary step in the design of any 

 contra-guide skeg ending, as it is for a deflection- 

 type bossing and a contra-rudder, is a flow test 

 in a circulating-water channel, using tufts, dye, 

 or the equivalent, to check freedom from separa- 

 tion, irregular or cross flow, and any other 

 questionable features. 



Design rules for deflection- type bossings, usually 

 more nearly horizontal than vertical, are given 

 in Sec. 73.10. 



Some notes applying to the incorporation of 

 contra-guide features and contra-rudders in 

 auxiliary saiUng yachts and propeller-driven small 

 boats are presented by F. A. Fenger [Rudder, 

 Jan 1954, pp. 76-79]. 



67.23 Shaping the Hull Adjacent to Propul- 

 sion-Device Positions; Hull, Skeg, and Bossing 

 Endings. One guiding principle in shaping a 

 hull form to produce efficient operation of a 



selected propulsion device is that the inflow jet 

 contracts in lateral dimensions and area as it 

 approaches the device, normal to the flow direc- 

 tion. Further, this contraction continues in the 

 outflow jet for an appreciable distance down- 

 stream from the device. These axial distances 

 are of the order of at least one diameter in the 

 case of a screw propeUer; of the blade length, 

 measured transversely, in the case of a paddle- 

 wheel; and of the "basket" diameter in the case 

 of a rotating-blade propeller. The contraction 

 ratio, as pointed out in Sec. 16.3, is a function of 

 the thrust-load factor Ctl . Since the thrust, the 

 propeUer-disc area, and the speed of advance are 

 known reasonably well at this stage of the design 

 the outhnes of the inflow and outflow jets for 

 open-water operation can be visualized by 

 reference to Fig. 59. G. 



A second guiding principle is that the hull 

 should produce, in the region selected for the 

 propulsion device, a flow of water which results 

 in the most efficient and most uniform loading of 

 the blades. For example, the swept volume of a 

 rotating-blade propeller and a paddlewheel in- 

 cludes the whole thickness of the boundary layer 

 next to the hull, as portrayed in diagrams 1 and 2 

 of Fig. 11. C, plus a region of potential flow outside 

 the blades. This is not particularly objectionable, 

 however, where the mechanism is able to take it. 

 Furthermore, the sum of the overloading forces 

 for all the immersed blades, as created within the 

 boundary layer, remains nearly constant through- 

 out each revolution of the device. 



A third principle, really a corollary of the 

 second, is that unavoidable local loading of the 

 blades, one at a time, with the resulting unequal 

 loading of the whole device, is to be reduced to a 

 minimum. This occurs particularly when the tip 

 or the outer portion of a single screw-propeller 

 blade passes through the region of high wake 

 velocity in a ship boundary layer or behind a 

 zone of separation. 



The commendable progress of the last two 

 decades, in which screw-propeller blade shapes 

 have been brought into close conformity with the 

 drawings, is at the time of writing (1955) be- 

 ginning to be matched by general refinements in 

 sternpost, skeg, and bossing terminations at the 

 forward edges of propeller apertures. The fact 

 that individual propeller shaft struts had to be 

 as thin as possible made it relatively easy to 

 specify and to obtain sharp terminations at their 

 traihng edges. Since the advent of cast-steel 



