666 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 72.8 



which acts to increase the friction resistance, 

 especiall}'- with a high backfiow velocity. However, 

 the gain from a small square-draft to water-depth 

 ratio yTAxlh is likely to be greater than the 

 loss from the increased wetted area. A ratio of 

 ■\/X^//i less than 1.05, as shown by the curve 

 of Fig. 61. G, produces a speed loss from aug- 

 mented potential flow of less than 10 per cent, 

 while a ratio below 0.375 is responsible for a 

 speed loss of only 1 per cent. 



If the length is limited, increase the beam but 

 hold a small draft. The additional bed clearance 

 provided for normal flow under the bottom 

 should more than compensate for the greater 

 waterUne slopes associated with the wide beam. 



It is comforting to know that, in general, a 

 form of hull suitable for confined waters is found 

 to give good performance in water of any extent 

 and depth, especially for low values of Cp and 

 of fatness ratio F/(0.10L)l Thus a vessel de- 

 signed to do well in the shallow portion of a 

 route of varying depth is by no means at a dis- 

 advantage when operating in the deep portion 

 of the route. 



72.8 Modifications to Normal Forms for 

 Shallow-Water Operation. For the vessel which 

 runs mostly in deep water but also has to per- 

 form well in shallow and confined waters the 

 concessions to the shallow-water requirements 

 depend upon the relative importance of ship 

 performance in one and in the other. A good 

 example of this situation is the ABC ship, for 

 which the design requirements are set forth in 

 Chap. 64. 



Considering the features of this vessel as 

 regards its operation in the canal leading from 

 Port Amalo to the sea, and in the river below 

 Port Correo, it is noted from Table 72. a that for 

 the 28-ft depth of the former the speed of the 

 wave of translation is 17.77 kt. For the 30-ft 

 depth of the latter the critical speed is 18.40 kt. 

 Both are well over the speeds contemplated in 

 those portions of the route, hence the ship will 

 be running in the subcritical range in both cases. 



The matter of providing room for the backflow 

 under and around the ship is handled in Sec. 

 66.13, when laying out the contour of the maxi- 

 mum-area section. 



A reserve of power is almost always available 

 to overcome the augmented resistance in confined 

 waters, because the deep-water speed at any draft 

 and trim is in excess of that permitted by local 

 regulations when traversing confined-water areas. 



For the 10-kt ABC s hip speed in the Port 

 Amalo canal, T, is 10/\/510 = 0.443; at a draft 

 of 26 ft, the value of depth /i/draft E = 28/ 

 26 = 1.08. From Fig. 58.E, using the T-2 tanker 

 as a basis, the estimated sinkage is 0.0043L at 

 the bow and 0.004L at the stern; or 2.19 ft and 

 2.04 ft, respectively. It is necessary to extra- 

 polate the graphs to obtain these values. 



For the 14-kt speed (through the wate r) in the 

 river below Port Correo, T, is 14/^/510 = 0.62; 

 at a draft of 26.5 ft in the fresh water, the value 

 of hlU = 30/26.5 = 1.13. The estimated sinkage 

 at the bow is 0.0068L or 3.47 ft; at the stern it is 

 0.0062L, or 3.16 ft. A much greater extrapolation 

 is required here, in Fig. 58. E, than for the Port 

 Amalo canal estimate. 



The Cp value of the T-S tanker is 0.74 compared 

 to 0.62 for the ABC ship, and the fatness ratio 

 is 5.76 compared to 4.327. Despite the greater 

 beam of the ABC ship, it appears that the 

 estimated sinkages could all be reduced to about 

 0.8 of the values given. Even so, the nominal 

 2-ft bed clearance in the Port Amalo canal is 

 reduced to only 2.0 — 1.75 = 0.25 ft; in the river 

 below Port Correo it is only 3.5 - 2.78 = 0.72 ft. 

 These are small but probably representative of 

 modern medium-speed operations in shallow- 

 water areas. 



72.9 The Adaptation of Straight-Element De- 

 sign to Shallow-Water Vessels. A vessel re- 

 quired to operate in confined waters, with restric- 

 tions to flow imposed by channel bed and bound- 

 aries, should logically receive more than the usual 

 amount of careful hull shaping. It needs every- 

 thing that can be done to improve the flow around 

 the hull. Nevertheless, the bed and side clearances 

 for most of these vessels approach zero in some 

 part of their operating areas; often in many parts 

 of those areas. 



The practical impossibility of shaping the hull 

 to compensate for more than a fraction of these 

 handicaps makes it good design, as well as good 

 engineering, to take this opportunity of incor- 

 porating straight-element features in the under- 

 water form. A judicious use of chines, coves, and 

 developable surfaces affords a surprising degree 

 of flexibility to the designer, as is evidenced by 

 the Hillman shallow-water pushboat. A body 

 plan of a craft of this type, 115 ft long by 27 ft 

 beam, is reproduced in Fig. 72. E. Its outboard 

 profile, less numerous details, is drawn in Fig. 

 72. F. While this vessel was designed primarily 

 for pushing, the general shape should serve well 



