742 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 75.4 



contact with the water during normal running. 

 This calls for all underwater butts to be flush. 

 All seams, if lapped and riveted, are to be as 

 nearly as practicable parallel to the lines of flow 

 in their respective regions. Tliis is especially the 

 case in the leading 0.2 or 0.3 of the length and 

 in the after half or two-thirds of the run. If this 

 can not be accomplished the seams in these 

 regions, lying at angles greater than about 30 deg 

 to the flow, should be faired with a suitable filler 

 compound as described earher in this section. 

 In fact, for the leading 0.2 of the length, flush 

 welded plating throughout promises the best 

 possible service performance. 



75.4 The Utilization of Casting or Welding 

 Fillets. The working of generous fillets into 

 the coves or inside corners of castings for hull 

 components is almost mandatory as a matter of 

 good foundry practice. Furthermore, many inside 

 corners occur where thin sections meet heavy 

 sections. Proper gradation in the distribution of 

 material calls for transition regions that are 

 improved by the use of large-radius corners. 

 Four pairs of corners filleted in this manner are 

 shown around the strut hub in Fig. 73. F. 



When large appendages and structural parts 

 which form a portion of the outer hull are made 

 up as weldments, good design precludes the use 

 of masses of welding beads in inside corners. 

 Certainly not enough beads can be added to 

 produce the equivalent of the generous-radius 

 fillets in a large casting. If fillets in weldments 

 are really necessary, for hydrodynamic reasons, 

 they should be worked into the adjacent structural 

 parts, more or less independent of the welding. 

 It may be necessary either to machine the fillets 

 into the weldment or to modify the design so that 

 excessive weld metal need not be deposited. An 

 example of this design is illustrated in Fig. 73. F, 

 where stubs for attaching the strut arms are 

 incorporated in the strut-hub casting. The fillets 

 are cast integral with the stub arms and the hub. 



75.5 Inside Comers Requiring Negligible 

 Fillets. As background information for the 

 design features discussed in this section, the 

 principal characteristics of flow about longitudinal 

 discontinuities are described in Sees. 8.2, 27.8, 

 and 28.2, particularly that encountered around 

 long chines and coves. It is not possible to assess 

 quantitatively the effect of these longitudinal 

 discontinuities by any method yet developed, or to 

 give design rules with numbers. 



When flow takes place past two fixed intersect- 



ing surfaces with a reentrant angle not less than 

 about 80 deg, and the flow is generally parallel 

 to the intersection of these surfaces, no fillet is 

 required, for hydrodynamic reasons at least, along 

 the cove thus formed. The water in this case 

 does its own fairing, explained in Sec. 6.7 and 

 illustrated in diagram B of Fig. 6.D. Common 

 examples of intersections of this type are to be 

 found where shaft struts enter the hull, dia- 

 grammed in Fig. 36. B for a pair of V-struts and 

 in Fig. 73. F for the ABC arch-stern ship. Similar 

 intersections occur where small skegs, horns, and 

 the like project below the main hull. A not-so- 

 comnion example, but one of much greater size, 

 is the long right-angled cove formed where a 

 large deck erection, such as a conning-tower 

 fairwater, rises from the flat superstructure or 

 upper deck of a submarine. The longest and the 

 most common pair of coves are those to be found 

 alongside a single-plate type of roll-resisting keel. 

 Both reentrant angles are of the order of 90 deg, 

 yet the sharp cove at the intersection with the 

 hull appears not to develop excess resistance or 

 to interfere with the keel performance. 



For reentrant angles of less than 80 deg, good 

 design is a matter partly of judgment and partly 

 of circumstances. There is a blocking effect at the 

 inside corner, and this effect increases rapidly as 

 the acute angle becomes smaller. On the one hand, 

 the water can do its own blocking and slowing 

 down. On the other, a fairing can be added to 

 the inside corner, provided the leading and trailing 

 ends of the fairing can themselves be faired. 



In the case of movable appendages attached 

 to or projecting from a hull, such as rudders 

 fitting close under the stern, or diving planes 

 with small hull clearances, it is not practicable, 

 nor is it necessary, to fit fairings. A similar case 

 is that of the retractable sound dome which, 

 when in use, is lowered bodily below the keel of 

 a ship through a hole only slightly larger than 

 the planform of the dome. If no separation or 

 eddying is to be expected abreast or behind the 

 dome, there is no particular need for fairing the 

 90-deg inside corner where the dome meets the 

 hull. Taking account of the variation in trans- 

 lational velocity under the keel, due to the bound- 

 ary layer, does not change the situation or 

 require any means of improving the flow along the 

 inside corners. 



75.6 The Fairing of Appendages in General. 

 Supplementing the foregoing, there are given 

 here a few design notes applicable to the fairing 



