Sec. 76.9 



DESIGN OF SPECIAL-PURPOSE CRAFT 



769 



should this angle, measured on the water side, 

 be less than 90 deg. If the reentrant angle of a 

 cove is much larger than 120 deg and approaches 

 180 deg, the position and direction of the cove, 

 with reference to the adjacent streamlines, is 

 not too important. 



(f) The discontinuities should fade out at the 

 ends and merge gently into the fair form of the hull 



(g) Shell openings on ducts or pipes leading into 

 the hull proper are best kept clear from the region 

 close underneath the offsets in discontinuous 

 sections. This is particularly true where both 

 sides of the cove slope upward, forming a possible 

 trap for air. Such a cove in the entrance may 

 easily collect air coming down from the region 

 of the bow-wave crest. The length of the dis- 

 continuity may be so great that this air is not 

 readily carried along and released in the run, 

 where the cove slopes upward. 



Although they represent longitudinal discon- 

 tinuities of relatively small transverse section, 

 bulged fender sirakes may be classed with small 

 bulges and blisters. Discussing structural matters 

 for a moment, the combination of simplicity of 

 construction, ease of upkeep, and greater efficiency 

 dictates the use of heavy, single-thickness fender 

 strakes, conforming to the adjacent hull shape or 

 bulge, instead of built-up external fenders. 

 They may lie below the surface waterline on 

 submarines, as at 3 in Fig. 36. N and at 1 in Fig. 

 73. Q, or at or near the waterline on tugs, sketched 

 in Fig. 68. J. Diagrams 4 and 5 of Fig. 76. F illus- 

 trate several types of bulged fender strakes, not 

 including one of circular-arc section. These may 

 be worked in as part of the hull plating, either 

 above or below water, and in a fiat or curved side. 



Bhstered fender strakes should involve not- 

 too-small reentrant angles where they join the 

 main hull plating. As far as practicable they 

 should follow the actual lines of flow as modified 

 by surface waves, especially at that speed for a 

 given ship at which low resistance is considered 

 important. If bulged fender strakes must of 

 necessity be placed at the waterline the dis- 

 turbance to surface wave action must be accepted. 



It should be possible, when blister sections are 

 sketched in, to lay down suitable cove traces which 

 will, when the blister is in place, lie very nearly 

 along the resultant lines of flow. This is partic- 

 ularly true if the blister, as it should, fairs reason- 

 ably well into the original hull and does not make 

 an awkward bump upon it. 



Fig. 76.F Design Details for Discontinuous 

 Sections 



When designing a blister to be added to an 

 existing ship, for which the Hues of flow are 

 available, these may be used for guidance in 

 laying out the coves, chines, knuckles, and other 

 longitudinal discontinuities. It must be expected, 

 however, that the addition of a blister of relatively 

 large volume may change the flow pattern around 

 the combination. A check test is called for at an 

 early design stage to insure that the flow around 

 the ship-and-blister assembly is satisfactory. 



A special case arises when external bhsters are 

 added locally — and usually temporarily — in the 

 form of boxes, pontoons, and the like to give 

 added buoyancy, or transverse metacentric sta- 

 bihty, or both, to a hull which must be floated 

 through a region of exceptionally shallow water, 

 or which must be held upright when lifted to a 

 draft corresponding to that acceptable for the 

 shallow-water area [MESR, Jun 1952, p. 53; 

 Oct 1954, p. 58]. 



It is sometimes necessary to tow a vessel thus 



