Sec. 67.12 



UNDERWA lER HULL DESIGN 



517 



the broken section lines and the skeg tangent 

 line in the afterbody of Fig. 6G.P. 



To preserve a nearly constant waterline slope 

 at the stern, in the actual wave profile at trial 

 speed, the waterlines just above the 26-ft DWL 

 terminate in a vertical knuckle at the outer lower 

 corner of the transom. With a sharp horizontal 

 knuckle at Sta. 20 it seems preferable to carry 

 the latter knuckle forward until it fades out 

 between Stas. 16 and 17. 



67.10 Variation of Section Coefficient Along 

 the Length. One means of knowing whether the 

 lower corners of the sections ahead of and at the 

 forward quarterpoint are cut away sufficiently is 

 to plot the section coefficients on a base of ship 

 length. It is indicated in Fig. 24.1 that a normal 

 form of this curve passes through the given value 

 of Cx at the maximum-area section, diminishes 

 slowly toward the FP, and rapidly toward the AP. 

 Because of the parallel middlebody worked into 

 some ships it is also necessary to consider the 

 shape of the forward portion of the section- 

 coefficient curve with respect to the entrance 

 length Le rather than the ship length L. 



Many years ago D. W. Taylor emphasized the 

 importance of easy curvature in the section lines 

 at about the forward quarterpoint. He stated 

 then — and it appears to be true by present 

 knowledge — that "at about the point where the 

 water wants to go under the ship, you ought 

 not to have a full section — not over 85 per cent 

 coefficient of fullness at the outside" [SNAME, 

 1907, p. 11]. While a section coefficient of 0.85 

 at the forward quarterpoint (0.25L abaft the FP), 

 represents a good design for easily driven vessels, 

 it can and does rise and fall with the value of Cx ■ 

 It is perhaps better to say that the lowest value 

 of the section coefficient in the entrance should 

 fall within the range of 0.25 to 0.45Lb from the 

 FP. Within this interval of length it should have 

 a value of the order of 0.80 to 0.90 of the Cx value. 

 These ranges are taken from the data of Table 

 67. c, for the position and value of the minimum 

 section coefficient in the entrance of a number of 

 merchant ships and designs, and from unpublished 

 data on a rather wide variety of combatant 

 vessels. 



Fig. 67.1 is a plot of the section coefficient for 

 the ABC ship with both the single-skeg transom 

 stern and the twin-skeg arch stern. The shape of 

 the curve is typical for a vessel of this type. For 

 this ship, with a Cx of 0.956, the minimum value 

 of the section coefficient is 0.873. It occurs at 

 0.165L, or 0.320Lj; , abaft the FP. The ratio of 



£0 18 16 M 12 10 



Stations 



6 ') 2 



Fig. 67.1 Plot of Section Coefficient for ABC 

 Ship with Alternative Sterns 



the section coefficient at that point to Cx is 

 0.873/0.956 = 0.913, which is somewhat higher 

 than it should be. A reworking of the lines at and 

 near the forward quarterpoint, mentioned in 

 Sec. 67.9, would bring down this value. Curves 

 similar to those of Fig. 67.1, for many other 

 vessels of varying sizes and types, may be calcu- 

 lated and plotted from the tabulated B/Bx and 

 A/ Ax data and from the profiles shown on the 

 SNAME Resistance Data sheets. 



For a ship with a bulb bow the section coeffi- 

 cients reach a value of 1.00 somewhere between 

 0.05 and O.IOL. Forward of this point they rise 

 rapidly to an extremely large value, perhaps 10 

 or more, just abaft the FP. 



If the designed waterline in way of the forward 

 quarterpoint is relatively full, with section lines 

 which flare out from below the DWL, the section 

 coefficients can be low even though the curvature 

 at the lower corner is relatively sharp. This is 

 why the section coefficient remains only a partial 

 indication of easy flowlines around and under 

 the bottom. It needs supplementing or replacing 

 by a measure of flowline curvature or the equiv- 

 alent. 



67.11 Hull Shape Along the Bilge Diagonal. 

 It was customary at one time to make use of a 

 bilge-diagonal coefficient Cbd , corresponding to 

 the fullness ratio of the hull in a diagonal plane 

 through (1) the intersection of the DWL plane 

 with the centerplane and (2) the intersection of 

 the floor line at the maximum section and the 

 half-beam line at that section, sketched in Fig. 

 25.H. The bilge diagonal is still used for fairing 

 purposes but it is rarely used as a modern design 

 feature, probably because the water flow almost 

 never follows its trace in the forebody and only 

 rarely in the afterbody. 



67.12 Side Blisters or Bulges. There are 

 few, if any existing design rules, formulated or 

 unformulated, for the hydrodynamic design of 



