506 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.3 



Fig. 67.B After Portions of Afterbody Waterlines for the Transom-Stern ABC Ship 



considered that, by reason of the wide waterplane 

 aft, in way of the transom, shown in the lower 

 diagram of Fig. 67.A, enough square moment of 

 area would be gained there to more than compen- 

 sate for the rather drastic narrowing of the 

 whole hull in the entrance. A check of the designed- 

 waterline It value, which shoidd have been made 

 before, actually was made after the lines had been 

 sent to the model basin. It revealed a. C,t oi only 

 sUghtly over 0.52 whereas the preferred value 

 was 0.561 and the required value, from item (42) 

 of Table 64.f, was 0.55. A proposal to remedy 

 this situation is discussed in Sec. 78.18. 



67.3 Waterline Curvature Plots. A designed 

 watei'line is to be checked for uniformity of cur- 

 vatiu'e before it is considered acceptable and 

 before it is used as a basis for shaping the re- 

 mainder of the hull. Sees. 49.10 through 49.14 

 describe approximate and precise methods, both 

 graphical and mathematical, of accomplishing this 

 operation. 



The 0-diml curvature plots for the designed 

 waterlines of the ABC transom-stern, single-skeg 

 ship, as well as for the alternative arch-type stern 

 described in Sec. 67.16, are given in the lower 

 diagram of Fig. 67. C. The upper diagram gives 

 corresponding plots for the Taylor Standard 

 Series parent form, EMB model 632 (modified), 

 and for a merchant ship of good performance. All 

 three indicate certain correct features, so far 

 as they are known, for a good designed-waterline 



(a) Not-too-violent changes in curvature with 

 ^---distance along the length. As an indication of 



how suddenly this 0-diml curvature can change, 

 a straight line tangent to a circle of diameter 

 Bwx involves a sudden change of 0-diml curvature 

 of from to 114.6. 



(b) No great extremes of concave or convex 

 curvature at any point along the length 



(c) A curvature plot with the minimum of longi- 

 tudinal waviness. Generally such waviness indi- 

 cates poor fairing or inaccurate dra\ving of the 

 waterline. 



(d) A curvature plot with rather long portions 

 of constant or nearly constant curvature, as for 

 the TSS waterline, provided there are no abrupt 

 changes at the ends of these portions. 



67.4 Underwater Hull Profile. The bow pro- 

 file under water is determined from the desired 

 section and waterline shapes at the bow, extending 

 from the baseline up to the designed waterline, 

 rather than from an effort to achieve a particular 

 profile that supposedly has merit. In other words, 

 the bow profile is determined in a sort of auto- 

 matic fashion, just as if one whittled a wooden 

 model to the desired section shapes forward and 

 then cut away the model on each side until the 

 waterlines met the centerplane. Developed in 

 this way a ship bow with wall-sided sections all 

 the way to the stem terminates in a plumb stem. 

 One with pure triangular V-sections terminates 

 in a straight raked profile passing through the 

 lower vertexes of these sections. 



For many ships which operate in shallow 

 waters, temporary grounding forward is a not 

 unusual occurrence. It may be advisable on these 

 craft to cut up the forefoot and thus remove 



