Sec. 66.24 



STEPS IN PRELIMINARY DESIGN 



489 



the basis of present hydrodynamic knowledge. 



The guiding principles in this procedure are 

 based upon an understanding of the water 

 flow around a ship-shaped form, and the effects 

 of this flow. One such principle is based upon the 

 fact that, for the underwater hull of a surface 

 vessel which varies not too widely from the normal, 

 the upper layers of water met by the ship pass 

 around the sides of the entrance while the lower 

 layers pass under the bottom. This feature is 

 illustrated by Figs. 4.0, 22.E, 24.L, 25.G, 66.R, 

 and a number of diagrams in Chap. 52. In the 

 run, the water which has passed around the sides 

 rises rather rapidly toward the surface, so that 

 toward the stern the water flowing over the hull 

 is largely that which has come up from under 

 the bottom. The shortest paths from the bow to 

 the stern are, in general, those which cross the 

 section lines at right angles, indicated by the 

 position of flowlines when projected on the mid- 

 section and shown in a body plan. However, it is 

 not always possible for the water to flow in this 

 fashion under a more-or-less flat free surface and 

 around a hull which must meet requirements 

 other than those of minimum resistance. 



Since neither the section shapes nor the flowline 

 positions for an entirely new hull are known at 

 the outset, this means that both have to be 

 worked in simultaneously, as for the flowlines 

 (streamlines) and the equipotential lines of a 

 flow net, described in Sec. 2.20. 



Compliance with this shortest-path rule, con- 

 formity to the general flow pattern described in 

 the references listed, and consideration of curva- 

 ture changes along the flowlines, calls for V-shaped 

 sections in both the entrance and the run. Con- 

 sideration of pressure resistance due to wave- 

 making, and of the height of the bow-wave crest 

 in particular, calls for vertical-sided entrance 

 sections in way of the bow-wave crest. A bulb bow 

 does not work well into V-shaped bow sections, 

 nor is it easy to fashion a deep forefoot from them, 

 where the bulb must be. These considerations, 

 coupled with the division of flow described in 

 (1) of the following paragraph, indicate that 

 U-shaped bow sections are to be preferred to 

 V-shaped sections, for this ship at least. 



Specifically, a few more detailed rules may be 

 formulated for guidance in shaping the hull of 

 the ABC design, with its B/H ratio of about 2.8 

 and its T„ of about 0.9. These should be part of a 

 comprehensive set for a large range of B/H 

 values, hull shapes, and speed-length quotients 



but making up this set is a formidable task not 

 yet completed. The present additional rules are: 



(1) Along the region of the designed waterline, 

 extending below that line for the order of 0.20 

 to 0.25H at the bow, 0.9H amidships, and O.IOH 

 at the stern, the water flows primarily around the 

 sides. The position of the dividing line at the stem 

 between the side and the bottom water depends 

 also upon the height of the wave crest at the bow 

 and the change of level at the bow when underway. 



(2) To minimize surface wavemaking the changes 

 in longitudinal curvature along the flowlines in 

 this belt, as well as the curvature itself, should 

 be a minimum. This embraces the number of 

 curvature changes between bow and stern; the 

 lowest possible number is two. 



(3) Changes in curvature in the side-water 

 region indicated in (1), at successively deeper 

 depths below the designed waterline, should if 

 practicable be offset longitudinally, and should 

 not occur at any one transverse station. The 

 reason for this is explained in Sec. 4.8 and the 

 accompanying Fig. 4.1 



(4) In the region at the stem below about 0.20 

 to 0.25H, measured downward from the designed 

 waterline, the water flows outward but then 

 swings downward rather rapidly, to pass under 

 the bottom inside the turn of the bilge, below 

 about 0.9 to l.OH 



(5) The twisting of the stream tubes accompany- 

 ing this turning of the flow, depicted in Fig. 4.P, 

 should be accomphshed as easily and as gradually 

 as practicable. Some remarks by D. W. Taylor, 

 made many years ago [SNAME, 1907, p. 11], 

 are still pertinent at this point; comments in 

 parentheses are those of the present author: 



"... this work shows the importance of an easy bilge, 

 tolerably well forward, that is to say, at about one- 

 quarter the length of the ship from the bow. The water 

 is trying hard to get under the bottom, and if you have 

 a shape such that it is difficult to get around the sections, 

 you have a ship that is harder to drive. Some of our 

 analyses of model trials appear to indicate that at about 

 the point where the water wants to go under the ship, 

 you ought not to have (a) full section — not over eighty-five 

 per cent coefficient of fullness (section coefficient) at the 

 outside." 



(6) If the ship has a deep centerline skeg under 

 the stern the stream tubes passing out from under 

 the bottom must twist back again through 

 nearly 90 deg as they approach the skeg ending. 

 Assuming a single propeller carried by the 

 centerline skeg the flow should, somehow or 



