508 



HVDRODYNy\MIC.S IN SHIP DESIGN 



Sec. 67.5 



67.5 Stem Shape at Various Waterlines. For 

 minimum pressure resistance and reduction in 

 size of the bow-wave crest, and for the elimination 

 of spray and feather, the horizontal sections at 

 the stem of a vessel, just above and just below 

 the surface waterline, are made as sharp as 

 possible. Theoretically, they are formed by con- 

 tinuations of the entrance and bow waterlines, 

 with whatever hoUowness the latter may have, 

 when carried forward to their normal intersections 

 in the plane of symmetry of the vessel. 



The hull structure of a wooden or metal vessel 

 inside the extremely thin, tapering stem dictated 

 by these hydrodynamic considerations is difficult 

 and expensive to fabricate. A simple solution is 

 to cut the structure back and terminate the stem 

 in a blunt or large-radius section, sometimes with a 

 radius of 1 ft or more, producing the circular-arc 

 beginnings depicted in Fig. 25. A. The resulting 

 bow feather is then accepted. 



A circular-arc leading edge, because of the 

 sharp change in curvature at the point where it 

 joins the nearly straight side of the entrance, is 

 susceptible to both separation and cavitation. 

 On a high-speed vessel, therefore, the planforms 

 at various levels of the stem, especially near the 

 surface waterline, are made elliptical, with a 

 gradual change in curvature and an easy tran- 

 sition into the side. 



Blunt stems build up relatively high dynamic 

 resistance below the waterline, which adds 

 directly to the pressure resistance of the ship. 

 The blunt nose of a submerged bow bulb, and the 

 bluntness in the horizontal sections of the stem 

 for some distance above it, are accepted for the 

 sake of the benefit they afford in other respects. 

 They also enable the ship length to be kept to a 

 minimum. 



Both the hydrodynamic and the structural 

 problems described in the foregoing, at least on a 

 metal vessel, are solved by adding an appendage 

 in the form of a sharp, narrow cutwater. That 

 designed for the ABC ship is described in Sec. 

 73.3 and illustrated in Fig. 73.B. 



With modern fabrication and erection methods 

 there is no excuse for lapping the shell plating 

 on the outside of the stem to form a discontinuity, 

 diagrammed at D in Fig. 7.J. Indeed, there is a 

 definite disadvantage to this construction on a 

 high-speed vessel because of the cavitation that 

 takes place abaft the discontinuity in the regions 

 of low hydrostatic pressure, near the surface. 

 This may be accompanied by possible erosion of 



the shell plating. It is certain to tear or pound off 

 the paint coating. Harmful cavitation, setting up 

 erosion, causing noise, and inducing vibration 

 and panting of the plating has in fact occurred 

 abaft projecting welding beads on high-speed 

 vessels where flush shell plates were attached to 

 a rabbeted stem casting. Any measurable stem 

 radius, fair or not, is liable to cause separation at 

 the surface and cavitation farther down, if 

 pushed to speeds of the order of 40 or 50 kt. 



67.6 Design of a Bulb Bow. The purpose of 

 the bulb bow, explained in Sec. 25.3, is to reduce 

 the height of the bow-wave crest and the magni- 

 tude of the pressure resistance caused by it. 

 This does not mean that the blunt surface water- 

 lines producing the high crest are to be retained 

 just because there is to be a bulb to cut down the 

 high waves caused by them. By moving some 

 of the displacement volume from the region of 

 the surface to well below it, into the bulb, it is 

 possible to fine the surface waterlines. Both the 

 finer lines and the presence of the bulb act to 

 improve the hydrodynamic performance of the 

 ship. It may be said, therefore, that in general 

 the design of a proper bulb bow involves also a 

 definite fining of the surface and near-surface 

 regions in the entrance. 



When a normal form of bow is converted to a 

 bulb form, good design procedure based on hydro- 

 dynamics requires that the displacement volume 

 in the bulb be removed from a surface-waterline 

 region immediately abaft the stem, say from the 

 FP back to about 0.15 or 0.20 of the waterline 

 length. This reduces the angle of entrance and 

 the amount that the surface water is pushed 

 sideward in the vicinity of the first bow-wave 

 crest. Normally it need not and should not be 

 removed from the surface and near-surface 

 waterlines in the vicinity of the forward shoulder. 



When a relatively large bulb is fitted, displace- 

 ment volume is also removed from the lower 

 outer corners of the sections at and ahead of the 

 forward quarter point, for a region extending 

 from about 0.15L to 0.40 or 0.45L, depending 

 upon the shape of the original hull and the 

 amount of volume to be shifted. 



The manner in which both these changes are 

 made is well described and illustrated by E. S. 

 Dillon and E. V. Lewis in Figs. 7 through 11 of 

 their paper "Ships with Bulbous Bows in Smooth 

 Water and in Waves" [SNAME, 1955, pp. 

 726-766]. 



The following is quoted from page 731 of the 



