Sen. 76.r> 



DESIGN OF SPECIAL-PURPOSE CRAFT 



763 



amateur or inexperienced builder, to the lack of 

 equipment in small plants, or to the necessity 

 for rapid and economical fabrication and erection. 



It is often convenient to replace the sharp 

 corners of polygonal sections with rounded corners 

 but in this case the corners lying along any diag- 

 onal must generally be of fixed radius if rapid 

 production methods are employed. 



The principal design requirement for straight- 

 element hulls is that the chines (and coves) shall 

 lie in the adjacent lines of flow in such a manner 

 that water does not cross these external dis- 

 continuities. 



The principles set forth in Chaps. 4, 27, and 

 52, concerning the manner in which water flows 

 around a ship, are employed to estimate the 

 direction which the water may be expected to 

 take. It is well to remember that, if the beam- 

 draft ratio is large, most of the flow goes under the 

 bottom. 



In the present state of the art it is not always 

 easy to predict the flow around hulls of unusual 

 shape. Fortunately, it is possible, even with small 

 models, to check the flow directions experiment- 

 ally on a straight-element form, so that the 

 designer can obtain an idea of the flow pattern 

 while the hull form is still sufficiently "plastic" 

 to be changed readily. 



For the best performance a cove line is not 

 placed where the water flowing around the ship 

 is required to cross it. Fortunately, this is con- 

 siderably less important than for a similar crossing 

 of a projecting chine, because of the sensibly 

 inert water carried along in the cove and the 

 usually shallow depth of the reentrant portion. 



If a chine extends above the designed waterline 

 in the entrance, it is generally necessary that the 

 slope of the chine, when projected on the center- 

 plane with the craft at rest and reckoned with 

 respect to the at-rest WL, be reasonably large, 

 to insure upward dynamic lift when pitching 

 and encountering waves and to throw spray clear 

 of the hull. A suitable range of angles of slope 

 for exposed forward chines of planing craft, 

 apphcable to all straight-element forms, has not 

 as yet been laid down. However, a series of chine 

 shapes and positions, based partly on the intended 

 service and partly upon the maximum T^ and 

 F„ at which the craft is to run, is shown in Fig. 

 77.1. 



The minimum number of chines is one at each 

 diagonal bulge, as in the hull of the well-known 

 flat-bottomed skiff, punt, or dory. In this case 



the chine angle approaches 90 deg but as the 

 relative speeds are low the chine line can depart 

 from the hues of flow to facilitate construction. 



For a sailing yacht which is almost never 

 upright when moving through the water, the 

 shape of the underwater hull varies with the angle 

 of heel, as do the lines of flow for that particular 

 heel. The flow may also vary for the speed or 

 range of speed corresponding to that heel, 

 because of the considerable wavemaking. Never- 

 theless, it is to be expected that, by simple flow 

 tests or other means, a chine line may be found 

 which fits all these flow positions rather well. 



The maximum number of chines (or coves) 

 which can be worked longitudinally into a hull is 

 Umited only by the number of strakes desired to 

 meet construction requirements. As many as five 

 chines on each side have been employed success- 

 fully in some hulls, illustrated by the body plans 

 of Figs. 27. B and 68. J. As the projecting corners 

 in multiple-chine sections approach 180 deg 

 there is less need for placing them exactly in the 

 lines of flow. The chine positions therefore can 

 favor the lines of plating or the type of framing 

 to be used. 



Frame sections which are nominally straight 

 may be given a slight outward bow or camber, 

 corresponding to the camber in a weather deck. 

 This is not enough to involve forming or furnacing. 

 Metal plating applied to slightly bowed frames 

 lies flatter, or perhaps one should say with less 

 waviness, than if held to perfectly straight frame 

 lines [Baier, L. A., unpubl. Itr. to HES of 4 Aug 

 1950]. 



The degree to which a hull surface may vary 

 from one which is exactly of single curvature 

 depends upon the size of the surface in question 

 compared to the size or the area or the shape of 

 the sheet or plate of which that surface is to be 

 made. A wooden plank may be bent, twisted, 

 and sprung out of its natural shape, fastened to 

 the frame of a boat, and left in its sprung shape 

 for years provided it is not too wide or too thick 

 for the shape to be impressed upon it. A wide 

 sheet or plate of metal may often be pulled and 

 stretched to make it conform slightly to a second 

 degree of curvature. A wide sheet of plywood, 

 assembled in the flat, may not respond to such 

 treatment without definite and detrimental crack- 

 ing. 



If the straight-element form facilitates con- 

 struction in any particular case, it is equally 

 advantageous to incorporate it into the above- 



