554 



riYDRODYNy\MICS IN SHIP DESIGN 



Sec. 6S.9 



All Comber Heiqhts ore Exocjtjerated 



(About 10 times in Dioqroms I, 2, and 3) 



Ridqe Line Mq\j Be Held Fixed in Position ond Shape 

 (straight in this Cose) ond Sheer Line at Side Allowed 

 to Take Its Shape "Automaticallu" b\j the Geometric 

 Construction ~ 



Sheer Line at Side Held Fixed in Position and Shope 

 and Ridcje Line at Centerplanc Allowed to TaUe Its 

 Shape Automoticalli^, 



Constant Ridqe Slope Assumed in Dioorams 4,5 "^ 



FiQ. 68.D Straight and Curved Deck Camber Lines 



in the drafting, shipfitting, and fabricating stages 

 but no ship deck, certainly not one of metal, ever 

 finishes fiat nor does it remain flat. It always 

 bends downward under its own weight, if not 

 under compression loads due to riveting and 

 welding. Generally there are more plating buckles 

 downward than there are upward. Here is a case 

 where the greater stifi'ness of an aluminum deck 

 plate of equal or slightly less weight might be a 

 di,stinct advantage. 



Ridge-type decks, hke the low ridge roof of a 

 house, illustrated at 3 in Fig. 68. D, are composed 

 of two flat surfaces each lying at a small angle to 

 the horizontal, only large enough to insure 

 drainage in service, and joined in a low knuckle 

 at the centerUne. This knuckle is a straight line 

 if the ridge surfaces are flat [Dawson, A. J., 

 SNAME, 1950, Fig, 10, p. 13]. However, the 

 knuckle can be curved, with a sheer incorporated 

 in it, leaving the deck-beam lines composed of 



two straight elements, lying at a constant angle 

 to each other. The ridge surfaces are then develop- 

 able cyhnders of very large radius. 



In fact, if a circular or parabolic arc of constant 

 shape is employed for the deck-beam lines on a 

 cambered deck, and if the intersections of the 

 deck-beam Unes with the centerplane lie on a 

 straight line fore and aft, as at 4 in Fig. 68.D, the 

 whole deck is a developable cylindrical surface. 

 If the deck line at the center or at the side follows 

 a curved sheer, corresponding to 5 in the figure, 

 the departure from true cylindrical form, for 

 any one plate, is usually insignificant. 



A ridge-type deck, having the same rise at the 

 centerline, possesses considerably more slope 

 amidships and less slope at the sides of the vessel 

 than a camber with a circular arc or even with 

 a paraboUc arc. Whether this represents an 

 advantage for the straight-element deck in 

 shedding water is debatable, especially as at one 

 angle of list the high side has no slope at all. 



68.9 Bulwarks and Breakwaters. Bulwarks, 

 either partial or full, are appropriate for both 

 large and small vessels when it is desired to: 



(a) Afford some protection along a deck edge 

 against wind and spray blowing across the deck 



(b) Prevent marginal waves and crests from 

 slopping over onto the deck 



(c) Retain on board loose gear, small items of 

 deck cargo, fish dumped from nets, and the like. 



If the owner desires or permits, bulwarks may 

 be added solely for the sake of appearance, as 

 when carrying a graceful sheer line along a straight 

 deck edge. 



While bulwarks can hold back some marginal 

 water from coming over a deck edge they can 

 and do keep on deck large menacing weights of 

 water which need to be unloaded quickly, before 

 the next sea comes aboard. Freeing slots along 

 the lower edge of the bulwarks, or hinged-cover 

 freeing ports in the bulwarks, are provided for 

 this purpose but the average head to make water 

 run through them rapidly is rather low. Further- 

 more, the port edges are usually sharp and the 

 orifice coefficient is also low. 



It is customary to provide a port opening of 

 0.1 the bulwark area and to hmit the bulwark 

 height to 5 ft [Lovett, W. J., "Applied Naval 

 Architecture," 1920, p. 174]. This rule takes no 

 account of the width of the vessel and the volume 

 of water trapped between bulwarks of a given 

 height, apparently on the theory that for a given 



