Sec. 6S.9 



ABOVEWATER FORM LAYOUT 



555 



angle of heel, the greater part of the water on a 

 wide ship spills over the top of the bulwarks. 



To require that a deckload of water, up to the 

 top of the bulwarks, should run off completely 

 within the interval of one pitching cycle would 

 practically require taking away the bulwarks 

 altogether. It is therefore necessary to assume 

 that some roll angle or pitch angle, or both, will 

 unload most of the water over the bulwark rail. 

 To get rid of the rest of the water in one pitching 

 or rolling period it appears that the freeing-port 

 area should be more nearly 0.2 the bulwark area. 

 Furthermore, this freeing-port area should be 

 provided abreast the volume which needs to be 

 emptied if the vessel ships a deckload of solid 

 water. 



Bulwarks at the extreme bow can serve as an 

 effective increase in freeboard in that region, 

 over and above that provided by the intact hull. 

 If not extended too far aft, say to not farther 

 than the point where the local beam exceeds 

 0.55x , it should be possible to leave them solid, 

 without freeing ports or slots. 



Breakwaters require positioning and shaping so 

 that the maximum water is deflected for the 

 minimum of splash or spray over the top. This 

 calls for a deflecting surface which is never — 

 or rarely ever — normal to the onrushing water and 

 which does not form an objectionable spray- 

 thrower for water in quantities greater than the 

 breakwater is designed to handle. The water 

 deflected from the forward side should have no 

 upward component, and as great an outward 

 component as possible, to throw it toward the 

 gunwale and get it off the deck. Fig. 68.E illus- 

 trates, at 3 and 4, two alternative methods of 

 accomphshing this. The function of the horizontal 

 lip along the top of the barrier is to throw moderate 

 quantities of water back forward but to permit 

 large quantities to pass over the breakwater 

 without too violent obstruction. 



The breakwater on a forecastle is usually of 

 V-shape in plan, with its vertex forward and with 

 diagonal sides extending practically to the deck 

 edges. The planform angles may vary from 45 

 to 60 deg with the centerline, indicated at 1 and 2 

 on Fig. 68.E. The height at the center, where 

 the water can not run off the deck freely, should 

 be higher than at the sides. A breakwater having 

 an elliptic or parabolic planform, with the sharp 

 curvature forward, is shown for an early German 

 "schnellboote" (high-speed boat) in Schiffbau [26 

 Oct-2 Nov 1921, Fig. 4, p. 114]. There may be 



two breakwaters in tandem, separated by an 

 appreciable fore-and-aft distance, so that the 

 water spilling over one is trapped by the second. 

 A set of tandem breakwaters of concave section 

 is fitted on the French battleship Jean Bart 

 [The 111. London News, 9 Apr 1955, p. 661]. 



A breakwater of any type requires adequate 

 bracing against the hydrodynamic forces. These 

 are not accurately or even roughly known but 

 their order of magnitude may be estimated by 

 assuming a dynamic load imposed by solid water 

 striking the breakwater at a certain velocity. For 

 a head sea, or an angle of encounter Q:(alpha) of 

 180 deg, this is compounded of (1) the speed V 

 which it is estimated the ship can make in heavy 

 weather and (2) the orbital velocity Uorb of the 

 crest of a wave which breaks over the forecastle 

 and strikes the breakwater. While, strictly speak- 

 ing, the dynamic pressure is that due to the com- 

 ponent of (7 + (7 Orb) normal to the breakwater, 

 there is little assurance that the deck load of water 

 sliding aft on the forecastle will strike the break- 

 water from ahead. The blow may just as well 

 come from the side, striking against one face for 



Not Less Than 1.25 3l"ont *1 



J-enqth of Breakwoter | 



on One Side i 



Fig. 68.E Design Sketches fob Forecastle 

 Brbakwateks 



