Sec. ^.?.,'? 



ABOVEWATER-FORM LAYOUT 



653 



(j) For structural reasons the knuckle should be 

 well clear of all deck edges along the shell. 



The knuckle and the compound flare in the 

 forebody sections of the ABC ship, delineated in 

 Fig. 66. P, are laid out by the procedure described. 

 The knuckle at Sta. —0.5 lies shghtly above the 

 knuckle line because of discontinuities in way of 

 the single centerline bower-anchor position. 



For vessels carrying wing screw propellers it 

 may upon occasion seem wise to afford lateral 

 protection to the propellers by widening the 

 abovewater hull above them, rather than by 

 fitting abovewater propeller guards. 



68.6 Check of Range of Stability and Dynamic 

 Metacentric Stability. At this stage in the 

 preliminary design, if not before, a check is made 

 to insure that: 



(1) The range of positive metacentric stability, 

 in a transverse plane and for the static case only, 

 is adequate for the service expected of the vessel. 

 This operation is particularly important for a 

 vessel of special shape, such as sketched subse- 

 quently in diagram 1 of Fig. 68. K in Sec. 68.12. 

 The method of accomplishing this is set forth in 

 many text and reference books on naval archi- 

 tecture [PNA, 1939, Vol. I, p. 135]. 

 (b) The vessel possesses adequate dynamic meta- 

 centric stability in a transverse plane; in other 

 words, it has sufficient stored-up righting energy 

 to more than absorb the dynamic rolling energy 

 [Vincent, S. A., PNA, 1939, Vol. I, pp. 135-136]. 

 This matter is discussed further under wavegoing 

 in Part 6 of Volume III. 



68.7 Abovewater Profile and Deck Details. 



The abovewater profiles of a ship, like the section 

 shapes, are governed partly by the necessity of 

 meeting certain wavegoing requirements and 

 partly by utilitarian needs. They may result from 

 fairing the sections into the ends, or from a 

 desire to achieve a certain appearance. Centerline 

 anchor stowages at the bow and stern, propeller- 

 aperture clearances for single-screw vessels, and 

 other features usually play a part more important 

 than hydrodynamics [Coqueret, F., and Romano, 

 P., SNAME, 1936, pp. 131-132]. 



In the main, however, the abovewater profile 

 should, like the underwater profile described in 

 Sec. 67.4, be a sort of automatic result of first 

 determining the ship form desired, in transverse 

 planes, and then carrying the hull surfaces 

 forward and aft, in fair shapes, until they meet 

 at the centerplane. This is what happened when 



the flaring, slightly concave abovewater entrance 

 sections of the fast sailing ships of the 1840's were 

 carried forward to produce what is now known 

 as the clipper bow, pictured at 1 and 2 in Fig. 26. D. 



It is regular shipyard practice to camber a 

 straight stem to compensate for the optical illusion 

 of concavity inherent in a perfectly straight stem 

 bar [Baier, L. A., unpubl. Itr. to HES, 4 Aug 1950]. 



Profiles and planforms for transom sterns are 

 discussed and illustrated in Sec. 67.20. 



For the ABC ship the planform of the main deck 

 is made elliptical at the stern, solely as a matter 

 of appearance. Some additional abovewater 

 volume and deck space are achieved by carrying 

 the transom all the way up to the main deck, as 

 has been done on many U. S. warships, but at the 

 expense of some additional weight and an un- 

 questionably heavy, clumsy appearance at the 

 stern. 



The forebody portions of the ABC transom 

 and arch sterns, above the DWL, are exactly 

 alike back to Sta. 11. However, the afterbody 

 portion of the ABC arch-type stern above the 

 DWL is slightly different from that of the tran- 

 som-stern ship because of the greater waterline 

 beam at the AP. The main deck planform and 

 the uppermost level lines are rounded in the 

 same way, except to a larger radius. 



68.8 Selection of Deck Camber. A normal 

 degree of circular-arc or parabolic camber in a 

 weather deck, with a rise at the centerline amount- 

 ing to say 0.020 or 0.025Bx for the widest part 

 of the ship (0.25 inch per foot of beam corresponds 

 to 0.02085x), is some help in shedding water 

 during wavegoing but it is hardly to be classed 

 as a quick-unloading device for a boarding sea 

 in an emergency. Constructions for circular and 

 parabolic arcs are illustrated at 1 and 2 in Fig. 

 68. D; also by G. de Rooij in "Practical Ship- 

 building" [1953, Figs. 329a and 329b, p. 133]. 



No one camber shape among a number that 

 are available has any particular hydrodynamic 

 superiority or significance. The camber may be 

 appropriate, therefore, to the drafting as well as 

 to the shipfitting and fabricating procedures. For 

 this reason a fixed camber curve may be used for 

 all widths of deck along the length. The curvature 

 or slope need be sufficient only to insure that, 

 within the life of the ship, there will be no de- 

 -pressions in the deck at any small heel angle 

 because of ill-formed or buckled plates or minor 

 service damage. 



Flat, straight decks are admittedly economical 



