Sec. 6S.4 



ABOVEWATER-FORM LAYOUT 

 TABLE 68.a — Sheer Heights in Fractions op Wateblinb Length 



549 



(and aft) to be added to this value. The ratio 

 of sheer heights forward and aft to ship length, 

 set down in Table 68. a, indicate little change in 

 the past century for normal designs of merchant 

 vessels but a great spread for vessels of even the 

 same type. D. Arnott gives a diagram containing 

 six different standard sheer lines for a 400-ft 

 vessel, with ordinates in inches [Mar. Eng'g., 

 Apr 1921, Fig. 4, p. 309]. All these lines have their 

 low points at about 0.53L from the FP. The 

 highest of them in the forebody is marked "Maxi- 

 mum Allowable German Sheer," and has a value 

 of 140 in, or 0.0292L, at the FP. The highest in 

 the afterbody is marked "Proposed British 

 Standard Sheer (flush deck vessel)" with a value 

 of about 63 in, or 0.013 IL, at the AP. 



The great spread of values in the data of 

 Table 68. a also applies to the data upon which 

 the tentative line of Fig. 68. A was drawn. A 

 better shape and position for this line must 

 await further hydrodynaraic analysis and con- 

 firmation from reliable ship performance data. 



The ratio of (1) freeboard at the bow to (2) 

 length is found rarely to exceed 0.08 to 0.09. If 

 the ratio of freeboard amidships to length reaches 

 the range of 0.08 to 0.09, sheer forward is no 

 longer necessary for wavegoing. Regardless of 

 the freeboard amidships, a relatively high bow 



is built into special vessels like whale catchers, 

 where personnel have to man stations at the 

 forecastle head. Fig. 68. B is a recent excellent 

 example of this type, the Enern, built for whaling 

 in the rough waters of the Antarctic [MEN A, Mar 

 1953, pp. 114-119]. Another is the Japanese 

 whale catcher Konan Maru 11, with a very high 

 gunner's platform at the bow [The Motor Ship, 

 London, Oct 1954, p. 301; SBSR, 23 Sep 1954, 

 p. 4031. 



If steep deck slopes are objectionable, sheer 

 and freeboard are easily gained in large increments 

 by the addition of a forecastle or poop on top of 

 a deck with normal slope and sheer. This is 

 carried to the extreme in lightships by adding a 

 complete deck, so that the freeboard-amidships- 

 to-length ratio may approach or exceed a value 

 of 0.10. 



As might be expected, a ship which is still safe 

 with a low freeboard, like a tanker loaded with 

 cargo having a specific gravity less than water, 

 or a tuna boat on which heavy fish have to be 

 yanked over the side by manpower, needs and 

 profits by more sheer than the normal amount. 

 In fact, long experience with vessels performing 

 different duties indicates the need for at least 

 four or possibly five kinds of sheer profile. Sche- 

 matic diagrams of parabolic profiles, set down in 



