Srr. 66.1 r, 



STEPS IN PRFLTMTNARY nF.STCN 



'IRl 



of the past [Thompson, R. C, NECI, 1935-1936, 

 pp. 216-217], no such problems have presented 

 themselves with the multitude of full-stern and 

 transom-stern vessels of the U. S. Navy built 

 from the middle 1930's to the present. Since the 

 transom stern proposed here is on the small 

 side as transom sterns go, it is considered accept- 

 able for this preliminary design. 



It is now possible to sketch a tentative designed 

 waterline for the ABC ship on the basis of the 

 following: 



(a) Length, 510 ft 



(b) Slope at stem, 7 to 8 deg 



(c) Entrance offsets from S. A. Vincent [MESA, 

 Mar 1930, Fig. 5, p. 139], for T, = 0.8 to 0.85, 

 with hollow portion 



(d) No parallel waterline 



(e) Slope at stern, 13 deg, maximum 



(f) Beam, maximum, 73 ft 



(g) Position of B^x , 0.54L, or 275.4 ft abaft FP 

 (h) Nearly constant curvature amidships 



(i) Cr = 0.713; Aw = 510(73)0.713 = 26,545 ft'' 



0.90 



OSOM M I I II I I I 



£5 60 55 50 -4S 40 



Position of Maximum Designed Waterline Beam 

 Byjx in Percentaqe of Ship Lenoth from FP 



Fig. 66.K Foee-and-Aft Position of Maximum 

 Waterline Beam Bffx 



(j) Transom width Bu = 0.3(Bx) = 21.9 ft 

 (k) Transom radius in planform, tentative, O.IOL, 

 or say 50 ft. 



Several attempts produce a result that meets 

 the requirements fairly closely. The preUminary 

 sketches are not illustrated here but the final 

 designed waterline shape appears in Fig. 07. A. 

 A preliminary check of the curvature, by the 

 graphic method described in Chap. 49, indicates 

 that the early contours could stand some smooth- 

 ing. However, before making another try at the 

 designed waterline it is well to see how other 

 parts of the underwater form work out. 



66.16 Estimated Draft Variations. It is useful 

 at this stage, as an aid in developing other features 

 of the underwater body, to have some idea of the 

 variations in draft to be encountered in the several 

 variable-weight conditions. The first statement 

 of these conditions, given in Table 66.d of Sec. 

 66.3, requires modification because of the changes 

 in fuel weights. The second variable-weight 

 statement appears in Table 66. f. 



The tons per foot immersion for the designed- 

 waterline dimensions tentatively selected are 

 approximately 26,545/35 or 758.4 tons per ft, 

 equivalent to about 63.2 tons per in. This value 

 diminishes as the load decreases and the ship 

 comes up in the water. The change is allowed for 

 in a rough way by reducing the 758 tons per foot 

 progressively to a guessed value of 725 tons per 

 ft at the lighter drafts. On this basis the drafts 

 corresponding to the entries in Table 66. f are 

 about as set down in Table 66.g. These show that 

 when the vessel is returning to Port Amalo 

 through the canal it is some 975 t fighter than the 

 designed maximum service displacement. When 

 leaving Port Amalo it may be from 2,400 to 3,400 1 

 lighter or even more, depending upon the liquid 

 ballast carried. This decreases the mean draft 

 by from 3.26 to 4.63 ft. It may be expected to 

 reduce the maximum draft, with the stern down 

 to keep the propeller under water, by at least 1.0 

 ft. The minimum bed clearance under the middle 

 of the ship in the Port Amalo canal is then 

 28 - (26 - 3.26) = 5.26 ft, which is undoubtedly 

 more than enough for the limiting speeds of 8 

 and 10 kt. The designed draft of the vessel might 

 possibly be increased from 26 to 27 ft, but then 

 the bed clearance in the river when leaving Port 

 Correo would be 3 ft minus the fresh-water 

 sinkage correction, or about 2.35 ft. This is 

 smaller than the 3 ft indicated in Fig. 66. G. 



