482 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 66.17 



TABLE 66.f — Second Statement of Variable- Weight Conditions 

 This table, at the present stage of the design, supersedes Table 66.d. 



It appears too small for the proposed river speeds 

 of 13 and 14 kt. 



A conference with the owners and operators at 

 this point reveals that they have in mind amend- 

 ing the original requirements to call for taking on 

 board, during certain seasons of the year, a 

 substantial amount of the consumable food 

 stores at Port Correo rather than at Port Amalo. 

 This would increase the designed maximum 

 displacement by the same amount and put the 

 ship deeper in the water than the original 26.65-ft 

 mean draft when starting down the fresh-water 

 river. It is decided, therefore, not to increase the 

 salt-water draft beyond 26 ft for the molded 

 displacement of 16,400 t. 



It is possible to estimate the moment to alter 

 trim per unit value and the change of displace- 

 ment per unit change of trim by the stern, using 

 established methods [Owen, W. S., PNA, 1939, 

 Vol. I, pp. 42-43]. However, such an estimate is 

 not worth while at this stage because the under- 

 water hull is not yet roughed out. The method of 

 allowing for changes in trim for this particular 

 design is described in some detail in Sec. 66.32. 



Unloading all cargo at the same time at Port 

 Amalo hghtens the vessel by some 8,025 t, 

 indicated in Tables 66.f and 66.g, while the same 

 procedure at Port Correo reduces its displacement 

 by some 8,425 t. These conditions correspond to 

 mean draft reductions of the order of 11.07 to 



11.62 ft. Although it is not done here, the trans- 

 verse metacentric stability at these light loads 

 must be carefully checked in a complete prelimi- 

 nary design. 



66.17 Sketching the Section-Area Curve; The 

 Maximum-Area Position. Sketching a prelimi- 

 nary section-area curve involves first a fullness 

 ratio for the curve equal to the prismatic coefficient 

 Cp selected. For the ABC design, this is 0.62. 

 The section-area curves of the Taylor Standard 

 Series [Taylor, D. W., S and P, 1943, p. 182], 

 reproduced in Fig. 51. A, are good average curves. 

 They are quite satisfactory for prehminary 

 guides, with the hmitations that the section of 

 maximum area is at the midstation, there is no 

 parallel middlebody, and the areas near the stern 

 apply to a twin-screw vessel of normal form. 

 References giving other sets of typical section-area 

 curves for single-screw vessels, covering a rather 

 wide range of Cp values, are listed in Sec. 51.6. 



Working over the guide curve for the appro- 

 priate Cp requires prehminary selection of certain 

 parameters, as follows: 



(a) Position of the section of m aximum area 

 along the length, expressed as LIVIA and reckoned 

 as a fraction of the length L abaft the FP, for 

 the prismatic coefficient Cp that matches the 

 Froude number or Taylor quotient corresponding 

 to the designed speed 



