284 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 54.12 



TABLE 54. b — Nominal Force, Velocity, and Pressure Due to Natural Winds 

 The ram pressures are based on a p-value of 0.002378 slugs per ft'. The flat-plate pressures are based on a thin plate 



mounted in the open, with a separation zone on its leeward side. 

 Column 5 is calculated for a Co of 1.16; column 6 for a Cd of 1.90. 

 The vAnd. velocities correspond exactly to the Beaufort-scale numbers pubUshed in "Instructions for Keeping Ship's 



Deck Log," NavPers 15876 of July 1955, Bureau of Naval Persoimel, U. S. Navy Department. 



[BNA, 1948, p. 190]. The nominal wind force in 

 column 1 is not a force in the strict sense of the 

 word but a Beaufort-scale number used by 

 mariners to indicate a range of velocity, set down 

 in column 3. The Mond velocities corresponding 

 to the Beaufort-scale numbers in Table 54. b are 

 taken from "Instructions for Keeping Ship's 

 Deck Log," NavPers 15876 of July 1955, issued 

 by the Bureau of Naval Personnel of the U. S. 

 Navy Department. The ram or stagnation pres- 

 sures in column 4 are those calculated by the 

 expression Q.bpWl where p is taken as 0.002378 

 slugs per ft' for "standard" air and Wt is the 

 velocity of the true or natural wind, in ft per sec. 



The right-hand column of the referenced 

 Barnaby table, not reproduced here, was calcu- 

 lated for a drag coefficient Co of 1.18. Column 5 

 of Table 54. b is based upon a drag coefficient of 

 1.16, corresponding to hat for a thin, flat plate 

 of square outline and a length-breadth ratio of 

 1.0, normal to the A^dnd, as in Fig. 55. B. The 

 values in column 6 are for a thin, flat plate, 

 standing in the open, with air access to the back, 

 having a length-breadth or aspect ratio of infinity. 



The reason why the flat-plate pressures are 



greater than the ram pressures is because there 

 is a separation zone and a — Ap region on the 

 back of each plate, in addition to the -|-Ap region 

 on the front. 



The wind drags hsted in Table 54.b apply 

 generally to wind screens and windbreaks on a 

 ship, in much the same manner as to signboards 

 ashore. Whether the drags are valid for deckhouse 

 structures depends upon whether or not there is a 

 separation zone on the leeward side of each house, 

 the same as that behind a flat plate. 



54.12 Location of Center of Wind Pressure. 

 It is often convenient, and sometimes necessary, 

 to know the approximate location of the point 

 along the ship axis at which the total resultant 

 Avind force is applied. When combined in a 

 suitable manner with the center of pressure for 

 slow drift motion of the underwater hull, oriented 

 on a selected heading, this would give the point 

 at which the ship should be held by a single 

 mooring, say, so as to remain in position on that 

 heading. 



Unfortunately, data on center of mnd pressure 

 are limited but Fig. 54. G contains five plots of the 

 fore-and-aft CP positions for the wind-resistance 



