Sec. 54.11 



AIR AND WIND RESISTANCE OF SHIPS 



Calculated Lateral Area of Abovewater Form, 20,167 sq ft 



Silhouette from Abeam, Correspondincj to Layout in Fiq, 68.M 



Fig. 54.F Wind-Resistance and Cbnteb-of-Pressube Layout for the ABC Ship of Part 4 



again as large as the Aa = (0.5)5i value origin- 

 ally used. To keep the problem simple it is assumed 

 that the relative-wind velocity over all this area 

 is the same, eliminating any boundary-layer effect. 

 The value of kg is taken from the Santa Rosa 

 short-dash curve of Fig. 54. C, for 6 = 22 deg, 

 as approximately 1.15. Then, for Wr = 41.5 kt, 

 Eq. (54.viii) gives 



2?wud = {ke)KAA)Wl 



= (1.15)(0.004)(3,880)(41.5)' = 30,739 lb. 



From Fig. 78. Nc of Part 4, the effective power 

 Pe for the transom-stern design of ABC ship, 

 predicted from model tests, is about 9,940 horses. 

 For the water speed of 20.4 kt, equivalent to 

 34.45 ft per sec, the total resistance Rt , with 

 appendages, works out as 9,940(550)/34.45 = 

 158,694 lb. The predicted wind resistance then 

 represents an increase in the estimated total 

 resistance at 20.4 kt of (100)(30,739)/158,694 or 

 19.4 per cent. Unless the machinery could develop 



more than its rated maximum power it is doubtful 

 whether the ship could make 20.4 kt on the trial 

 course against a 41.5-kt relative wind on the bow. 



54.11 Magnitude of Wind Pressure. It is 

 helpful at times to have an idea of the magnitude 

 of the forces exerted by a natural wind on a flat 

 plate or a flat surface normal to the wind direction. 

 A number of authors have given tabulated data 

 of this kind in the past, among them Dixon Kemp 

 ["A Manual of Yacht and Boat Sailing," Cox, 

 London, 3rd ed., 1882, p. 599, on which there is a 

 table of wind-pressure intensity]. His values of 

 normal pressure in lb per ft^, for a range of wind 

 velocity of 1 to 100 kt, corresponding to the 

 complete range of the Beaufort scale from 1 to 

 12, were based upon a constant drag coefficient 

 Cd of 1.87. It is now known that this drag coefla- 

 cient varies from about 1.16 for a square plate 

 with free edges to about 1.90 for an infinitely 

 long strip, also with free edges. 



The figures iia Table 54.b are adapted from a 

 table published more recently by K. C. Barnaby 



