Sec. 54.9 



AIR AND WIND RESISTANCE OF StIIPS 



281 



faces in contact with the air are large and long, 

 many of the contours are reasonably uniform, 

 and friction drag is no longer negligible. If the 

 frontal area of such a ship hull is taken as equal 

 to the maximum underwater ai'ea Ax , or even 

 as B{H), with an L/B ratio of say 7, the "wetted" 

 abovewater area of the smooth side portions is of 

 the order of 2L{H) = 2{7B)H = 14B(i7). This 

 proportion of wetted to frontal area is about the 

 same as for a railway coach. Unfortunately, a 

 reasonably accurate prediction of the friction 

 drag for the coach must await more knowledge 

 as to the actual air flow around it [Hoerner, S. F., 

 AD, 1951, pp. 169-170]; the same is true of the 

 ship hull. 



54.9 Drag and Resistance with Wind on the 

 Bow. For the reasons explained in Sec. 26.15 

 and illustrated in Fig. 26.1 of Volume I, the 

 relative wind blowing at an angle on the bow 

 impinges separately on d-eck erections, stacks, 

 and certain other elements of the upper works 

 which normally benefit from shadowng when the 

 relative wind is dead ahead or nearly so. Further- 

 more, a ship hull, lying at an effective angle of 

 attack to the relative wind and acting as a short- 

 span airfoil, cantilevered above the water surface, 

 is creating an induced drag as well as the lift 

 depicted in diagram C of Fig. 26. H. This induced 

 drag, although not shown there, is additional to 

 the pressure drag. It is measured as part of the 

 wind drag and wind resistance when model tests 

 are made, and is included in the coefficients set 

 forth in this chapter. 



As a result, the axial component Rwi„d of the 

 lift and drag forces due to the relative-wind 

 velocity Wr at a range of relative-wind angles on 

 the bow usually exceeds the value of the wind 

 resistance i2wind when the relative wind is from 

 directly ahead. The ratio of these forces is ex- 

 pressed for convenience as kg . Then for any 

 angle of relative wind 6, measured toward the 

 right from ahead, /2wind = kgDw , when D^r is 

 measured at 6 = deg. This is equivalent to 



jRwind at angle d = fc9(i2wind at deg) (54.viii) 



The rates at which the coefficient kn vary with 

 the direction of the relative wind for several 

 ships of different types are illustrated in Figs. 

 54.C, 54. D, and 54. E. Two similar graphs, one 

 for a cargo vessel with forecastle, centercastle, 

 and poop, and the other for a passenger ship, 

 are given by W. P. A. van Lammeren, L. Troost, 

 and J. G. Koning [RPSS, 1948, Fig. 8, p. 25]. 



-Tanker SALINAS 

 -Carcjo Ship CLAIRTON 

 -Passenger Ship 5ANTA R05A ^ 



10 20 30 40 50 60 70^-80 90 

 Angle of Relative Wind f rom Aheod, deg 



Fig. 54. C Graphs of kg for Three Merchant Ships 



Anqle of Relative Wind from Aheod, deq 

 O 10 20 50 40 50 60 70 80 96 



1.7 

 1.6 



1.4 ° 



O 



1.3 -^ 



10 20 30 40 50 60 10 80 30 

 Angle of Relotive Wind from Ahead, deq 



Fig. 54. D Graph of kg for Three Destroyers 



