Sec. 54.17 



AIR AND WIND RESISTANCE OF SHIPS 



287 



bottom. In any case, too high a value of Co for 

 the lateral water resistance represents an unsafe 

 estimate, since the calculated rate of downwind 

 drift is then smaller than is found on the ship. 

 The resistance to drift may be expressed by 





(54.x) 



The force causing drift is the wind drag of the 

 ship at the relative wind velocity Wr , expressed 

 by 



D^ = C^(Air>(0.5p^)4^Tf„' (54.iv) 



The lateral wind force is calculated from the 

 latter equation, substituted for the lateral 

 resistance to drift Rorut of Eq. (54.x), and this 

 equation is then solved for the drifting speed 



FDri.t . 



If the abovewater lateral projected area A a 

 is assumed equal to the underwater lateral area 

 A I, , and Co for wind drag is taken as 1.18, then 

 for equal wind and drift drags 



(1.18) I (for air) TF^ = (1.15) | (for water)(7Dri,t)' 



whence 



FLift 1.18(0.001189) 



TFi 



1.15(0.99525) 



Wr 



= 0.035 



It is to be remembered that, since the ship is 

 drifting downwind, 



Wr = IFt.u, - F:,n,t. 



54. 1 6 Estimating the Forces on a M cored Ship. 



A brief discussion of the forces on a moored ship 

 is included in Sec. 12.8. It is pointed out by T. 

 Thorpe and K. P. Farrell [INA, 1948, p. 116] 

 that a ship lying at a mooring is subject to forces 

 due to wind, waves, and tidal current. Sec. 12.8 

 mentions that it is also subject to slope drag 

 when the ship rides at anchor in an appreciable 

 current, with the moored end higher than the free 

 end. To the usual resistance forces derived from 

 relative motion of the ship and the water there 

 is added the drag of the non-rotating or non- 



operating propulsion device (s) due to the current 

 flowing by them. 



A ship with a normal proportion of its total 

 bulk volume under water usually rides to the 

 current rather than to the wind. This means 

 that the wind may blow at any bearing relative 

 to the ship, and that the greatest drag due to 

 both current and wind may be expected when the 

 ship is riding head to the current, with the wind 

 about 30 deg on either bow. T. Thorpe and K. P. 

 Farrell, in the reference cited, emphasize the effect 

 of gusts and squalls, because the wind drag varies 

 as the square of the maximum instantaneous 

 velocity, assuming that it blows with this aug- 

 mented velocity on the whole ship at once. 



Wind-drag forces on groups of moored ships, 

 lying alongside each other, are given by M. E. 

 Long in TMB Report R-332 of December 1945, 

 entitled "Wind Tunnel Tests to Determine Air 

 Loads on Multiple-Ship Moorings for Destroyers 

 of the DD692 Class." 



The naval architect will require drag data on 

 moored vessels only infrequently. No attempt is 

 therefore made here to include, with or without 

 adaptation, any of the tables, graphs, or diagrams 

 given in the references. 



54.17 Surf ace- Water Currents due to Natural 

 Wind. A discussion of drift and leeway in 

 particular, and of wind resistance in general, is 

 not complete without some mention of the surface- 

 water currents produced by a natural wind blow- 

 ing over a body of water. Some data are available 

 to relate the magnitude of this current to the 

 wind velocity but without the necessary informa- 

 tion as to the height above the water surface at 

 which the velocity is measured. 



E. F. Eggert states that this surface current, 

 presumably more-or-less uniform for the draft 

 of a surface ship of moderate size, has a mag- 

 nitude of 0.015 times the wind velocity [EMB 

 Rep. 264, Aug 1930, p. 1, based on data furnished 

 by the U. S. Coast and Geodetic Survey]. C. O'D. 

 Iselin states that on the average the surface water 

 moves at about 3 per cent of the wind velocity. 

 This is twice the value just quoted. Further, 

 Iselin reports that the surface-water current 

 moves in a direction about 30 deg to the right of 

 the wind in the northern hemisphere (30 deg to 

 the left below the equator) ["Oceanography and 

 Naval Architecture," SNAME, New Engl. Sect., 

 Jun 1954]. 



