CHAPTER 54 



Estimating the Air and Wind Resistance of Ships 



54.1 Scope of This Chapter; Definitions .... 274 54.8 



54 . 2 Increase of Wind Velocity with Height Above 



Water Surface 274 54.9 



54.3 Flow Diagrams for Upper- Works Configura- 54.10 



tions 276 



54.4 General Formulas for the Wind Drag of 54.11 



Irregular Ship Hulls and Superstructures . 276 54.12 



54.5 Notes on Wind-Resistance Models and 54.13 



Testing Techniques 278 54.14 



54.6 Bibliography of Model Wind-Resistance 54.15 



Tests 278 54.15 



54.7 Drag Coefficients for Typical Abovewater 54.17 



Hulls and Upper Works 279 



Comments Concerning Wind-Friction Re- 

 sistance of an Abovewater Hull 280 



Drag and Resistance with Wind on the Bow . 281 

 Prediction of Wind Resistance for ABC Ship 



of Part 4 282 



Magnitude of Wind Pressure 283 



Location of Center of Wind Pressure . . . 284 



Lateral Wind Drag 285 



Lateral Wind Moments and Angle of Heel . 285 



Estimated Drift and Leeway 286 



Estimating the Forces on a Moored Ship . 287 



Surface-Water Currents due to Natural Wind 287 



54.1 Scope of This Chapter; Definitions. 



The general phenomena and the effects of the 

 flow of air over the abovewater hull and the upper 

 works of a ship are described in Sees. 26.15 and 

 26.16 of Volume I in purely qualitative fashion. 

 The winds of nature, powerful enough to propel 

 saihng vessels, produce sizable quantitative effects 

 on mechanically driven ships, often of the order 

 of tenths of the power and whole knots or more of 

 speed. Reasonably accurate estimates or predic- 

 tions of these effects are required, especially when 

 analyzing ship-trial data [Eggert, E. F., EMB 

 Rep. 264, Aug 1930; SNAME, 1932, pp. 17-44; 

 1933, pp. 243-295; Taylor, D. W., S and P, 1943, 

 pp. 167-169]. 



Definitions applying to these phenomena and 

 effects are given in Sec. 26.15, supplemented by 

 Figs. 26. G and 26. H. It is most important to 

 keep clear the distinction made there between the 

 wind drag D„r , which always acts dovmwind from 

 the relative wind direction, and the wind re- 

 sistance ffwind • The latter is the sum of the fore- 

 and-aft components of both the mnd drag and 

 the wind lift, acting always along the principal 

 ship axis, opposite to the direction of motion. 

 This distinction is necessary because the wind 

 forces are impressed on the ship separately from 

 the hydrodynamic forces. 



54.2 Increase of Wind Velocity with Height 

 Above Water Surface. The boundary layer 

 formed by the -wind blo\ving over moderately 

 long stretches of water is thick in proportion to 

 the vertical dimensions of a ship hull, or even of its 



masts. The thickness 5 (delta) may attain values 

 greater than a thousand feet [Matveyey, R. T., 

 Meteorologiya i Gidrologiya, No. 3, 1949, pp. 

 20-29; ASCIL Transl. 490]. Large wind velocities 

 at high airplane altitudes are caused by move- 

 ments of huge air masses and are not strictly a 

 boundary-layer effect. 



For the ordinary boundary layer in air or water 

 the so-called reference velocity, represented by 

 [/„ , is that at a great distance from the body or 

 ship. Since this velocity is rarely known for the 

 thick and high boundary layer in the atmos- 

 phere above a large water surface, it is customary 

 to use as a reference, for ship-design and ship- 

 operation purposes, some velocity that is easily 

 measured. In the past, the reference velocity has 

 often been considered to be that observed at a 

 height of 50 ft above the water. However, there 

 is no accepted standard height for measuring the 

 wind velocity which is assumed to be acting on a 

 boat or a ship as a whole. Nevertheless the mnd 

 velocities of interest to marine architects and ship 

 operators are usually e.xpressed as multiples, 

 greater than 1.0, of whatever velocity near the 

 surface is taken as the reference or the standard. 



Theoretically, the air velocity over smooth 

 water is zero at the water surface, as for a hquid 

 flonang over a solid surface. Practically, even for 

 model sail boats and sailing yachts having mast 

 heights of 1 ft or less, the actual air velocities at 

 measurable distances above the water surface are 

 large. Taking all things into consideration, a 

 reference height of 6 ft above the water surface is 



274 



