EM 1110-2-1100 (Part II)
30 Apr 02
(c) During a storm, it is likely that some water will be entrained in the air. This would significantly
increase the air density. Some investigators have argued that the entrained water will also slow down the air
so there would be no net increase in wind drag. This issue is still unresolved. Current practice is to neglect
entrained water when specifying the air density in the wind drag calculation.
(d) Values for the drag coefficient depend on the vessel geometry and orientation to the wind, and have
been determined from wind tunnel tests. CD values show significant variation, so reference should be made
to Benham et al. (1977), Gaythwaite (1990), Isherwood (1973), Naval Facilities Engineering Command
(1968), Owens and Palo (1981), and Palo (1983) for information on drag coefficients for a variety of vessels.
These references also give some information on projected areas for common types of vessels.
Table II-7-6 lists drag coefficients for wind (Bruun 1989).
Table II-7-6
Drag Coefficients for Wind Force
CD
Wind
Max.
Min.
Mean
Direction
Crosswise
1.40
0.80
1.11
Bow
1.04
0.62
0.82
Stern
1.02
0.64
0.77
(e) The wind speed at the standard elevation (10 m) is commonly used because this would be a good
reference elevation for most larger vessels. If the center of pressure of the vessel is at a significantly different
elevation than 10 m, the 1/7th power velocity profile law may be used to correct the wind velocity. It is
defined by
0.11
z
Vz ' V10
(II-7-32)
10
where Vz is the velocity at the desired elevation z. Typically, a 50-year return period wind speed is used or
the limiting wind speed for which a vessel might remain moored would be used.
(f) The gustiness of the wind must also be considered; i.e., what duration wind gust is sufficient to
envelop the vessel. To account for gustiness, the drag coefficient is often increased by a factor of about 1.4
(Tsinker 1986).
(g) Commonly, current drag forces are determined with less certainty than wind drag forces. Current
speeds and directions at the mooring site may be difficult to predict, particularly in a harbor having a complex
layout. Currents may be continually shifting if dominated by the tide or river flow. The Reynolds number
for currents acting on a vessel is usually in the fully turbulent region, but often close to the transition point
where drag coefficients show a wider range of scatter. The vessel draft and resulting bottom clearance can
also have a significant impact on the resulting drag force. Gaythwaite (1990) presents several sources for
current drag coefficient and vessel projected area information. Seelig, Kreibel, and Headland (1992) have
published a recent analysis of new scale-model drag studies and other data collected over the past five
decades.
II-7-70
Harbor Hydrodynamics
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