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From: (no name) (no email)
Date: Sat Apr 21 2007 - 10:11:21 EDT
Wind drag on a boat is a function of the projected area at right angles to
the wind, the square of the wind speed, the density of the air, and the
dimensionless coefficient of drag which depends on the shape of the boat. Drag
coefficients have been determined from wind tunnel tests. Some representative drag
coefficients are:
Open parachute (or efficient spinnaker) = 2.0
Hollow hemisphere, concave to wind = 1.7
Flat rectangular plate = 1.28
Wires, cylinders, masts = 1.0
Cargo ship, wind dead ahead = .95
Fishing trawler, wind dead ahead = .9 to 1.05, depending on superstructure,
outriggers, etc.
Streamlined passenger vessel = .70
Recreational trawler = .70 to 1.2, depending on superstructure, masts,
outriggers, etc.
Sphere = .47
Hollow hemisphere, convex to wind = .38
Modern automobile = .26 to .35
Airplane = .09
Using Area in sq. ft., wind Velocity in knots, and the U.S. Standard
Atmosphere for air density, the equation for Drag in lbs. can be written as:
Drag = .00339 x Coefficient of Drag x Knots^2 x Area
In a 20 kt. wind, a boat with an area of 100 sq. ft. at right angles to the
wind with a drag coefficient of 1.0 will have 135.6 lbs. of wind pressure on
its surface. The drag goes up as the square of the wind velocity. A
conservative way to estimate frontal area is to multiply the beam by the height of the
superstructure. An even simpler way is to multiply the beam by 3/4 of the beam.
By this calculation, my Willard has about 100 sq. ft. of area. A Nordhavn 40
has 160 sq. ft. of area. A Nordhavn 47 has about 195 sq. ft. of area. A
Nordhavn 72 has 330 sq. ft. of area.
For each 100 sq. feet of area:
20 Kts = 136 Lbs.
40 Kts = 542 Lbs.
60 Kts = 1220 Lbs.
80 Kts = 2170 Lbs.
100 Kts = 3990 Lbs.
120 Kts = 4882 Lbs.
140 Kts = 6644 Lbs.
160 Kts = 8678 Lbs.
180 Kts = 10984 Lbs.
The breaking strength of proof coil 5/16 chain is 7600 lbs., of 3/8 chain is
10,600 lbs.
The breaking strength of 1/2 nylon is 7500 lbs., of 5/8 nylon is 12,200 lbs.,
but nylon rodes should only be loaded to 1/2 breaking strength to assure
adequate stretch with a margin of safety. All this assumes that the anchor doesn't
drag, the pilings don't break, and the cleats don't pull out of the deck.
Larry Z
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