In a message dated 10/9/04 12:01:47 AM, trawlers-and-trawlering-request@lists.samurai.com writes: << Question: Before finding T&T post, I did a Google search and found a ABYC table on the topic. There is a substantial difference - ABYC shows a 30-footer at 80 knots as having approx 5400 pounds of pressure (extrapolate - 42 kts of wind produces 1400 lbs, as wind speed doubles, pressure quadruples thus the 5400 pounds). Assuming the 30 footer has 100 sq ft of broadside area, the formulaic approach renders 2170 pounds at 80 knots, a substantial difference. Any thoughts on reconciling the difference? >> Peter, It all depends on the coefficient of drag assumed. 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 I suspect that the ABYC is using the flat plate coefficient of drag of 1.28 in their calculations to give a conservative estimate.. This might be appropriate for a boat with a broadside aspect to the wind but almost certainly too high for a boat with the wind on the bow. In this case the coefficient of drag for a moderately streamlined recreational boat may be as low as .70. To be on the safe side, choose an anchor rode that is never stressed to more than 25% of its breaking strength at the highest expected wind velocity. This should give you plenty of margin for error. In case you are interested in the power requirements to move against a wind, it takes between 10 and 12 hp. to hold 100 sq. ft. of frontal area with a 1.0 coefficient of drag against a 20 kt. wind. Larry Z