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Ask Sail: The Relationship Between A Boat’s Windage and Anchor Load

In your October 2013 Ask SAIL section (“Bigger is Better”), Don Casey states that doubling a boat’s windage increases the load on an anchor approximately fourfold.
Author:

Robert Britson Panama City Beach, Florida

Q: In your October 2013 Ask SAIL section (“Bigger is Better”), Don Casey states that doubling a boat’s windage increases the load on an anchor approximately fourfold. As a professional engineer designing structures to resist wind loads, all building codes I work with indicate that wind pressure is directly proportional to surface area and is proportional to the square of the wind velocity. Applying the equations to a sailboat, doubling windage would double wind pressure, not increase it fourfold. Doubling wind speed is what would increase pressure fourfold. Am I missing something?

Don Casey Replies

A: You are correct. Wind load is more or less directly proportional to area, so doubling a boat’s wind profile essentially doubles the load on the anchor. However, the point I was trying to make is that when you have two boats of the same length, but one presents a much larger profile to the wind, the difference in the load on the anchor is going to increase exponentially as wind strength increases. When the wind goes from 15 knots to 30 knots, a boat with twice the windage will exert four times the pull on the anchor—a reality not accounted for in anchor charts based on boat length.

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