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:
Publish date:
Updated on

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.

HR-D-Casey-gray_2

Got a question for our experts? Send it to sailmail@sailmagazine.com

Related

sailingabove-2

Swan Flyer: A Hot New One-design

In a racing scene that’s bristling with innovation, legacy builder Nautor’s Swan refuses to be left behind in its quest to dominate the champagne end of one-design sailing. Arriving hard on the heels of the radical Juan Kouyoumdjian-designed ClubSwan 50, the ClubSwan 36 offers a ...read more

Maiden sets out for Uruguay during the 1989-90 Whitbread race

Making Maiden and the 1989-90 Whitbread Race

As recently as the late 1980s, the idea of an all-female sailing team in the Whitbread Round the World Race (the predecessor to the Volvo Ocean Race) seemed laughable to many. How could women handle the competition? They weren’t strong enough. They wouldn’t be able to take the ...read more

01-LEAD-J99

Four Very Different New Boat Designs

Following up on the J/121, which won a SAIL Best Boats award in 2018, the new J/99 represents a similar concept in a smaller package. Specifically, the new 32-footer’s deck layout and rig have been optimized for smaller and even doublehanded crews, with an eye toward meeting the ...read more

Dinghy Suggested CROP

Ask Sail: Dinghy Dilemma

DINGHY DILEMMA Q: We are in the throes of choosing a dinghy, and I would like to ask if you would recommend buying a RIB with a double-skinned hull rather than a single-skinned hull. Which provides better handling or stays drier? Also, aside from the heavier weight of a ...read more

Groupe Beneteau charging its boats in NEOLINE vessel

Transporting Sailboats Under Sail

Transporting sailboats under sail? That sounds like a cool concept, and it’s one that looks set to become reality in 2021 when shipping company Neoline brings its sailing cargo ships into service. Groupe Beneteau has committed to transporting boats between Europe and the United ...read more

01_vor120612_ross_0644

Sailing Master Ken Read

Images trigger memory. Preparing to interview the golden boy of American sailing, I thought I would find a picture that would show Ken Read at the peak of his sailing career, his heyday, to share and have a warm and fuzzy start to our conversation. It was a commanding image from ...read more

New-engine-being-lifted-over-stern

Know how: Replacing the Auxiliary Power System

One of the most complex tasks undertaken during Passion’s refit was the complete replacement of her auxiliary power system—engine, V-drive and fuel tanks. I needed more horsepower, which drove the need for more fuel capacity and a larger V-drive to handle the higher engine ...read more