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Ask SAIL: New Boat, New Anchor

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Robert Weisberg, Frisco, Texas asks:

I recently purchased a Beneteau Oceanis 41 and keep it on a lake just outside Dallas that has mainly a mud bottom. I have a 35lb Delta anchor (which, according to the manufacturer’s website, should be appropriate for sailboats from 44 to 52 feet), 20ft of 3/8in chain and 130ft of additional rope rode. I usually anchor in 8 to 12 feet of water and put out 7:1 scope (measured from where rode enters water, not from the stem of boat). Since buying this boat two months ago, I have dragged anchor on numerous occasions. I had a Catalina 350 before this and never dragged. I have owned boats for 40 years and have done more than 45 bareboat charters on catamarans up to 46 feet and monohulls up to 52 feet and have never had an issue with dragging. I am always very careful to back down to be sure the anchor is holding firmly. It’s hard for me to believe I need a larger anchor, as this one already seems oversized.

 

 

DON CASEY REPLIES

 As a rule, given adequate scope, if your anchor still drags, you need a bigger anchor. Not more scope. Not more chain (although that can help). Not a different anchor, unless you are matching the anchor to a single specific bottom type. You need a bigger anchor. Period.

Tables published by anchor manufacturers are typically wildly optimistic and are based mostly on the need to claim more holding power than competitors. Consider the potential difference in windage between a flush-deck 40-footer and one with high freeboard, a 30-inch stack-pack, a big dodger or cockpit enclosure, a loaded radar arch and an inflatable dinghy hoisted in davits. Doubling windage increases the load on an anchor approximately fourfold, without regard to boat length. Claiming that the same anchor is suitable for both boats is ludicrous. The minimum anchor weight for today’s high-profile sailboats should not be less than 1.5 pounds per foot of boat length, which makes your 35-pound Delta seriously undersized.

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Anchor force vs windage drag

Good response, Don. One thing though, bsbearden has a keen ear... 
The force on the anchor will multiply fourfold with a doubling in wind speed. There are many variables of course, but the force on the anchor only doubles when the frontal area is doubled.
The drag equation defines a linear relationship between the drag force and the surface area.
Just thought you'd want to keep up your usual standard of excellence and accuracy. : )

Doubious

"Doubling windage increases the load on an anchor approximately fourfold". Really? That does not sound right.

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