Gone Outdoors

How to Calculate Chain Length

by Will Charpentier

When you're anchoring your boat, you know that you should let out anchor chain equal to four to seven times the depth of the water you're anchoring in. If you know how the length of chain is calculated, this will make your anchoring process more precise, allowing you to let out the correct length of chain for the depth of water you're anchoring in at a glance. The basis for calculating that length lies between your hands.

Grasp the end of a length of chain in your hands, then spread your arms wide while holding the chain steady in your left hand: this is called, "pulling a fathom." The original measure of a "fathom" was the distance between a seaman's hands when they were spread thusly. Since that time, while the length of a fathom has been standardized to six feet, the measure between your hands is close enough.

"Pull" 15 fathoms of chain (wear gloves while you do this). Fifteen fathoms of chain equals one "shot." Take the paint brush and paint one of the links in your right hand red.

Pull 15 more fathoms of chain. After you pull that shot, paint two of the links in your right hand red. Pull another shot of chain and, with the paint brush and red paint, paint three links in your right hand red.

Use your calculator to figure out how much chain you've pulled through your hands: 1 fathom = 6 feet; 15 fathoms (1 shot) = 90 feet. Three shots is 270 feet. Don Casey of the Boat Owners Association of the United States says that using an all-chain anchor means that a 4:1 chain to depth ratio is suitable for most conditions. Divide this length by 4 (for a calm anchorage) and you have enough chain to provide a stable anchorage in about 67 feet of water.

Divide 3 shots of chain (270 feet) by 7 (for a rougher anchorage, or for an overnight anchorage where the sea might build) and you have enough anchor chain to anchor securely in about 38 feet of water. This means that you should let out about one shot of chain for each 13 feet of water depth.

As you prepare to lower your anchor, measure the length of the chain (called the "scope" of your anchor) by counting the number painted links, letting out a shot of chain for each 13 feet of water below your keel.: one painted link on the first, two painted links on the second, and three on the third.

Items you will need
  • Paint brush
  • Red enamel paint

Tip

  • Other colors of paint may be more visible; the color isn't material, the number of painted links is.

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.

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