Seamanship
Anchoring
How much anchor cable to pay out for the depth and weather, in shackles, and how much room the ship needs to swing.
1 shackle = 27.5 m (15 fathoms)
Fill in the fields to see the result.
How it works
Rules of thumb for the length of cable, with D the depth of water in metres (charted depth plus tide): 3D + 90 m in fair weather, 4D + 145 m in bad weather, or 39 √D. A shackle is 15 fathoms, about 27.5 m. More cable holds better; the holding ground, current, wind, the ship's size and the anchor's maker matter as much.
Swinging circle radius ≈ length overall + the cable's horizontal reach (√(length² − depth²)). Keep that clear of shoals and other ships' circles, and check it on the ECDIS anchor watch.
For training and cross-checking. Not a substitute for approved navigation equipment, nautical publications and the ship’s own procedures. Calculations run in your browser and nothing you type is stored; a PDF you download is made on our server and not kept.
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