The Anchor Doesn't Hold the Ship. The Chain Does.
A 200,000-tonne ship is not held by the hook on the seabed. Most of the holding comes from the chain lying flat on the bottom. The anchor's job is to pin down its end.
Picture a ship at anchor and you probably picture a giant steel hook, dug into the seabed, bravely holding 200,000 tonnes against the wind.
That is not really what is happening.
What holds the ship is mostly the chain, and there is far more of it down there than you would think. A ship anchoring in 20 metres of water does not let out 20 metres of chain. She lets out 100 to 140 metres: five to seven times the depth, as a rough rule.
All that extra chain is not hanging in the water. It is lying flat along the seabed. And ship chain is huge. On a big ship, one metre of it weighs about a quarter of a tonne. That is three or four grown men. Now picture a hundred metres and more of that, pressed into the mud, with the seabed gripping every link. That is what the ship is really tied to.
The anchor does matter. Its main job is to pin down the end of the chain. But it can only do that while the pull comes flat along the seabed. If the chain is too short, the pull comes from above and lifts the anchor out of the bottom. It is like lifting a garden fork by the handle. Then the ship starts to drift, and that is exactly what the officers on anchor watch are looking for.
So the old sailors were right: the anchor holds the chain, and the chain holds the ship.
Next time you see a ship "at anchor" off a port, remember: she is not hanging from a hook. She is resting on a long, heavy chain, and the seabed is holding down metre after metre of it.
Sources: IACS Unified Requirement A1, "Anchoring Equipment" (the table of anchor and chain sizes, and its design assumption of a chain scope of at least six); IACS Recommendation No. 10, Table 3, "Mass of stud link chain cables" (where the weight per metre comes from); and the Admiralty Manual of Seamanship (BR 67, Royal Navy) on anchoring practice.