Windlass & Anchoring — The Chain Holds the Ship, Not the Anchor

The windlass drops and weighs the anchor — but it is not what holds the ship, and neither, on its own, is the anchor. The holding power lives in the pair: flukes buried in the seabed plus a length of heavy chain lying flat on the bottom, its sag — the catenary — keeping the pull on the anchor horizontal and soaking up the gusts. Tap a step or press “Play” to follow the machinery from the motor and dog clutch to the brake, stopper and hawse, and the physics from the flukes to the scope rule — then take the simulator and try to hold a ship in a rising wind with too little cable.

Ground tackle · flukes buried · pull horizontal

Step 1 / 9

The anchor is not a hook

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Key —

Tap a step

Scope, worked — 20 m of water is not 20 m of depth7 shackles, not 4½

Common mistaketaking depth from the echo sounder alone — the catenary hangs from the hawse, 10–20 m above the water on a big ship. In 20 m of water your scope depth is nearer 30–40 m.

Anchoring in a charted 20 m: the echo sounder is honest about the water, but the cable does not hang from the waterline — it hangs from the hawse pipe, and on a large ship in ballast that is another 10 m or more up. The depth that sets your catenary is hawse to seabed: 30 m. Six times scope gives 180 m of cable; at 27.5 m per shackle that is 6.5 — and since you cannot veer half a shackle short, seven shackles in the water. Work from the sounder alone and you would call it “4½, say 5” — some 60 m of chain that should have been lying on the bottom as your spring and your horizontal pull, and is not there the night the wind gets up.

Try it — hold her in a rising wind

You are anchored with the wind on the bow. Set the depth, veer or heave shackles of cable, and bring the wind up, then press Start — the physics does the rest: the wind load tries to straighten the catenary, and the chain lying on the bottom is your reserve. Keep chain on the ground and the pull at the anchor stays horizontal; run out of reserve and the shank lifts, the flukes lever out, and she drags. Pay out less cable than the depth and you have not anchored at all.

Depth m · charted water

20 m

Cable shackles · 27.5 m each

5 (138 m)

Wind knots · steady + gusts

15 kt

Holding ground good / poor

MUD · GOOD

cable138m · depth to hawse30m · scope4.6×

wind load 10 t vs holding 108 t → HOLDING

holding = buried anchor (derated as the shank lifts) + friction of the chain lying on the bottom — and with no catenary reserve left, gusts arrive as snatch loads, so the effective wind load climbs too

Scope4.6× depth Chain on bottom69m Wind load10t Holding108t Dragged0m

Rules of thumb on the gauges — scope 5–7× depth-to-hawse (10× heavy weather) · beyond ~50 m walk the anchor out · brake ≈45% · stopper ≈80% of cable breaking load.

The model is the real trade-off, simplified: wind squares with speed; the catenary needs a certain hanging length for a given pull and depth, and whatever cable is left over lies on the bottom as reserve and friction. Too little scope → the chain comes bar-taut, the pull at the anchor turns upward, the flukes lever out and she drags. Cable shorter than the depth → the anchor never touches down at all. Veer cable to fix both — that is the whole lesson.

READY — set the controls, then press Start

Set the depth, the cable and the wind to build your scenario — then press Start to bring the weather up and see whether the ground tackle holds.

Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; varies by ship/model. Reviewed and verified by a Master Mariner.