Mooring Winches — Why the Brake Is Set to Slip

A moored ship does not hang on her winch motors — she hangs on the friction of hand-set band brakes, and those brakes are deliberately set weaker than the lines so the drum slips before a line can part. Tap a step or press "Play" to follow the load from the drum, through the band and its anchor, out along the line to the shore bollard — then take the simulator and set the brake yourself against a rising surge: get it right and the winch renders; get it wrong and the line comes back across the deck.

brake · line: all fast · motor off · on the brake

Step 1 / 10

One job — hang on

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Key —

Tap a step

Why tighter is not safer — the over-wound brakerender = 0.60 × MBL

Common mistakewinding the brake down "as hard as it will go". An over-set brake cannot render — the next big surge takes the line to 100% and it parts at full stretch, with full snap-back.

Take a 64 t MBL line. The brake is set — and proved with the jack-type brake test kit, annually and after any lining work — to render at 0.60 × 64 ≈ 38 t, measured with the line on the first layer of the drum. That leaves a 40% reserve: in the worst surge the drum slips at 38 t and the line never gets near its breaking load. Now wind the same brake down as hard as two sailors can swing the wheel and it may well hold 70 t or more — above the MBL. Nothing slips any more, so the next overload has only one place to go: the line itself parts at 100%, at full stretch, with all that energy released across the deck. A mooring brake is the one machine on board you deliberately set to lose — and remember the figure only holds on one layer with the line reeved the marked way: three layers can quietly turn your 38 t into under 30.

Try it — set the brake against the surge

A moored ship, one line to a shore bollard, and a swell working her along the berth. You set the brake (as % of the line's 64 t MBL), the layers on the drum, and the surge load. Keep the brake at 60% and an overload makes the drum render — tension capped, line safe. Wind it to 100%+ and the brake can never slip: the line takes everything and parts at 64 t. Or switch to AUTO (self-tension) and watch the winch hold its little setpoint perfectly while the ship walks away down the berth.

Brake setting % of MBL

60%

Layers on the drum working section

Surge load t · swell, tide, passing ship

15 t

Mode brake / self-tension

BRAKE

eff. brake hold38.4t · surge peak15t · margin to parting+49t

line tension 15 t → drum IDLE

the brake holds until the line load beats its render point — then the drum slips and pays line out, capping the tension below the line's strength. wound to 100%+ it can never slip: the line takes everything. layers move the render point down without touching the handwheel

Line tension15t Eff. hold38.4t Paid out0.0m Ship drift0.0m Peak load15t

Marks on the tension column — brake render 38 · line parts 64 t. The render mark moves with your brake setting and the layers — the parting mark never moves.

Effective holding = setting × 64 t × the layer penalty (≈ −14% per extra layer). While the surge stays below it, the drum stands still. Above it — if the brake is set under 100% — the drum renders: tension is capped, line pays out, the line survives. In AUTO the winch renders at its ~10 t working setpoint and heaves the slack back in the lulls — the tension always looks fine while the ship drift gauge quietly climbs: tension held, position lost.

READY — set the controls, then press Start

Set the brake, layers and surge — then press Start to run the surge against your settings. Try 60% and the drum renders; wind it past 100% and the line parts.

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.