Jacklines and Tethers Explained: How to Rig Them and When to Clip On

What a Jackline Is Actually For

A jackline — a jackstay, in older and British usage — is a line run along the deck that you clip a tether to, so that you can move from the cockpit to the bow and back without ever being unattached. The tether runs from a harness on your body to a hook on the jackline. That is the whole system: harness, tether, jackline, strong point.

What it is for is more specific than "safety". It is for keeping your body on the deck when the deck tries to throw you off it. A boat heeled at 25 degrees, pitching into a sea at night, with a wet, sloping foredeck and both your hands full of sail, is a machine for putting a person in the water. The lifelines are knee-high. A tether short enough that you physically cannot reach the rail defeats the machine. A tether long enough to let you over the rail does not.

This is where the usual rig fails. On most cruising boats the jacklines run along the side decks, outboard of the shrouds, because that is where the deck is clear. Clip a 2 m (6.5 ft) tether to a side-deck jackline on a 3.5 m (11.5 ft) beam boat, go over the low side, and there is you, the tether and about a metre to spare, in the water, being dragged. At 6 knots the water is moving past at 3 m/s. The drag on a clothed body at that speed is well over 100 kg; you cannot pull yourself up the tether against it, your crew cannot lift you over the rail against it, and your face is being pushed under by the bow wave of your own body. People have drowned this way while still attached to the boat, and their crew have been on deck trying to get them back aboard while it happened.

A run of offshore-race fatalities between 2011 and 2017, on both sides of the Atlantic, involved sailors who were clipped on and went over the side anyway — some dragged, at least one whose tether hook failed. A UK Marine Accident Investigation Branch report into a 2017 death found the tether's snap hook had distorted and opened when loaded sideways across its gate rather than along its length. World Sailing responded by tightening the Offshore Special Regulations on tether hooks, jackstay construction and the release at the harness end. The lesson the sport drew, and the one to carry aboard a cruising boat, was not "tethers are dangerous". It was: rig it so you cannot reach the water, and make sure you can get free if you do.

Rigging the Jacklines

Where they run

Bow to stern, as far inboard as the deck allows. The best position is the centreline: along the coachroof, or from a padeye at the mast base aft to the cockpit and forward to the foredeck, so that a person on a short tether is held in the middle of the boat and cannot reach either rail. Where the deck layout forces side-deck jacklines, run them inboard of the shrouds wherever you can. Outboard of the shrouds you are already a third of the way to the water before you have started.

Stop short of both ends. A jackline that runs to the bow pulpit or to the stern rail lets a tethered person go straight off the end of the boat: over the bow into the path of the hull, or off the stern into the propeller wash. End the forward run a couple of metres short of the stem, and the aft run at the cockpit. A simple check: with the jackline rigged and your longest tether clipped to it, walk the full length of the boat and lean out over the rail everywhere you can. If your shoulders pass the lifelines anywhere, the run is wrong there, or the tether is too long.

What they attach to

Strong points through-bolted to the structure with backing plates: a mooring cleat, a dedicated padeye, the base of a sheet winch or the mast collar. Never a stanchion base: stanchions are designed to be leaned on, not to arrest a 90 kg person who has just been thrown 2 m across the deck, and the fastenings tear out of the deck under a shock load a fraction of what the jackline itself would carry. The World Sailing rules ask for a jackstay breaking strength of roughly 20 kN (about 2,000 kgf); that figure is meaningless if the padeye at the end is held by two self-tappers.

Tension them so that they lie flat on the deck with little sag. A slack jackline is a trip hazard and lets the hook slide outboard. Webbing jacklines usually end in a sewn loop at one end and a lashing at the other; lash with a Dyneema or polyester cord, not the webbing itself, so you can adjust and inspect it.

Material

The table above is the summary. The reason webbing wins is the one that matters at 0300: it does not roll. A round line — wire or Dyneema — underfoot on a wet, heeled deck is a marble. Flat webbing is not. The price is UV: polyester webbing left rigged in the sun loses strength steadily and gives no dramatic warning, which is why the standard advice is to replace it every two or three seasons, sooner in the tropics, and to stow it below when the boat is not sailing. Dyneema resists UV far better and is stronger by a wide margin, but it rolls and it chafes: every place it crosses a deck fitting is a place where the load-bearing outer fibres are being sawn through. Wire is the old answer and is still legal; plastic-covered wire is the one to take off the boat, because the cover hides the corrosion that is eating it.

The Tether

Length

The Offshore Special Regulations cap a tether at 2 m (6.5 ft), and require either a second short leg of about 1 m (3.3 ft) or a separate short tether. That two-leg tether — one long, one short, both from a single point on the harness — is the right kit for a cruising boat too. The long leg is for moving along the jackline. The short leg is for staying put: clipped to a cockpit hard point at the helm, or to the jackline when you are working at the mast, so that you cannot reach the rail at all. Moving forward, you clip the short leg to the next point before you unclip the long one, and you are never unattached.

Hooks

Both ends carry a snap hook. The jackline end must be a double-action hook: one that needs two distinct movements to open, so that a lifeline or a sheet brushing the gate cannot pop it. Look for a hook shaped so that a sideways load cannot bear on the gate; that side-loading was the failure mode identified after the 2017 death, and hooks sold as compliant with the current OSR are designed against it. Plain carabiners and single-action hooks do not belong on a tether.

The harness end is the other half of the argument. The OSR requires a means of quick release under load at the chest, so that a person in the water can free themselves from a tether that is drowning them. It is usually a hook with a release tab or a soft shackle with a pull loop, and it must be operable with one hand by someone being dragged face-down. Check yours does that on a calm day. Then check the tether has the coloured overload flag the OSR requires stitched into it: a tether that has once arrested a fall has stretched, the flag shows, and it is retired.

The harness

On a modern boat the harness is built into the inflatable lifejacket, and that is the right arrangement, because it means you are never wearing one without the other. Two things make it work. The D-ring must sit at the sternum, not the belly, so a fall loads your chest and not your spine. And the crotch straps must be done up. Without them an inflated lifejacket rides up over the head of a person in the water, and a harness loaded from above slides up to the armpits and can pull clean off. Crotch straps are the part of the kit everybody owns and half the fleet leaves undone. Do them up before you clip on the first time and they stop being a decision.

When to Clip On

The commonest way to fall off a boat is not a knockdown. It is stepping out of the companionway in the dark to check something small, in conditions that seemed too easy to bother clipping on for. So the rule that works is not a judgement about the weather. It is a fixed list, and the first item is the one that matters: the hook goes onto the cockpit hard point from inside the companionway, before you climb out.

To that list add: whenever you are reefed. Reefed means the wind is up enough that you shortened sail, and that is the working definition of tether weather. A skipper who says "clip on" once, at the start of the passage, as a standing order for the night watches, has done more for the crew's safety than any single piece of gear aboard.

Cockpit Hard Points

The jackline gets you along the deck. The cockpit needs its own attachments, because the cockpit is where people spend the watch, and a 2 m tether clipped to a side-deck jackline from the helm seat reaches the water on almost any boat. Fit padeyes, through-bolted with backing plates, at the helm and on each side of the companionway, positioned so that the short leg of the tether holds a seated person clear of the rail. Rig them so that the watch can clip on from the companionway steps before coming up, and so that the helm can be handed over without anyone unclipping.

The same thinking answers the question on a powerboat, where jacklines are rare and the deck is not a place people work under way. An open-cockpit fishing boat or a planing motor cruiser running offshore at night still has the same failure case: a person over the side, unnoticed, from a boat doing 20 knots. The answer is not a jackline but a hard point at the helm and at each crew position, with a short tether, used when running at night or in a sea, and an engine kill cord on the driver's leg regardless. The concept is identical: attach to structure, short enough that the water is out of reach.

The Standards

Two documents are worth knowing by name. The World Sailing Offshore Special Regulations (OSR) govern equipment on offshore racing yachts and are revised every two years. They are not law for a cruising boat, but they are the best-researched statement of what works: jackstays of webbing or wire attached to strong points, tethers no longer than 2 m with a short leg or short second tether, a snap hook at each end, a coloured overload indicator, and a means of quick release at the harness end operable under load. Read the current edition's sections on jackstays and on harnesses and tethers; they are short.

ISO 12401 is the international standard for deck safety harnesses and safety lines on small craft. It sets the strength, the hook behaviour, the marking and the testing that a harness and tether must pass, and the OSR requires harnesses and tethers to meet it. When you buy, look for ISO 12401 on the label, and for a lifejacket with an integral harness that is certified to it. Anything sold as a "safety line" without that mark is a length of rope with hooks on.

The Hours Nobody Is Watching

Clipping on is a rule about darkness and inattention more than it is a rule about wind. The tether earns its keep on the quiet night watch when one person is on deck and the rest are asleep, and the same is true of the anchor alarm. Both are for the hours nobody is watching, when the thing that goes wrong goes wrong silently. The night you set an anchor watch on the phone is the same night, an hour earlier, that everyone on the foredeck was clipped on while the chain went down — and the habit is the same one: decide it before it is dark, so that nobody has to decide it in the dark.

The Bottom Line

A jackline's job is to keep you on the boat, not to stop you drowning beside it. Run it inboard, centreline if you can, from strong point to strong point, stopping short of the ends so that a full tether cannot reach the water. Use flat webbing and replace it every two or three seasons. Carry a two-leg tether, short leg in the cockpit, double-action hooks, quick release at the chest, crotch straps done up. Then clip on before you leave the companionway at night, when reefed, when alone, in fog and when told to, and the question of getting a dragged crew member back aboard never arises, because nobody left the deck.