SailingSeptember 26, 2026• 11 min read• By Safety Anchor Alarm Team

How to Reef a Sail: When, Which System, and the Sequence Step by Step

You reef for the wind you will have in ten minutes, not the wind you have now. A reef put in early costs you perhaps a knot in the lulls. A reef put in late costs you the sail, and sometimes a crew member's fingers on the way to the mast. Every other rule about reefing follows from that one. Here is why a reefed boat is usually faster, when to do it, how the five common systems differ, the slab reefing sequence step by step, how to cope downwind and single-handed, and how to shake a reef out without tearing the sail on a reef tie.

The short answer

Reef when you first think about it, and for most cruising boats that is the first reef around 15–18 knots apparent and the second around 22–25, adjusted for your boat. To slab reef a mainsail: ease the sheet and vang, support the boom with the topping lift or a rigid vang, lower the halyard to its mark, hook the tack cringle, tension the halyard hard, wind the reefing line in until the clew is down and the foot flat, retrim, then tie the bunt up loosely. To shake it out, untie the reef ties first, then reverse the sequence.

Five Reefing Systems Compared

Every system shortens sail. They differ in where you stand to do it, how good the sail looks afterwards, and what happens when something jams. The last column is the one that matters at 2 a.m.

System	How it reefs	Good	Bad	Watch for
Slab (jiffy) reefing	Halyard lowered to a mark; tack cringle hooked at the gooseneck; a reefing line pulls the clew cringle down to the boom.	Simple, strong, cheap to maintain, gives the best sail shape of any system, and the sail can still be dropped if anything jams.	Someone usually goes to the mast. Two or three separate lines to handle per reef.	Chafe at the cringles and where the reefing line turns into the boom. Reef ties taking load.
Single-line reefing	One line runs through both tack and clew cringles inside the boom, so a single pull from the cockpit sets the whole reef.	Everything from the cockpit. Good for short-handed sailing and cold, wet nights.	High friction, so the line is hard to pull and the tack often does not come fully down. Rarely offered for a third reef.	A slack tack with a tight clew, which pulls the draft aft and gives a baggy, powerful shape exactly when you want a flat one.
In-mast furling	The sail rolls around a vertical foil inside the mast; you ease the outhaul and wind in the furling line to any depth of reef.	Infinitely adjustable, all from the cockpit, no bunt to tie. The easiest system to reef early with.	No battens or vertical battens only, so no roach and a poorer shape. A jam with the sail half out is a serious problem in a rising wind.	Furl with the boom at right angles to the mast and steady tension on the outhaul, or the sail bunches and jams. Never let the outhaul run free.
In-boom furling	The sail rolls around a mandrel inside the boom while the halyard is eased.	Full-roach, fully battened main with a good shape at every reef. Can be dropped conventionally if the system fails.	Expensive. Boom angle to the mast must be exact when furling or the sail creeps forward or aft on the mandrel.	Mark the vang or topping lift setting that gives the correct boom angle and use it every time. Halyard tension while furling.
Headsail roller furling (partial furl)	Wind in the furling line to roll away part of the genoa; the sheet lead moves forward on the track.	Quick, from the cockpit, no foredeck work. Fine for taking the edge off a gust.	A partly furled genoa is a baggy, powerful sail with the draft in the wrong place. Beyond about a third furled it flogs and drives poorly upwind.	A foam or rope luff pad reduces the bagginess. Move the sheet cars forward as you furl. For real wind, a smaller headsail or a staysail beats a rolled-up genoa.

Why Reef: the Heeled Boat Is the Slow Boat

The instinct to hang on to full sail comes from a belief that more canvas means more speed. Up to a point it does. Past that point, and on most cruising monohulls the point is somewhere around 20 degrees of heel, three things go wrong at once. The keel and rudder are working at an angle and generating less lift for more drag. The hull is pushing an asymmetrical shape through the water, which makes the boat want to round up. And the sails, tipped over, are presenting less area to the wind than their size suggests while spilling the rest as turbulence.

The rounding up is what you feel as weather helm: the tiller or wheel held permanently to leeward to keep the boat on course. A few degrees of weather helm is healthy. A rudder held at ten or fifteen degrees is a brake the size of a door, and in a gust it stalls, the boat rounds up regardless, and everything flogs. Put a reef in and the boat stands up, the helm goes light, the rudder straightens, and the log usually shows the same speed or more. Racing crews know this and reef for speed. Cruisers should reef for comfort and arrive at the same conclusion.

When to Reef: the Rule and the Numbers

The rule every instructor gives is the right one: if you are thinking about reefing, reef. The thought arrives because the boat is already telling you something, and the cost of acting on it early is trivial. The cost of acting late is a sail flogging itself to pieces while somebody tries to hook a cringle on a boom that is trying to hit them.

For those who want a number, most moderate cruising boats of 10–13 m (33–43 ft) take the first mainsail reef somewhere around 15–18 knots of apparent wind and the second around 22–25, with a third reef or a trysail question arising past 30. Treat these as a starting range, not a spec. A light, beamy modern hull with a big roach main reefs earlier; a heavy long-keeler carries full sail longer; a catamaran, which does not heel to warn you, needs numbers written on the bulkhead and obeyed because it will not give you the feel. Points of sail matter too: a boat comfortable under full main on a broad reach at 20 knots will be overpressed the moment it hardens up onto a beat, because the apparent wind climbs as you turn towards it. Reef before the turn, not after.

Which brings us back to the ten-minute rule. Wind is not a value, it is a trend. A forecast for a front, a squall line on the horizon, a headland ahead that will compress the breeze, a course change onto the wind, or sunset in an area that gets a nightly katabatic: each of these tells you what the wind will be shortly. Reef for that. If you reef at 15 knots and it stays at 15, you have lost half a knot. If you wait until 25, you are doing the hardest job on the boat in the worst conditions of the day.

The Systems, and What Each One Asks of You

The comparison table above gives the summary. A few points deserve more room.

Slab reefing

Slab, or jiffy, reefing is still the reference system. The sail has one, two or three rows of reinforced eyes, the cringles, at the luff and leech. The luff cringle drops onto a hook, the ram's horn, at the gooseneck; the leech cringle is pulled down and aft by a reefing line that runs inside or along the boom. Two lines, one hook, one person at the mast or none if the halyard and reefing lines are led aft and the hook is replaced by a tack line. The shape it produces is excellent because you control luff and foot tension separately, and if anything jams the halyard still runs and the sail still comes down.

Single-line reefing

Single-line reefing threads one line through both the tack and clew cringles so that a single pull from the cockpit does the whole job. The convenience is real. So is the friction: the line turns through six or eight blocks and sheaves, and on many boats the clew comes down beautifully while the tack stops a hand's width short of the boom because the line will not pull any further. The result is a sail with the draft too far aft, which is the opposite of what a reef is for. If you have this system, check the tack after every reef and be prepared to grind the halyard down a touch and pull again.

In-mast and in-boom furling

In-mast furling rolls the mainsail around a foil inside the mast. You can furl away any amount you like, from the cockpit, with no bunt to tie, which makes it the easiest system to reef early with, and early reefing is the whole game. The price is a sail with no roach and at best vertical battens, and a failure mode that is genuinely dangerous: a jam with the sail half out in a rising wind, with no way to drop it. The discipline is procedural. Keep the boom roughly square to the mast, keep steady tension on the outhaul as you wind the furling line, and never let the sail flog while you furl. In-boom furling rolls the sail into the boom instead, keeps full battens and a full roach, and can be dropped conventionally if it fails, but it is fussy about boom angle: mark the vang or topping lift setting that gives the right angle and use it every single time.

Reefing a headsail

Almost every cruising headsail is now on a roller furler, and rolling away a third of the genoa is the first reef most people ever take. It works, up to a point. Past a third furled, a genoa cut for full hoist becomes a deep, baggy sail with the draft in the middle and a clew that has climbed so high the sheet lead is wrong. A foam or rope luff pad sewn into the sail rolls up fatter in the middle and flattens the result noticeably; if your genoa lacks one, ask the sailmaker. Move the sheet cars forward as you furl to keep the leech from twisting open. And accept that for a real blow the answer is a smaller sail, a staysail on a cutter or a storm jib on an inner stay, not a genoa rolled to a handkerchief.

Slab Reefing, Step by Step

The sequence below is for a conventional slab-reefed mainsail. Say it aloud on the mooring until it is automatic, because the first time you need it at speed you will not be thinking clearly. The third column is the point of the table: every step exists because of what happens without it.

The slab reefing sequence. Steps 2 and 8 are the ones that hurt people and sails respectively when they are skipped.
Step	What you do	What goes wrong if skipped
1. Depower	Ease the mainsheet and vang. Head up towards a close reach if you are running.	The sail stays loaded against the rigging; the halyard will not come down and the cringle cannot be reached.
2. Support the boom	Take up the topping lift, or confirm a rigid vang is holding the boom.	The boom drops onto the coachroof or a head when the halyard goes. This is how people get hurt reefing.
3. Lower the halyard	Ease the halyard to the pre-marked position for that reef.	Without a mark you go too far or not far enough, and the whole sequence is done twice in the wind you were trying to escape.
4. Hook the tack	Drop the reef tack cringle onto the ram's horn at the gooseneck, or clip it.	The luff is unsupported; when you tension the reefing line the sail pulls aft and the shape is ruined.
5. Tension the halyard	Grind the halyard up hard until the luff is tight and the tack is held on the hook.	A soft luff gives a scalloped, baggy sail with the draft aft; the boat heels more, not less.
6. Wind in the reefing line	Take the clew cringle down to the boom and along it until the foot is flat and tight.	The clew floats, the leech flogs, and the reef delivers no flattening at all.
7. Retrim	Ease the topping lift, take up the vang and mainsheet, bear away to course.	A tight topping lift leaves the leech twisted open and spilling; you have shortened the sail but not shaped it.
8. Tidy the bunt	Gather the loose sail along the boom and tie it with the reef ties, loosely.	The bunt fills with water and wind and flogs. Tied too tight it carries load and tears the sail when shaken out.

Two details are worth more than they look. First, the halyard goes back up hard. A reefed sail with a soft luff is baggy and powerful, which is precisely what you were trying to get rid of; many boats heel more after a lazy reef than before it. Second, the reef ties, the short cords or the lacing through the small eyelets along the reef row, are there to tidy the bunt and nothing else. They are tied loosely, around the bunt only or around bunt and boom, and they must never carry sailcloth load. The eyelets are not reinforced for it. A reef tie that takes load when the halyard goes up will tear a hole in the sail at that eyelet, and a sailmaker will tell you it is the commonest repair they see.

Reefing Single-Handed: Heave To

Alone, or with a crew you would rather keep in the cockpit, the problem is that the boat needs steering while the sail needs two hands. The answer is to heave to: tack without releasing the jib sheet, so the backed headsail pushes the bow off while the helm, lashed to windward, tries to bring it back. The boat sits roughly 60 degrees off the wind, making a little leeway, quiet and nearly level. The mainsail is in the lee of the backed jib, depowered, and you have as long as you need to go forward and reef without the boat doing anything surprising. Hove to, the whole sequence is a two-minute job. On an autopilot-steered boat the temptation is to reef under autopilot on a close reach instead. It works in flat water; in a sea the pilot will lose the plot at the worst moment and you will be at the mast when the boom comes across.

Reefing Downwind

Running before the wind is where the ten-minute rule bites hardest, because the apparent wind is low and the boat feels fine right up until it does not. When you finally want the reef, the mainsail is plastered against the shrouds and spreaders and the halyard will not come down under any force you can apply. The orthodox fix is to head up to a close reach. The main unloads, the halyard runs, the reef goes in, and you bear away again having lost five minutes and gained a manageable boat. The apparent wind you meet when you head up is a shock after the run, which is why you reef for the wind you will meet, not the wind on the run.

If heading up is not an option, a lee shore close aboard or a sea too big to turn into, rig a preventer from the boom end forward to the bow so the boom cannot gybe while you work, ease the vang and sheet enough to take some pressure off, and accept that the reef will be rough. Or gybe deliberately, under control, so the sail is now on the side you can reach and the spreaders are on the other. Better than both: look at the forecast before you bear away, and put the reef in while it is still a close-reach job in a modest breeze. A reef you do not need on a run is easy to shake out later. A reef you need and cannot put in is a broach waiting to happen.

Shaking a Reef Out

Reverse the sequence, with one absolute rule at the front: reef ties first. Every one of them. Walk the boom and check. Then ease the reefing line, unhook the tack cringle, hoist the halyard to the full-hoist mark, ease the topping lift if you used it, and trim vang and sheet. If a tie is missed and the halyard goes up, the whole weight of the sail is briefly on a cord through an eyelet designed for none of it, and you will hear the result. Shake reefs out one at a time and sail a few minutes between; the wind that made you reef rarely leaves as fast as it arrived, and a reef shaken out at the first lull is often back in ten minutes later.

Marks, Chafe and the Third Reef

Three pieces of preparation make all of the above faster.

Mark the halyard and reefing lines. With the boat quiet, put each reef in properly and mark the halyard at the winch or the clutch for each reef, and the full hoist, with whipping twine or a marker in a colour per reef. Mark the reefing lines the same way. At night, in oilskins, the marks are the difference between a reef that takes two minutes and a reef that takes three attempts.
Check the cringles for chafe. The reef cringles and the cloth around them carry the entire load of the reefed sail and rub on the hook, the boom and the reefing line every time the sail moves. Look at them, and at the leech tape above the clew cringle, whenever the sail is down. A reefing line that has hardened or fuzzed where it turns into the boom is a line about to fail on the night you need it.
Decide about the third reef. Many production mainsails come with two reefs. For coastal sailing in forecastable weather two is defensible. For offshore passages, where you will be out in whatever arrives, you want either a deep third reef that leaves roughly a third of the sail, or a separate trysail on its own track that can be set without touching the main. Offshore racing rules under World Sailing's Offshore Special Regulations require heavy-weather sails for the relevant categories, and cruisers heading offshore do well to meet the same standard. The third reef is the one you hope never to use and the one that, used once, pays for itself.

Deciding Before the Weather Does

The habit that makes a good reefer is not strength or speed. It is a willingness to act on a trend before it becomes an event, and to accept a small, certain cost now against a large, uncertain one later. That habit does not stop when the sails come down. In an anchorage the same front that had you reefing at dusk will swing the boat through 180 degrees at 3 a.m. and load the ground tackle from a direction it was not set for. You cannot reef an anchor. What you can do is decide, before you go below, what movement counts as dragging and have something wake you when it happens, which is what a GPS anchor alarm is for. Set it before you sleep, the way you reef before the gust, and the situation is one you chose rather than one that chose you.

The Bottom Line

Reef for the wind you will have in ten minutes, not the wind you have now. A boat heeled past 20 degrees with the helm hard over is slower than the same boat reefed and upright, so the early reef costs you almost nothing and the late one costs the sail. Know your system's failure mode, mark your halyard, hook the tack before you tension anything, keep the reef ties loose, and take them off first when you shake it out. Do it while it is still easy. That is the whole skill.

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Frequently Asked Questions

When should you reef a sail?

Before you need to. The old rule is that if you are wondering whether to reef, the answer is yes. As a numerical prompt, many cruising boats want the first reef in the mainsail somewhere around 15–18 knots of apparent wind and the second around 22–25, but the figures depend on the boat, the sail area, the crew and the point of sail. The better signals are the boat's own: heel much past 20 degrees, a helm that needs constant weather correction, and a rudder that stalls in gusts. Reef for the wind you will have in ten minutes.

How do you reef a mainsail with slab reefing?

Ease the mainsheet and vang so the sail is depowered, take up the topping lift or rely on a rigid vang so the boom cannot drop, and lower the halyard to the pre-marked position. Hook the reef tack cringle onto the ram's horn at the gooseneck, then tension the halyard hard. Wind in the reefing line until the clew cringle is down to the boom and the foot is tight. Trim the sheet and vang again. Tie the loose bunt of sail up with the reef ties, loosely, so they carry no load at all.

Does reefing make a sailboat slower?

Usually not, and often the opposite. A monohull heeled beyond about 20 degrees is dragging its rudder at an angle, presenting a less efficient sail shape and pushing a larger asymmetrical hull through the water. Weather helm, the boat's attempt to round up, is drag. Reef and the boat stands up, the helm goes light, the rudder straightens and the speed frequently comes back or improves. What you lose is a fraction of a knot in the lulls. What you gain is a boat that is steering rather than fighting.

Can you reef a sail while sailing downwind?

It is difficult and often unwise, because the sail is pressed against the shrouds and spreaders and the halyard will not come down cleanly. The standard practice is to head up to a close reach, which unloads the mainsail enough for the halyard to run and the reef to be hooked in, then bear away again. If you must stay on a downwind course, rig a preventer so the boom cannot gybe, or gybe deliberately first so that you are working on the sail from the side you can reach. Better still, put the reef in before you bear away.

How do you shake out a reef?

In reverse, with one rule that comes first: untie every reef tie before you touch anything else. A reef tie left in when the halyard goes up and the reefing line is eased puts the full load of the sail on a thin cord through a small eyelet, and the sail tears at that point. Once the ties are off, ease the reefing line, unhook the tack cringle, hoist the halyard to its full-hoist mark, then trim the sheet and vang. Then check the reef cringles and the leech for chafe while the sail is up and quiet.

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