Some sailors swear by mainsail furling systems, others swear at them. Graham Snook looks at way to keep your furling mainsail in check

Mainsail furling systems have come on a long way.

Sails no longer need to be wrapped around a boom, nor does an in-mast mainsail need to be the hollow-leached, baggy triangle we first saw decades ago.

Furling mainsail systems can now offer more sail area than a traditional slab-reefed mainsail.

Be that as it may, every slab-reefed sailor knows a horror story about in-mast or in-boom furling that is enough to make them steer well clear, while those that have furling mains wouldn’t put to sea without one.

There is little doubt that furling mainsails are gaining in popularity, even for serious offshore cruising boats.

A damaged furling mainsail

Get mainsail furling wrong and it can lead to costly and potentially dangerous problems. Credit: Graham Snook

In 2018, 38% of boats sailing round the world in the World ARC had furling mainsail.

Hallberg Rassy reports that almost all new owners buying boats over 40ft opt for furling mainsail systems, with Discovery reporting a similar trend.

So has the reputation of furling mainsails been unfairly tainted, and are they more prone to user error, or have the systems ironed out the glitches?

Whether you’ve got a furling main on your own boat, or if you’ll be using one when you charter, it’s worth knowing how to avoid the pitfalls of furling mainsails.

How do furling mainsail systems work?

In an age where we expect everything just to be simple and to work, letting off one line, and pulling another to make the mainsail appear or disappear sounds appealing, but what is the best way to furl the mainsail?

Is there a correct way to do it?

‘Carefully,’ replies Jeremy White,of Elvstrøm Sails UK. ‘They’re mechanical systems and they need to be operated correctly.’

Whether you have in-boom or in-mast, they both work on a similar principle which anyone with a furling genoa will be familiar with.

Inside the mast is an aluminium foil that takes the luff of the mainsail, and in a boom a mandrel takes the foot of the mainsail; both the foil and the mandrel revolve to roll up the sail.

A lot of issues with in-mast furling are caused by the sail not furling properly inside the mast and the furl being too bulky or the sail rubbing on the inside of the mast.

Many in-boom problems are caused by an uneven furl with the sail bunching at one end or the other.

There are a number of issues to look out for with each system to ensure stress-free furling.

Different options

If you’ve bought a new boat that was ‘good value’ and it came with sails, question how good those sails really are.

Many original Dacron sails are built to a price that will get you on the water and get you sailing, but they may not be built for longevity or performance unless you’ve specified them and know what you’re getting.

There has been much advancement in furling mainsail design, improved materials, vertical battens, and increased sail area.

Many new furling mainsail systems present a larger sail area than that of a conventional slab reefing sail.

But what should you be looking for when buying a new sail?

‘Whichever sailmaker you choose, get the highest quality material you can afford,’ advises Jeremy.

It’s a false economy to buy cheaper sailcloth as it will stretch and you’ll be left with a baggy sail after a few seasons.

For example, the luff of laminate sail (on a 45ft yacht) might only stretch 15mm over its lifetime, but on a polyester sail that might be as much as 15 cm.

That excess sail has to roll up in the same space as did when it was new.

For those wanting maximum sail area, and sail support, full-length vertical battens are the way forward.

These support the leech giving a good full roach, and importantly, they support the sail over its full height which gives it rigidity while it’s being furled, whereas shorter, vertical roach battens can leave the sail unsupported at their base causing furling problems.

For those without the budget or desire for a battened sail using modern materials, a sail with a hollow leech still offers many advantages over a slab reefing system, namely ease of reefing, the ability to set exactly the right amount of sail, and the simplicity of stowing, even if you do lose some power from a smaller sail area and a less perfect aerofoil sail shape.

If you’re having new sails made consider getting them silicone-coated.

This helps the sail slide over itself making the furl inside the mast tighter.

In-mast furling

It may be a simple system, but how you unfurl and furl the main will help avoid problems


If you were to look down from the top of the mast, the foil usually rolls onto the foil in anti-clockwise direction, that is, the unfurled sail comes off the starboard side of the foil, though it’s worth checking on yours.

This is the key to getting in-mast furling to work correctly; trying to furl on a port tack drags the full height of the sail over the side of the mast slot, adding friction where there shouldn’t be any.

Furling on starboard tack obviates most of this friction while you furl.

Batten on a furling mainsail system

Full-height battens support a larger roach, but make the furled sail more bulky. Credit: Graham Snook

Whether letting the sail in or out, the first thing is to release the backstay (to straighten the mast so the foil doesn’t rub) and put the boat on a starboard tack – with the wind slightly forward of
the beam – this is so the sail feeds cleanly into the mast and around the furler inside.

Unfurling the sail is usually pain-free if the sail was furled correctly.

A diagram showing an in-mast furling system

With the yacht on a starboard tack and the wind forward of the beam, release the mainsheet and vang.

Ensure the furling line is released then pull out the sail using the outhaul.

A man easing a main sail via winches on a yacht

Ease the outhaul as your furl so the sail doesn’t flog but isn’t loaded up either. Credit: Theo Stocker

You shouldn’t need to control the furling line as there should be no pressure on the sail, even on a windy day.

If you intend to be reefed, however, don’t let it run unchecked. When the right amount of sail is out, make off the furling line. If you’re reefed, tension the outhaul to give the sail the correct shape (flatter in stronger winds and when close-hauled) then set the mainsheet and vang and away you sail.

To unfurl

  • Release the backstay (if you have one)
  • Put the boat on a starboard tack – with the wind slightly forward of the beam
  • Release the mainsheet and vang
  • When the right amount of sail is out, make off the furling line
  • Tension the outhaul to give the sail a correct shape, then sheet in


To furl the sail, after letting off the backstay and putting her on a starboard tack with the wind slightly forward of the beam, let off the mainsheet and then ease the outhaul a little and start to furl.

Always look at the sail as you’re furling – you’ll be able to notice issues as they happen and not after you’ve wound an inch-thick clump of sail through a half-inch gap.

A man sorting out reefing lines on a yacht

Having different coloured lines can make things simpler for your crew. Credit: Theo Stocker

If your sail has full-length vertical battens ensure the first batten is parallel with the mast when it enters, and if reefing, leave a batten just outside the mast groove.

Keeping too much tension on the outhaul will drag the foil aft in the mast, bending it and causing the sail to rub against the inside of the mast, creating friction.

Once you’ve taken the slack out of the sail, ease the outhaul and take in on the furling line again.

Try not to let the sail flog as this also bends the foil and causes more friction.

Repeat the ease-furl process until only the UV protection strip is showing.

A yacht undersail

Sail on starboard with wind forward of the beam for easy furling. Credit: Theo Stocker

If you have laminate sails, and they have been furled away wet, try to dry them at the first opportunity.

If you’re having problems furling using the lines, don’t be afraid to go to the mast with a winch handle and furl the sail at the mast.

Try it one day, it is remarkably easy.

If you’re having to do anything different, such as raising the boom or chanting a prayer to the god of furling fails, it’s worth looking at your system in detail for problems.

To furl

  • Release the backstay
  • Put the boat on a starboard tack – with the wind slightly forward of the beam
  • Release the mainsheet and vang
  • Ease the outhaul a little
  • Take in on the furling line
  • Keep easing the outhaul and taking in on the furling line
  • Furl the sail until the UV strip is showing at the mast

Problem solving

If furling the right way still isn’t working for you, there are a number of things to consider…

1. Understand your system

First to check is to have a look inside your mast at which way your system should furl.

If your furling system has the option, put a winch handle in the furling mechanism at the mast and turn it the direction indicated to make sure the sail is going into the mast in the correct direction.

Clicking over the ratchet at the mast before it’s time to furl will ensure it always rolls in the right direction.

2. Assess your sails

The biggest cause of problems is the sail itself – how old it is and the material it is made from.

Stretch in the cloth makes baggy sails, which furling systems will happily munch on.

Furling mainsails are cut flatter than conventional slab-reefing sails as accommodating the belly of the sail is problematic.

A baggy sail

An old or baggy sail may cause jams, as will creases from not enough halyard tension. Credit: Theo Stocker

Some older furling mains may have be made with an inappropriate, fully-bellied shape.

If your polyester sails have a deep belly, think about getting new ones as you’ll be fighting a losing battle.

As the belly folds, it doubles the thickness of the furl, causing unsightly and inefficient creases at best, and hideous sail jams or rips at worst.

3. Adjust halyard tension

Excess halyard tension can also cause the fabric to bunch up; vertical creases at the luff cause the sail to fold over itself.

A sail showing halyard tension

Vertical creases at the luff reveal too much halyard tension, and potential for more snags. Credit: Theo Stocker

To resolve this, release the halyard until you have horizontal creases at the luff, then add just enough tension to remove them, though you may need to adjust this when underway.

4. Check the backstay

While the mast is bending, the foil inside it remains straight; the furled sail will bind at the apex of the mast’s bend.

If all of this fails, it’s worth calling a rigger to check the foil tension.

If this has gone slack, as you furl the foil will bend and rub against the mast.

5. Smooth it out

The next thing to look at is reducing friction.

As is often the case, the lines to your furling gear and outhaul are led through various fairleads and blocks across the deck and up the mast.

Reefing lines on a boat

Move deck organisers to give slacker turning angles. Credit: Theo Stocker

Make sure all the angles they have to go through are a wide as possible – consider moving them if not – and all blocks and sheaves are running smoothly.

A good wash with fresh water and a squirt of dry lubricant can work wonders.

In-boom furling

Not a new concept, in-boom furling is an elegant solution, but brings its own challenges

Unlike in-mast furling, in-boom systems can be retrofitted in place of conventional slab reefing.

For an in-boom furling system to work efficiently, however, it has to overcome a number of problems.

In boom furling system on a yacht

To start with, have a crew on deck to watch the sail as it furls to spot any issues. Credit: Advanced Rigging and Hydraulics

First of all, the sail has to be led from the boom to the mast, but there needs to be space for the bearings for the central mandrel and the boom’s gooseneck fitting, so the whole sail has to move aft along the boom.

To combat this, many in-boom systems have a protruding track on the trailing edge of the mast, while other units have the reefing mechanism at the aft end of the boom, or sometimes you’ll find a combination of both.

Whatever the system, there is usually a flexible feeder to guide the sail from the boom and feed it into the mast track.

A furling drum on a sail

The furling drum at the outboard end of the boom brings the main closer to the mast. Credit: Advanced Rigging and Hydraulics

Another issue with in-boom furling is the bolt rope, as Andy Cross from Crusader sails explains.

‘The sail has to use a bolt rope, and with it comes friction. Unlike a furling genoa that may only be raised and lowered once a season, the mainsail is nearly always used so the luff tape has to be reinforced.’

Any wear or damage to the bolt rope also requires a new bolt rope along the full luff of the sail, as any repairs would soon wear through and increase the friction.

The necessary extra reinforcement at the luff brings with it another problem: extra cloth thickness at the front end of the sail.

Boom on a yacht

When reefing, stop when a batten is just above the mandrel, then take in on the furling line. Credit: Graham Snook/Yachting Monthly

As the sail rolls around the mandrel there is more sail material at the luff than across the rest of the sail.

The solution?

To slightly raise the aft end of the boom, allowing the extra sailcloth at the luff to roll at a rate that matches the leech.

The angle from the mast to the top of the boom has to be 87° to the mast, 3° above perpendicular.

The full-length battens in the mainsail help stabilise the sail as it furls and the batten pockets have been attached to the sail to match the mandrel angle.

It’s the thickness of cloth at the luff and the battens that make the correct boom angle the most important part of the system.

sail on a yacht

The necessary bulk of the boltrope makes the boom angle critical.

Get that right and your life suddenly becomes a whole lot easier.

It’s essential to mark the vang when the boom angle is correct.

Some owners choose to make a strop, running next to the vang, out of a low-stretch material like Dyneema, so the topping lift can be pulled taught and the strop prevents the boom raising higher than it should.

When marking or limiting the boom angle, it must be easy for any crew to see, by day or night.


How to get it right

1. Prepare to set sail

‘Before raising, lowering, or reefing the mainsail,’ explains Kim Petersen, Elvstrøm Denmark’s in-boom sail specialist, ‘get into the habit of always releasing the backstay tension – this will successfully straighten the mast and takes any flattening tension out of the sail – and then making sure the boom is at the correct angle – this is extremely important.’

Electric winch control systems at the helm

Be careful with electric winches not to over-tighten halyards. Credit: Graham Snook

To raise the sail, after slackening the back stay and adjusting the boom level, point the boat into the wind, release the mainsheet and take up on the mainsail halyard, making sure that the furling line can run free as you hoist the mainsail.

Once set, increase the halyard tension until the horizontal creases at the luff have just gone.

2. Reducing sail power

If you don’t need full sail, only raise the sail until the nearest batten is at the mandrel.

Rather than being able to reef at any point, where the lower battens are fitted, the sailmaker will have reinforced the sail to take the clew loads.

Not reefing at these points means an area of unreinforced leech could be required to take a load it was not designed for.

A yacht with a furling mainsail

Each batten represents a reefing point. Credit: Graham Snook

You’ll end up with fullness at the foot of the sail and a stretched leech, or a damaged sail.

If you need to flatten the sail, for better pointing or in stronger winds, take in on the furling line without adjusting the halyard tension.

This will give the same result as using a cunningham to tighten the luff.

Furling the main

When lowering the sail, release the backstay and set the boom at the correct angle.

It’s then best to take all the pressure off the sail by heading into the wind.

It doesn’t matter if the sail is flogging; the battens keep the sail rigid and support it as it furls.

If there is any pressure on the sail, this will cause it to furl unevenly.

In-boom reefing on a yacht

With in-boom reefing you should still have a good sail shape even when deeply reefed. Credit: Graham Snook/Yachting Monthly

Unlike an in-mast furling system, where the sail is visible, on an in-boom system everything is happening on top of the boom, overhead and out of sight.

If you have a crew member spare and it’s safe, sending them to the mast to keep an eye on the sail as it furls can prevent damage, at least for the first few times using the system.

Pull in on the furling line, slowly and smoothly releasing the mainsail halyard, but keep a bit of tension on it.

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If you have too much tension the sail will want to roll away from the mast; if you’re seeing creases running from the bottom of the track to the boom at 45°, release the halyard a little more.

If the sail is rolling up toward the mast, you’ll need to increase the tension a little.

As the sail furls, the luff tape will naturally first roll aft, then move forward and repeat this – it’s all perfectly normal.

Once the sail is fully down, tuck the head into the boom if your system allows it, and add the sail cover.

Reefing underway

To reef when sailing, release the backstay and set the boom angle.

In rough weather, or when there are big seas, it’s best to take up on the boom’s topping lift to secure the boom and stop it rising and falling as the yacht goes over the wave crest.

A sail being protected from the sun with a metal cover

Cover the sail once stowed to protect from UV. Credit: Advanced Rigging and Hydraulics

You’ll find it easier if you can bring the boat onto the wind and release the mainsheet to remove all drive from the mainsail; if the sail is flogging it’s depowered and can still be furled.

Next, take in on the furling line while slowly and smoothly easing the halyard as the sail furls.

Graham Snook

Graham Snook is a marine photographer and journalist who has been involved in testing yachts and equipment for over 20 years. Credit: Graham Snook

Once you’ve reached a point where the batten is at the mandrel on top of the boom, make off the halyard and then furl the sail until the batten is under the mandrel.

Without reefing pennants to hold and support the clew of the sail, the loads are transferred to the batten and the cloth around it.

Because of this, it’s recommended that for in-boom reefing mainsails, a stronger stretch-resistant cloth like Dacron reinforced with Vectran or Dyneema or a tough cruising laminate cloth be used.

Whichever mainsail furling system you have or choose, spending a bit of time practising what works and what doesn’t on your system, at a time when it’s convenient to you, will pay dividends when you find you do need to reef.

Mainsail furling has had a bad reputation in the past, but used properly and with a little care, there’s no reason why it shouldn’t give you trouble-free sailing for years to come.

In-boom furling

To raise the sail

  • Release the backstay tension
  • Make sure the boom is at the correct angle – use a strop or mark the vang if necessary
  • Head up into the wind – it doesn’t matter if the main flogs as it goes up
  • Release the mainsheet
  • Take up on the mainsail halyard, but do not overtighten

To lower the sail

  • Release the backstay tension
  • Make sure the boom is at the correct angle
  • Point the yacht into the wind
  • Take in on the furling line as you ease the halyard at a steady rate