Yachting Monthly|August 2020
Some sailors swear by furling mainsail systems, others swear at them. Graham Snook looks at ways to keep your furling mainsail in check.
Graham Snook

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 mainsails can now offer more sail area than a traditional slab-reefed mainsail. Be that as it may, every slabreefed 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. In 2018, 38% of boats sailing round the world in the World ARC had furling mainsail, and Hallberg Rassy reports that almost all new owners buying boats over 40ft opt for furling mains, 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.


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 aluminum 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.


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 mains 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.

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.


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August 2020