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.
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.
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. 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. 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.