The engineering of in-boom furling systems has been considerably refined over the years and today’s systems are much better than early iterations, writes Rupert Holmes
Key advantages include one person being able to handle the mainsail entirely from the cockpit, especially if powered coachroof winches are available. There are no lazyjacks to snag battens and no messing with the miles of reefing pennants that are needed for slab reefing.
Yet these systems don’t need a sail with the hollow leech and very flat cut that’s necessary for in-mast furling. They therefore have the potential to offer similar sailing performance to boats with conventional slab-reefed mainsails.
Given how neatly the sail is furled, and the absence of highly loaded reef points in the leech, in-boom systems also have potential to increase a sail’s lifespan.
Unlike in-mast furling, all the mechanical elements can be reached from the deck, which facilitates inspection, service and repair, while minimising weight aloft.
If the system does fail, the sail can be dropped and secured around the boom in the same way as a conventional mainsail without lazyjacks. However, the lack of luff slides or cars to keep the front of the sail under control can make this a much more difficult task than for a conventional cruising yacht mainsail.
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Despite the obvious advantages there’s still a lot of bar-room discussion about the pros and cons of in-boom furling. Most issues are caused by problems with the initial set up, or by incorrect handling procedures.
It’s critical to have a correct lead for the sail’s luff tape as it transitions from the rolled up position within the boom to the track on the aft face of the mast. To achieve this the boom must be held at exactly the correct vertical angle to the mast when the sail is hoisted, lowered or reefed.
If the angle of the boom is wrong the luff tape walks backwards or forward on the mandrel. If the outer end of the boom is too low the sail moves forward, whereas if it creeps aft the boom is set too high.
At first sight the latter may appear to be a less obvious problem, but it risks ripping the luff tape from the sail. I’ve seen owners blame sailmakers for this, when clearly the issue was one of initial set up.
For most boats this magic angle between mast and boom is in the range of 86-89°. Some systems are set up with a solid vang that holds the boom permanently at the correct angle, but this means twist in the sail can’t be controlled. Others have systems that enable the boom to be reliably, quickly and repeatedly returned to the correct level.
Most manufacturers include a suitable system when supplying their equipment, although old-school systems, without a gas vang, might simply rely on a mark on the topping lift. If the kicking strap is then pulled on tight the boom will be at the correct angle, although creep and stretch in the lines have the potential to make this solution a little hit and miss.
Boats fitted with a rigid vang that supports the boom may have a Dyneema strop of exactly the right length that stops the boom rising higher when the correct angle is reached as the kicking strap is eased.
“Furling booms are heavier than normal booms so it might be that the existing vang is a bit too weak for the new furling boom, which can lead to it ‘dancing’ while trying to furl the sail,” cautions Johan Mulder, the CEO of Romar Leisure Furl.
He also advises using extra thickness in the leech to help the sail furl neatly. “This can be in the form of a sacrificial UV-strip, which then obviates the need for rigging a boom cover to protect the sail.”
After fitting in-boom furling it’s clearly important to spend time checking and, if necessary, refining the set up. It’s worth getting the sailmaker and rigger who installs the system to collaborate on this aspect and demonstrate how to use the system as configured for your boat on a sea trial.
After this initial set up and handover it’s also worth practicing on your own so that any queries can be dealt with before undertaking a major trip.
Perfect set up, however, is no guarantee on its own that the system will be trouble free. The process of using in-boom systems is not complex, but the correct procedures must be followed every time.
As already mentioned, for boats with adjustable vangs the boom must be set at the correct height before undertaking any operation. Then when hoisting the sail (or letting out a reef) a little tension must be kept on the furling line to avoid the luff tape sagging in the gap between the mandrel and the bottom of the mast track. Equally, when lowering the sail a little tension must be kept on the main halyard – in effect, the sail is pulled down, rather than dropped.
If the system works well in flat water, but not in a seaway, this may be indicative of the end of the boom moving up and down and therefore not remaining at a constant vertical angle to the mast. The topping lift may therefore need to be tensioned more tightly against the kicking strap, or the vang’s gas spring may need to be replaced, so that the boom is held more solidly in position.
A lot of systems offer control of the draft in the bottom third of the sail, although this is rarely achieved with a conventional clew outhaul. A common method is to rotate the mandrel just enough to wind in some material from the middle of the foot, but not at the tack or clew, thus flattening the middle of the lower part of the sail.
In theory in-boom systems can give an infinitely variable amount of sail area. However, in practice, reefing down to each batten position gives a better shape for a sail with full battens.
The downsides of a well-set up system appear to generally be fairly small, with the exception that some systems can’t be used to reduce sail when sailing downwind. Rounding up in a big sea to bring the apparent wind forward of the beam is never fun, so in my view the lack of ability to reef a cruising yacht when running downwind is a serious weakness.
In addition, hoisting and lowering sail may take longer than with conventional systems. This process is best done with a view of the sail, especially the gooseneck area, which may be problematic for single-person operation on boats with fixed sprayhoods.
Batten compression on the mast track can create friction and accelerate wear of the luff tape, although some systems are designed to minimise this problem. Arguably a bigger issue is that in-boom reefing is still produced in small volumes, even by marine industry standards, and is therefore expensive. This is especially true for the best kit that’s engineered to be robust, snag free and look good.
Owners looking at retrofitting in-boom furling will also need to factor in the cost of a new mainsail. This needs to be fitted with a luff tape that fits the internal diameter of the mast track, while a different amount of luff round may be needed compared to a standard mainsail. The foot also needs to be cut to match the precise angle between the boom and mast and battens must lie parallel to the mandrel.