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Mainsail Handling Systems: In-Boom Furling
by Dave Flynn, Quantum Atlantic
As mainsails get larger, the struggle to raise, lower, reef, flake, and cover them becomes more and more of a battle. Attempts to tame the beast have gone on since man first began to sail. Up until recently, these efforts have focused on breaking sail area down into smaller sizes to keep it manageable. This is one of the key factors behind “split rigs,” like the classic ketch, yawl, and cutter configurations, dominating larger cruising boat design. The last two decades have witnessed a revolution in sail handling systems. Roller furling systems for headsails are now standard equipment on virtually every cruising boat (less than 20% used them in 1980. These systems have been adapted to furl mainsails as well, by putting them inside the mast. While functional from a handling standpoint, In-Mast furling systems limit sail performance, primarily since the sails can’t take advantage of the structural benefits of battens. No longer does the modern cruising sailor have to accept this compromise. In-Boom furling systems not only furl the sail, but they allow the mainsail to be built with no concessions to size, shape, or performance.
Unfortunately, the reputation of In-Boom suffered from early attempts. The Stow boom worked reasonably well under ideal conditions on smaller boats, but was plagued with mechanical and reliability issues. Today, several respected manufacturers build In-Boom systems that work, including Schaeffer, Profurl, and Leisure Furl. Insuring that the system furls properly, and to guarantee reliability, puts special demands on sail design, engineering, and construction. More often than not, a problem with “the system,” is really a problem with the sail. Quantum has focused considerable resources and energy on solving the engineering and design problems involved, and has had the advantage of working with the systems since the earliest iterations.
Today, based on extensive experience, Quantum has developed special expertise in the design and construction of these special sails, including the recent completion of the largest ever built (Frers 140’)
HOW THEY WORK
Modern In-Boom systems are not just headsail furling systems which have been adapted; they are designed from the ground up for the job. They are made up of a rigid mandril set inside a hollow boom. The foot of the sail is attached along a portion of the mandril, and tack and clew are secured at either end.
The mandrill is driven by a powerful drum. The line control for the drum is typically led aft on the deck alongside the mainsail halyard. A single electric winch is usually used for the mandril control line and the halyard (the halyard on the hoist, the control line when furling). Both control lines are run through sheet stoppers.
A special feeder guides the luff of the sail into a sail track attached to the aft face of the mast.
To hoist, ease the control line and raise the halyard. To lower, ease the halyard and wind in the control line. The critical issue is keeping the luff aligned underneath the feeder as the sail is raised and lowered.
The beauty of In-Boom furling is not just in the never leave the cockpit ease of handling, but in the sail itself. A full batten structure is used which allows the designer to create a full-sized sail, with as much roach (area outside the straight line between head and clew), as the sail needs. Sail shape, while it must be tailored to the demands of the system, is as good as a conventional sail. Shape can be controlled with mastbend and by partially furling the sail.
In-Boom furling also allows for infinite reefing, with a wonderfully flat, smooth shape.
THE DEVIL IS IN THE DETAILS
Construction of a sail to fit an in-boom furling system demands special engineering. Batten pocket angles, corner patch assembly, plying to maintain uniform roll tension, luff tape, headboard, tack and clew fittings, batten pockets; in short, almost every detail must be specially adapted to the individual furling system. The most common problem with in-boom systems is not the system itself, but the sail. Not only is it critical that the sail’s geometry is perfect, it is also essential that nothing move initially or over time. This is why composite construction, with its lower rate of stretch, is often used (particularly as the sail size increases).
Frers 140’ custom; the world’s largest in-boom furling mainsail.