After nearly two decades of being frozen out of mainstream yacht racing (as rule-makers, rightly or wrongly, sought to slow the cycle of obsolescence), carbon-fiber spars have finally made it to center stage. IMS yachts and the hottest offshore one designs now feature carbon rigs, although the Whitbread 60 class still prohibits them.
Interestingly, experience with the latter has demonstrated that a highly customized alloy rig with tailored panels, multiple spreaders, etc., is only marginally heavier and more limber than its carbon equivalents. But carbon fiber is proving spectacularly successful when it comes to unconventional rigsconfigurations that would never have been viable without this exceptionally strong, stiff, and lightweight material. In 1986, Garry Hoyt’s book Ready About was published. A stinging critique of the sailing industry, Ready About contends that conventional rigs are as antiquated as the struts-and-wires wings of the biplane era.
Hoyt’s pioneering work with the Freedom line of cat ketch cruisers failed to demonstrate a performance advantage for unstayed rigs based upon large, round sections. However, it did establish that carbon fiber could be used to build reliable, exceptionally low-maintenance rigs with convenient handling characteristics. For a clear-cut efficiency gain, most designers now believe it is necessary to progress to a rotating mast with an airfoil cross sectiona step that usually requires advanced materials and fabrication techniques.
Wing masts, stayed and otherwise
Most airfoils, including thin membrane sails, generate their peak pressure gradients (i.e., lift forces) in a narrow zone just 25% to 30% of the chord length behind the leading edge. For this reason, the turbulence produced by an ordinary fixed mast does disproportionate harm to a mainsails efficiencyan estimated 20% to 40% loss of drive when compared with a headsail of equal area. This loss can be greatly reduced by mounting a teardrop section mast on a pivot and causing it to rotate until the curvature of the leeward surface of the spar fairs smoothly into the leeward surface of the sail. This is the principle of the over-rotated maststandard equipment on countless beach cats and quite a few larger multihulls.
A typical rotating mast will have about a 2:1 chord-to-depth ratio (cross-sectional length to width). The next step in a logical progression is to increase the chord-to-depth ratio, so the mast itself will more closely resemble the symmetrical wing section of an aerobatic aircraft. Chord-to-depth ratios for contemporary wing masts typically fall between 3:1 and 5:1.
A true wing mast represents worthwhile sail area in its own right, and in many cases will generate forward drive that is disproportionate to its actual area. In extremely high-wind conditions, this advantage can become a serious liability because the typical three-wire staying arrangement that supports most rotating rigs also prevents the mast from being weathercocked to a neutral position when the wind is appreciably aft of the beam. When this happens, the boat can become dangerously over-canvassed, even with no sail set.
Monohull sailboats as a group have much less sail-carrying power than multihulls and could therefore benefit significantly from the improved efficiency of rotating rigs. But perhaps the aforementioned safety concerns have discouraged some designers. However, the chief reason why these rigs have very rarely been used on monohulls is a troublesome engineering problem: how do you go about staying a mast on a narrow hull without using spreaders to open up the shroud angles? In a typical rotating rig for a multihull sailboat, side forces are sustained by just two shrouds that lead far outboard to the edges of a wide platform. The resulting shroud angles are very favorable, typically between 30° and 40°. Shroud wire tension and mast compression stay comfortably low. But even with the help of fixed spreaders, monohull shroud angles rarely exceed 15°. And without spreaders, compression loads on most monohull masts would skyrocket to critical levels and beyond. To use a stayed rotating mast on a monohull sailboat, the designer must find a way to alter the basic rigging geometryeither by shortening the mast(s), leading the shrouds to outriggers, or a bit of both.
One of the first contemporary monohulls to be equipped with rotating wing masts is a 50-foot racer designed and built in 1994 by New Zealand’s Greg Elliott. In this case, the boat was fitted with two identical 59-foot carbon-fiber wing masts rather than a single, much taller rig. Other innovative features included all-Kevlar standing rigging, a deep bulb keel positioned almost two thirds of the LWL aft, and a retractable daggerboard forward for upwind work. The boat is a spectacular performer, particularly on reaches, and currently holds several course records. It isn’t difficult to imagine how a similar rig, most likely with proportionally even shorter masts, might work very nicely for a performance-oriented offshore voyager.
To stay a single 75-foot wing mast on his open 60 Aquataine Innovations, French singlehander Yves Parlier devised a pair of long outriggers that slope diagonally upward from the deck near the mast step, and spread the shroud base to around 35 feet. More recently, Giovanni Soldini adapted the same system for his new open 60 Fila. It’s a neat engineering solution, but it is cumbersome looking and unlikely to catch on with the voyaging fraternity.
On the other hand, performance voyaging was certainly part of the brief when naval architect Eric Sponberg began work five years ago on the design for Project Amazonthe offbeat open 60 that features twin unstayed wing masts, a vee-bottom hull with lifting strakes, and a canting keel with large, moveable trim tab. Her Hungarian/Canadian owner, Sebastian Reidl, plans to race the Around Alone (former BOC) this September. Due to its relatively high freeboard and aluminum construction, Project Amazon is about 40% heavier than a typical full race 60 and carries about 15% more working sail on two identical 85-foot wing masts with 3:1 chord-to-depth ratios. The masts are built around carbon-fiber box beams, heavily reinforced near deck level where the peak bending loads occur.
Reidl’s twin masts have a symmetrical lens-shaped cross sectionnot the usual teardrop shape. The leading and trailing edges were designed to be interchangeable, and each is equipped with its own sail track. With this arrangement it should be feasible to weathercock the masts to de-power more effectively when running before a storm under bare poles. Unlike most stayed wing masts, Project Amazon’s masts are designed to bend. The leading and trailing edge fairings were fabricated using more resilient S-glass fiberglass because, according to Riedl, rig builder Ted van Duesen calculated that the compression fore-and-aft would be too much in carbon, and it would probably splinter. Under full sail in 25 knots, Riedl reports about three feet of fore-and-aft bend and about half as much athwartships.
In terms of its sailing performance, Project Amazon has proven to be even stiffer than anticipatedonly 8.5° of static heel with full keel offset. During the pre-race work up in Charleston, Riedl plans to lighten ship almost two tons by reducing ballast and changing from twin rudders (extending through massive drums) to a single rudder system. If the boat performs respectably in Around Alone, the cruising potential of this unique design approach will be firmly established.
The canting wing mast
Petit Navire, one of several 60-foot monohulls just launched in France, is attracting disproportionate attention thanks largely to a unique unstayed wing mast rig that can be inclined up to 20° to windward. Petit Navire, like a majority of other recent open class boats, also sports a canting keel. However, in this case the keel swings 45° to each sideconsiderably farther than the 25° to 30° that’s typical for open 60s. In fact, Petit Navire’s design parameters fall well outside the open 60 class limitations because her static heel changes almost twice the allowable 20° maximum when both the canting keel and the canting rig are shifted from one extreme to the other. No doubt she could be de-tuned to fit the class, but her principle purpose is outright record attemptsthe monohull transatlantic and round-the-world marks.
Both of these record attempts are to be tackled with a full crew of five aboard. With enough manpower on hand for frequent headsail changes, there is no need to have twin headsails permanently set on furlers or, for that matter, even permanent forestays. Luff tension for both the fractional or masthead headsails is generated by 3:1 halyards working against running backstays. These, of course, put substantial compression loads on the mast and in turn upon the custom-built traveler car within the hull that permits the mast butt to be dragged from side to side by a pair of 14:1 tackles led to winches on deck. To allow the 80-foot wing mast to rotate freely and simultaneously swing through 40° port-to-starboard, the spar passes through a huge ball-and-socket joint that is recessed into the top of the long, low cabin.
As in the case of Project Amazon, the unstayed wing mast on Petit Navire can rotate far enough to align with the wind when sailing downwind in storm conditions under bare polesan important safety advantage. In early light-wind sailing trials Petit Navire has sometimes doubled the true wind speeda display of sheer sailing efficiency that in the past has only been achieved by extreme, high-performance multihulls. The aerodynamic shape of the wing mast and complete absence of spreaders or side rigging is part of this efficiency story.
Equally significant, however, is the canting feature that allows the rig to remain perpendicular to the water while the hull is heeling up to 20°. This keeps sail forces in the horizontal plane, eliminating the usual tendency to press the hull downward and simultaneously boosting the forward force component. In addition, the canting feature keeps the rig weight to windward of the heeled center of buoyancy where it actively contributes to sail-carrying power instead of reducing it. Most of the 60-foot trimarans racing in the European Gran Prix circuit are equipped with hydraulic shroud adjusters so the rig can remain bolt upright while the crew is flying two hulls at a heel angle of around 15°. However, Petit Navire is one of the first monohulls to pursue this route to greater sailing efficiency.
Does Petit Navire’s canting rig make her any more dangerous than other open 60s? This is an uncertain call, because, in most respects, she conforms to revised class rules, only recently changed to reduce the likelihood of capsize and extended periods of inverted equilibrium. These new rules require a boat to have positive righting moment (with the ballast centered) at heel angles up to at least 125°. Many of the earlier 60s developed a capsizing moment at 110° or less. Petit Navire (and other newly launched 60s) are equipped with heavier keel bulbs to meet this more rigorous stability requirement, but the combination of canting keel and canting rig is something of a wild card.
Unlike a typical flush-decked 60, Petit Navire has a long, low trunk cabin that not only provides a bit more living space for the five-person crew, but would substantially reduces stability in the fully inverted attitude. In light of recent experiences, Petit Navire’s canting keel with its generous 45° per side swing range, probably qualifies as a positive safety feature. When Soldini’s Fila recently pitch-poled and turtled during a fierce Atlantic storm, the boat was righted by releasing a limit stop so the keel could flop over to a special emergency position. All of the 60s that remained bottom-up for substantial periods during the last Vendee Globe were fixed-keel models, and on several occasions sailors reported using the hydraulic releases on canting keels to enable their boats to self-right.
The 20° static heel limitation in the open 60 rule made good sense back when the only moveable ballast was seawater carried in side tanks. But with the advent of canting keels (and now canting rigs), it’s probably time to re-evaluate this restriction. Certainly from the perspective of a the progressive voyaging sailor, any technology that can give a moderately proportion monohull the efficiency and sail-carrying power of a beamy race boat without compromising its self-righting characteristics is a development well worth watching. It may be quite some time before the canting keel/canting wing mast combination trickles down to the voyaging mainstream, but, without question, this development has real merit.
The AeroRig was refined by British designer Ian Howlett and introduced to the market some eight years ago by Carbospars in the U.K. It’s unique among advanced rigs in that it was conceived and developed primarily for voyaging applications rather than for racing. That said, solo sailing star Pete Goss and his designer Adrian Thompson have selected twin AeroRigs for the 125-foot wave-piercing catamaran that Goss and his crew will campaign in The Race 2000 round-the-world extravaganza. With issues of controllability, the big unknown for these outsized ocean thoroughbreds, the AeroRigs ability to weathercock (so reefs can be tied in during a squall from astern) could prove vital in the Southern Ocean. The AeroRig is basically a scaled-up variant of the swing rigs often used for radio-controlled model sailboats.
The idea isn’t new; solo sailing legend Blondie Hasler used a swing rig three decades ago, and some large racing multihulls enjoyed success with similar rigs in the 1980s.
Carbospars’ version is based upon a round, unstayed epoxy/carbon-fiber mast that pivots in a hull socket like the mast of a Laser dinghy. A broad horizontal boom or yard forms an integral part of the mast structure. This boom, which cantilevers fore and aft like an inverted crucifix, carries a masthead backstay at its aft end and a fractional forestay forward. In other respects the AeroRig is superficially quite ordinary: a conventional mainsail set on a luff track at the back of the mast, plus a non-overlapping, self-tacking jib, often with roller-furling gear. However, both the main and jib are part of an integrated whole that rotates independently of the hull, and is controlled by a single sheet led to the boom. Forces generated by the jib serve to partially counterbalance forces from the main, so sheet loads are exceptionally low and gybes extraordinarily gentle.
From an aerodynamicist’s viewpoint, the appeal of the swing rig concept is that air flow always meets the sails at the optimal angle, irrespective of the boat’s point of sail. This results in pronounced performance gains on broad reaches and runs, particularly when sailing sans spinnaker. Several series of sailing tests involving AeroRig’s boats and conventional sisterships have been reported in the British yachting press during the past five years or so. The boats involved have ranged from a chunky 23-foot cruising catamaran to several species of mid-sized production keelboats. In each case, the reviewers documented markedly superior speed for the AeroRig boat when the wind was aft of the beam. They liked the single line sheeting, very low sheet loads, and docile, highly predictable handling. With an AeroRig it’s even possible to brake or back up by swinging the rig until the sails fill from the opposite side.
Upwind, the AeroRig either performed on par with the standard rig or was slightly inferior, depending upon boat type. Other shortcomings include a forward boom extension that sweeps across the foredeck on tacks or gybes, and the need to take special care while hoisting or dousing sail to avoid inverting the rig. Finally there is the matter of costsignificantly higher than for a conventional rigalthough this is partially offset by the elimination of some winches, mainsheet traveler, and aft-led control lines.
All in all, the AeroRig is really catching on, with about 100 boats currently equipped and numerous new projects in the works. Big voyagers in the 70-foot-plus class are particularly good candidates, but boats as small as 22 feet have also been fitted with AeroRigs. Pete Goss’s huge catamaran is currently under construction in the village of Totnes, Devon, for launch in spring 1999. Despite the enormous size of the platform, this boat is actually very lean and spare if contrasted with a conventional catamaran scaled up to comparable size. The two AeroRigs are stepped in the main hulls, so the crossbeams are not required to sustain large mast compression loads. The rig’s center of effort is also much lower than normalanother factor that should contribute to confidence (and high average speeds) when the going gets rough. Finally, Goss intends to sail with a crew of fivefar fewer than his rivals plan onfor a major saving in accommodations, stores etc.
Nevertheless, light-air performance remains a question mark because the AeroRigs are relatively short. But Pete Goss has opted for an open programno secrecy. With sailing trials planned for next summer and a Jules Verne attempt toward the end of 1999, it shouldn’t be long before we have a verdict on this bold experiment.
Contributing editor Sven Donaldson, a former sailmaker, is a marine technical writer based on the West Coast.