Two companies, Hobie and HydroSail, have built the TriFoiler and the WindRider Rave, respectively. These are new sail-powered hydrofoils that are earning raves from sailors and racers. Addition of new technologies to the boats have lessened the problems faced by previous models.
“Fast is fun” is sled maven Bill Lee’s favorite half-liner, but there’s a growing group of beach-cat sailors, windsurfers, and even one-design racers poised to lay claim to his slogan. This group is flying hydrofoils at automotive speeds, aided by a couple of boatbuilders who have launched reasonably affordable production foilers. Sail-powered hydrofoils have been around for a while, but up until now, they’ve been considered more or less quirky prototypes. The bad rap seems to be going away, however, and at recent boat shows speed aficionados crowded around WindRider’s Rave and Hobie’s TriFoiler with more than just idle curiosity.
Sailboats have dug holes in the water for centuries. For most, in fact, there’s only a slim hope for a momentary surge onto a plane. Climbing out of a wet trench is like expecting a Sumo wrestler to perform floor exercise. In most cases, the displacement hull plows aside and pulls along surrounding water, a process that makes waves and consumes most of the energy provided by the sail plan.
The few boats that can actually get over the hump of their own bow waves and transition into planing mode introduce a new set of hydrodynamic concerns. At this point, skin drag becomes the heaviest of shackles that bind, a reality of fluid dynamics that restrains further increases in speed.
Friction is an unrelenting foe of all machines, and sailboats are far from immune to its effects. Large, flat surface areas plane easily, but once on top of the water, they suffer from excess skin drag, which limits their high-end performance. The law of hydrodynamic lift defines an indirect correlation between speed and the optimum area of a planing surface. In short, the faster you go, the less boat needs to be in the water.
The dilemma facing hard-core speed sailors revolves around the design compromises necessary to provide enough flat surface to get a boat on to a plane, and then, once it’s skimming across the water, to provide a hull shape that lessens the effect of skin drag.
Rising Above the Drag
Hydrofoils are a natural evolution in the quest to rise above the drag-inducing effect of water. Their primary advantage is the way that they decrease surface area as speed increases and lift ensues. These wings in the water are like any foil – deriving lift through the pressure differential between the surfaces.
Just as a boat needs to be steered in a port/starboard context, a hydrofoil needs a means of controlling its up/down incidence angle. Most popular are mechanical feedback systems. These sense boat trim and actuate linkage connected to trim tabs that cause a foil to climb toward the surface or dive toward the bottom. The latter is advantageous as the breeze stiffens and the foil on the windward side of the hull (or weather ama) adds righting moment by automatically pulling downward. This auto-stabilization effect contributes significantly to a vessel’s ability to carry sail.
Until recently, fast boats and big dollars were synonymous. Budget-bound hydrofoil fans had to roll up their sleeves and experiment in their own garages while others plunked down a lot of money for esoteric custom boats.
One-off composite construction in exotic materials paid off in performance, but the hefty price tag chased away less well-heeled speed enthusiasts. That began to change when Hobie started building the TriFoiler, a production version of the Greg Ketterman design that Russell Long used to set world speed records in the early ’90s. Last year, Wilderness Systems joined the quest by engaging the design expertise of Sam Bradfield and his firm, HydroSail. The result is the WindRider Rave, an innovative “medium-tech” design that’s less complex, easier to rig, and less expensive than the TriFoiler.
The Rave has a 25-knot-plus top end but doesn’t require a crew ready for an Olympic campaign. Its success begins with the marriage of rotomolded, soft-skin hulls and a stiff, load-carrying frame. Bradfield (who once set a world speed record with a hydrofoil that sported inflatable hulls) saw the potential value in using rotomolded polyethylene hulls. He designed the boat like the human body: a rigid skeleton carries the loads and the softer skin keeps the water in, or in this case, keeps it out.
What makes the combination a success is the way the foil, rig, and sailing loads are focused onto an aluminum grid rather than onto the weaker, soft, flexible skins of the hulls. There’s a space frame in the main hull and tubular supports for the amas. The shrouds, as well as the upside-down T-shaped daggerboard/foil combination, all stress the light but hefty alloy tube and space frame.
The large-roach, full-batten mainsail is boomless, and after going through an intentional jibe at 25 knots, I could see why it makes sense to keep the rig that way. Each warp-speed pivot jibe feels like the crescendo of an amusement park ride that’s over in a nanosecond. When you finally realize that you’re on the other tack and that the boat never faltered from its foil-flying stance, you’re on your way to understanding what sailing at warp speed is all about.
In light air, there’s a feeling akin to what you might expect from a highly maneuverable beach cat. As you approach the threshold of hull-flying conditions, typically around 12 knots, the boat needs to be coaxed onto its feet. Usually this means turning a bit off a beam reach with the puffs, and then a small but quick turn back up to a beam reach. The acceleration of the leeward foil is enhanced by the longer turning radius, adding more lift and helping to get the hull out of the water.
As the hulls start to fly, there’s a distinct feeling of acceleration, the ride smooths out and dries out, and first-timers can’t hold back hoots of exhilaration. It’s a wet ride; when the water temp is chilly, your PFD better be over a spray top, or better yet, a full wet suit and booties.
Factory rep Mike McGarry, part of HydroSail’s design team, told me that there’s a group writing up one-design rules for the Rave. Reach mark roundings will probably feel like being between the uprights of a sling shot. Footage of these boats battling around a racecourse could rival Indy car action.
Sailing to weather is a “six of one, half dozen of the other” decision process. On one hand, when in a displacement mode, the high-aspect-ratio daggerboard/foils allow you to sail at about 40 to 45 degrees to the apparent wind. Once you’re up on the foils, 55 to 60 degrees becomes the best you can do to weather. Windward legs in one-design racing will likely reveal just when it makes sense to crack off and fly above the water versus staying stuck and pointing higher.
The Rave’s rotating aluminum foil-shaped spar and Neil Pryde sails come standard with the boat and are a cost-effective package that performs well. The small self-tending jib and larger, pole-mounted roller-furling reacher (a removable option) extend the usable wind range of this boat. Besides being able to climb out of the water in as little as 11 or 12 knots of true wind, the Rave is a fun boat to ghost along in 5 knots. Yes, a carbon spar and high-end composite hulls could squeak out a few extra knots, but they would undoubtedly skyrocket today’s $7,995 base price. Add to this the facts that even in 20-plus conditions you never think of hiking and that you can solo sail with ease, passing fleets of Melges 24s and Mumm 30s with their chutes up, and it’s easy to see why there’s growing interest in boats that fly.