The design of transportation systems is inevitably a matter of trading off desired features or parameters in the process of optimization. The more importance placed on performance, the more profound the trade-offs will be. I was invited to comment on a conceptual sailing hydrofoil trimaran intended to introduce some new and not-commonly-seen technology. This was at the inception of the Rave V development program. Apparently my comments and suggestions carried some merit and I was invited to join in on the design process.
The initial baseline for the vessel was for the use of “V” type foil systems with “sonic tubes’ at the junctions of the two individual foils that make up the V, a pair forward for lift and lateral resistance to sail propulsive force, and one aft for lift and steerage. Sail propulsion was to be provided by a pair of Marconi type sail plans, side by side, supported by an “A” frame mast system. It was intended that all four sails would be carried on roller furler systems and that only one mainsail and jib set would be used at a time for each tack, with the other set furled. Much of the structure was to be of carbon fiber composites for the high strength and stiffness to weight ratio of the material. The main center hull, called the waka in trimaran terminology, was to be of a tandem seating type like the original WindRider Rave. The A frame mast concept was anticipated to provide sail propulsion with a force bias resistant to heeling moment, obviating the traditional need for crew to hike the respective windward rails of the outer hulls, called amas in trimaran terms, on the two tacks. The tandem or kayak type seating is perceived to be comfortable and convenient. My immediate impression, based on my years of general sailing experience, and especially my most recent multihull racing experience, was that the furling of two sails and unfurling of the other two sails at each tack would quickly prove to be more burden than most pleasure sailors would care to indulge in. Further, I was doubtful that the dynamic roll moment balance provided by the eccentric driving sail plan would sufficiently offset the aerodynamic drag losses imposed by the furled pair of sails so as to be worth the effort. The technical challenges of designing a vertical-axis furling system for an efficient high aspect ratio mainsail, with a high roach shape and the accompanying stiff battens of virtual necessity, would likely prove prohibitive. The on-the-water effort of roller-furling and unfurling fully battened mainsails around booms is known to be too cumbersome to be done as frequently as each tack. A pair of rotating wing masts without jibs, with fully battened sails that are deployed throughout the operational sailing realm was proposed and soon enough accepted as a viable way to operate the vessel as efficiently as the double main and jib rig but with far greater simplicity. The original masts were to be simply linked at their tops. With both sails to be flying simultaneously it was clear that they would likely interfere with each other at the top of the rig. A short spar of symmetrical foil section was added to the rig, mast head to mast head, to space the sails far enough to avoid this interference, with the added benefit of fencing the roll off of vertical spanwise airflow known as vortex generation, a form of induced drag. Some minor technical issues with the articulation of the masts to the linking spar, and the observation that it would be preferable to present the leeward sail forward into the lead of the airflow (much like the upper wing of a biplane is staggered forward), led to Larry Knauer’s excellent suggestion to replace the linking spar with an arch shaped spar. With a radius of curvature equal to that of its and the masts’ chord, this shape is the most efficient currently know for vortex generation reduction, and is used on many other forms of modern efficient aircraft (due to the nature of the Rave V in its intended operational realm it is difficult to think of it as something not related to aircraft). With the arch rigidly attached to each mast top the natural rotating motion of each mast should automatically twist the leeward mast forward ahead of the windward mast to enhance slot effect and maximize the interactive drive force of the sails.
The length overall of the boat was originally arbitrarily specified to be 18 feet with a beam of 12 feet. The original mast height was specified at 31 feet. With the sail plan designed to be apparently proportional to these dimensions, the area of each sail including the wing mast ended up being about 247 square feet for a combined total of 494 square feet, likely overwhelming in any but the lightest breeze. A length to beam ratio closer to that of the original Rave was adopted stretching the beam to a more realistic 15 ½ feet. A decision was made early to more-or-less reapply the length and beam dimensions of the original Rave. This shortened the waka hull to 17 feet and decreased the beam to 14 1/3 feet. The newly specified combined sail area was set at 320 square feet, representing a 50 percent increase over that of the working sail area of the original Rave. For the subsequent variations to the design that have been explored, the mast length has since stayed between 25 ½ and 26 feet. It was initially planned to mount the straight-arm aft aka across the stern of the waka and the amas to facilitate a strong carry-through of mainsheet, hull, rudder (steering foils) and backstay loads. The masts were to mount on top of the amas at the location of the forward straight-arm aka mounts. The original configuration of the aka to ama interface was simply to use a variant of the conventional multihull bolt-on crossbeam type mount. A practical and safety aimed specification was set for floatation in each ama to be approximately equal to the weight of both standard crew standing on the same ama with its top pushed down to water level. The specification for standard crew weight was set at 200 lbs. each. The target all-up empty weight of the boat was set at 300 lbs. In order not to exceed the 400 lb. floatation capacity of the amas and maintain a more-or-less proper longitudinal floatation balance to the sail plan, crew weight distribution in the waka, foil mount, etc., the ama would need to be excessively long and impractically thin to carry to aka mounts at the aft ends of each respective hull. Also, such a thin hull form would likely create negative manufacturing issues. The mounts would be located at the ends of amas so thin that the quest for a strong load carry-through would be inherently defeated. The aft aka was consequently moved forward of the aft cockpit and the amas were reconfigured to minimally meet the + 400 lb. floatation capacity requirement at a more workable length. The backstay bridle mount was moved forward with the aka sleeve mount in the waka, reducing the potential for sail hang-up during low speed gybing. Since the amas have, in practice, under way, a rather minimal role in providing buoyancy to the vessel, up to the point wherein the foils and sonic tubes provide enough lift to carry the hulls out of the water. Therefore, any excess ama hull weight and drag (hydrodynamic and aerodynamic) is, by design, simply a wasteful detriment to performance and production cost. In short, the amas are there for structure, docking, low speed handling, and safety. An end-on look at a modern foil-borne Moth class boat reveals a trimaran minus the amas. The original target liftoff speed for the boat to become fully foil-borne was specified at 7 miles per hour, and later relaxed to 8 miles per hour, at the full specified standard gross weight of 700 lb. The amas were shaped to provide fine entry buoyancy with minimized surface area-to-volume of the wetted bottom of their hulls, in small waves, while the boat is still heeled over in typical trimaran fashion at pre-foil-borne speeds. This criterion is about as subjective as it gets. The waka, as it is required to float most of the weight of the entire boat plus a crew at 400 lbs., was designed with a trade-off slightly away from the fine minimized surface-to-wetted-area shape towards a fine entry type shape that should promote early hydrodynamic lift, as it is done on the classical planing hull dinghies, but with a decidedly long-on-the-water-line narrow section. The super sleek forward bow top section (“foredeck”) suggested in the publicity 3-D drawings and animation would have been fine for aerodynamic purposes and likely would have shed splashed water efficiently as well but was rejected, at least for the present, for practical and safety reasons. With no mast or any other structure to hold on to aft of the head stay and forward of the back stay, a crew going forward to receive a dock, tow line or a less than ideal shore would do so at risk of going overboard, potentially into the path of foils. A flatter top with curved edges and a downward slope forward for aerodynamic purposes is hoped to provide at least minimum footing with reasonably good aerodynamic form properties.
From the onset the forward foils were baselined to attach to the amas and waka above the waterline so that they could be hinged at the amas for detachment and retraction from their waka-end mounts, accessible from the forward cockpit. A 6 inch chord section was the initial baseline for the foil blades and the length of each making up the V shape was set to allow the sonic tubes to clear the waka upon V foil retraction. The sonic tubes were initially sized to a diameter equal to that of the foil chord, pending change to be determined by calculation and experimentation. The original specification for draft at full foil depth was arbitrarily set at four feet. This was not achievable even with the full 15 ½ foot beam as the foil angles would have been substantially steeper than 45 degrees, judged at the time to be inefficient for lift, and sonic tube-to-hull clearance on retraction was not achievable even if the foil blade lengths were adjusted to relocate the tubes to directly beneath the foil pivot mounts on the amas. The foils were therefore designed to look correct and the draft was determined by that. The steering foil system aft was originally specified to be also a V foil, retractable for beaching by way of a hinging trunnion built into the waka hull transom, allowing the entire foil system to swing aft and up. Subsequent calculation strongly indicated that the lift force from the two foil blades plus the sonic tube would be too strong to balance out the component of boat weight apportioned to it. This was anticipated as a review of the state of the art demonstrated by other foiling sailboats suggested that steering foils tended to be much smaller in lifting surface area than their primary lifting foil counterparts. Thereafter the V foil blades were replaced by a single symmetrical section narrower chord (4 ½ inch) rudder leaving only the sonic tube for lift. Set angle of attack (angle of incidence) was baselined at 2 degrees for the sonic tubes, and expected to be about the same for the foils. The baseline angle was reduced to one degree for the rudder’s tube when it was apparent that the lift forces would still be higher than needed to balance the preferred load distribution between the forward primary lifting foils and the aft lifting tube. The forward foil systems were located so as to provide a balanced lateral resistance to sail force and a near neutral helm based upon a standard una-rig sail plan center of effort. This was subsequently reviewed and corrected to reflect a slightly more forward center of effort consistent with a slotted sail plan (main and jib) that the rotating wing mast should more closely resemble. Consequently the foils were moved 4 inches further forward much improving the calculated static force balance between the primary lifting foils forward and the lifting tube aft.
It was assumed from the onset that some method of pitch control and balance would be necessary in the form of water ballast or steering foil pitch angle trim or both. The transition from a V foil aft to a rudder with lift tube likely improved the simplification of the trim method as it reduced the size of the steering system mountings. Two trim angle methods currently under consideration include a screw jack type rudder head pitch angle adjuster or a slider crank/link type adjuster to the same type of rudder head. The rudder blade is now intended to be retracted by lifting it straight vertically through a dagger board style rudder head.
A number of scenarios were considered for the rigging of the boat from trailer to sailor. Of those pertaining to hull geometry, the method of mounting the amas to the wakas with the crossbeams, know in trimaran parlance as akas, has been baselined as simple ferrule plug-in types. An effort was made to investigate the possibility of “telescoping” the akas into the hull for trailering, and out for sailing. This couldn’t be accommodated with a simple two section concentric tube system per aka as the beam of the assembled vessel is too great to allow for it and still meet the U. S. Department of Transportation’s 8 ½ foot maximum road vehicle width requirement for transport without special procedures and requirements. Clever and convenient hull folding or retracting systems or methods tend to carry a weight penalty to the detriment of performance. The idea of attaching the akas to the amas from above in a reversible fashion was abandoned in favor of a lighter permanent internal epoxy bonded attachment. The accompanying reshaping of the ama’s hull should also offer the slight benefit of aerodynamic drag reduction.
