from which the weight in pounds is found as
This type of construction is the best for racing craft at present, not because its strength/weight figure is the best one can do, but because its stiffness/weight is. This, of course, is quite important in order to keep a taut forestay. Using a self-stressed pyramid rig, GRP foam sandwich construction becomes optimal. In a catamaran, exotic materials should be used in the hull connecting structure. In the 'trimaran, carbon fibre in epoxy should be used in the outriggers as well as in the beams.
We now have the information we need to see how speed scales with size. If the overall beam scales with the square root of L as implied by Eq. (9-7) for proportional increase in the cross section of the connectors, then the weight scales as L2,5 and the ratio VB/VT defined by Eq. (8-20) scales as y/L. The maximum possible speed defined by Eq. (8-27) remains almost constant with change of scale, however. If the hull connector cross section is increased by greater than L2 in order to cause the overall beam to scale linearly, then the weight will go up approximately as L3 and VB/Vr will be approximately invariant and the upper speed limit will increase as y/L.
An example of a trimaran in the 50 foot size range is shown in Fig. 10-2. The lateral plane is supplied by the dagger-type skeg and a pair of leeboards on either side of the hull.
* Hydrofoil application to offshore trimarans can definitely be contemplated at the present time. Aeroplane and canard configurations both appear to be feasible. With the canard configuration, experience has shown that the bow foil should carry about 15 percent of the weight and the Bruce foils about 85 percent. In this way the higher angle of attack of the bow foil and its consequent less than optimal Y7.9 has only a small degrading effect on the overall drag angle of the system. The outriggers on a hydrofoil trimaran may be somewhat smaller than those intended for an ordinary trimaran. Careful calculations must be made for the loading of these outriggers at sub-flying speeds in order to ensure that the DLR does not attain such a value as to build up a wave resistance hump. The foils should be designed 80 that the bow lifts out first. This will lead to an increased angle of attack on the Bruce foils and cause them to lilt the stern smoothly 99
to bring the boat to level flight. The horizontal axis about which the trimarans boat pivots in pitch will be somewhat ahead of the bow. This arrangement is very stable. It is important, however to use a bow foil with ample reserve foil area above the foil-borne waterline in order to cope with head seas.
On paper, the canard configuration looks best. The overall drag angle is lower owing to the load distribution and the pitching stability is superior. On the other hand we are faced with the fact of Mayfly's undeniable success with the aeroplane configuration.
Eric Tabarly, a man who must always be taken seriously, is presently (1978) involved in the planning of a 59 foot aeroplane trimaran. This is shown in Fig. 10-3. This boat, to be christened Pen Duick VII will be of aluminium construction, albeit of a rather special sort known as an integrated structure system. In this scheme the alloy plates are milled to various thicknesses in order to reduce unneeded weight to a minimum. The entire boat is planned to weigh
12100 pounds. The span of the beam linking the two forward foils is 66 feet, 7 feet greater than the length. These foils can be set at any dihedral angle up to 45 degrees and serve at zero heel angle to cancel almost completely the heeling torque. The stern foil which serves as rudder and longitudinal trim adjuster will probably carry about 35 percent of the weight with the remaining 65 percent borne by the Bruce foils. Propulsive power is developed by a 2150 ft2 sloop rig mounted on a 59 foot rotating wing mast. This mast has no shrouds but is supported by two tubular struts to the main beam. This structure is unusual in that it puts the mast into tension rather than compression. Model tests by the Higher National School of Aeronautics at Poiters has indicated that the boat should lift onto its foils at a speed of 12 knots after which a speed of 20-25 knots should be attainable with very little extra effort. Tabarly has been testing a high speed 20 foot version offshore and reckons that if he can keep the big boat sailing foil-borne for 5 or 6 days, he stands a good chance of being able to make a singlehanded Atlantic crossing in 15 days.
At the time of writing, the only hydrofoil sailing craft to have crossed oceans successfully is David Keiper's Williwaw. This yacht has a trimaran configuration and was designed initially as a hydrofoil craft. Williwaw has an overall length of 31 feet and weighs about 2100 pounds empty. The aluminium hydrofoils account for about 400 pounds of this total. The boat is rigged as a sloop with a sail area of 380 ft2. The hydrofoil configuration is neither canard nor aeroplane but a four-foil combination. The photo of Fig. 10-4 showing Williwaw with foils retracted shows the layout. The small outriggers shown were later replaced by larger ones. The bow foil is a single blade of 6 inch chord that spans the boat. The minimum dihedral angle is 30 degrees.
104 Fig. 10-4. Williwaw with all four foils retracted.
The stern foil is a three-rung ladder which combines the functions of lifter and rudder. The lateral foils are four-rung ladders with a dihedral angle of 35 degrees. Heeling torque is compensated at a heel angle of 5°-10° which usually suffices to lift the windward dihedral foil clear of the water. The hydrofoil configuration when sailing is a sort of reverse proa configuration with two foils to windward on the hull at a distance of 26 feet apart and a single dihedral ladder foil to leeward 10 feet from the symmetry axis. In practice, everything works together very well.
Williwaw's conception began in 1963. She was built in 1966, first became foil-borne in April, 1968, and underwent her first trials offshore in May of the following year. In September of 1970 she made a 16 day passage from Sausalito, California to Kahului Harbour, Maui under disadvantageous conditions. Since then Williwaw has circumnavigated
Fig lOfeS. Williwaw foil-borne ut ncii.
¡gh speed the Pacific, cruising 20,000 miles between California and New Zealand. sailing Williwaw is designed to lift off at a speed VB = 12 knots in winds of 13 knots or greater. At takeoff the horizontal projection of the hydrofoil area is about 12 ft2. The boat operates at a minimal leeway angle except during sharp gusts. The original outriggers had a buoyancy capability of only 600 pounds each. It was found necessary to increase this in two steps to a bit more than 2000 pounds each for take-off in gusty conditions. The bow foil has a forward sweep angle of 10 degrees and the lateral foils a forward sweep angle of 14 degrees. This causes the water encountering the struts at high speed to climb the struts and very effectively prevents the onset of ventilation effects. Keiper has also found it advantageous to add torpedo shapes at the junction of foil blades and struts to alleviate a mild ventilation problem there.
Williwaw is fast under ideal conditions and has flown at better than 20 knots. Her offshore passages have not been remarkable for their speed although stability and seakeeping are greatly enhanced. So far as future development is concerned, Williwaw has shown that fixed high aspect ladder foils are a viable proposition for an off-shore sailing machine. David Keiper is a real pioneer and we all owe him a great debt of appreciation. Figure 10-5 shows Williwaw foil-borne at sea.
1 Alexander, Alan; James Grogono, and Lonald Nigg, Hydrofoil Sailing. London: Juanita Kalerghi, 1972.
2 Barrault, Jean-Michel, Sail, 8, 88 (September, 1977).
3 Clarke, D. H., Trimarans. London: Adlard Coles, Ltd, 1969.
4 Cotter, Edward F., Multihull Sailboats. New York: Crown Publishers, Inc., 1971.
5 Gougeon, Meade and Ty Knoy, The Evolution of Modern Sailboat Design. New York: Winchester Press, 1973.
6 Harris, Robert B., Racing and Cruising Trimarans. New York: Charles Scribner's Sons, 1970.
7 Keiper, David, AYRS, 74 (Sailing Hydrofoils) (1970); AYRS Airs, II, 34 (1975); AYRS, 83B, 36 (1976); AYRS, 85B, 44 (1976).
8 McMullen, Michael, Multihull Seamanship. New York: David McKay Company, Inc., 1976.
Proas represent an approach to sailing that is, for the most part, foreign to Western experience. As a result, this type of multihull has not been explored in terms of modern materials and technology to anything like the extent that catamarans and trimarans have.
The flying proa originated in Micronesia and reached its highest development in the Mariana Islands in the fourteenth or fifteenth centuries. These craft were built in lengths of 70 feet and more and, driven by woven pandanus sails, could sustain speeds of 20 knots on a reach under favourable conditions. During the time that Spain occupied the area, proas were used to carry mail between the Caroline, Mariana, and Philippine Island groups. One of these craft is said to have made the 1700 mile run from Guam to Manila in 6 days.
The drawing of a Marianas flying proa in Fig. 11-1 is a composite of information obtained from historical sources and private communication with Professor Edwin Doran of Texas A & M University. The hull is asymmetric with the curved side to windward. The outrigger is a solid log of considerable weight and has little reserve of buoyancy. If caught aback, the outrigger is rapidly driven under and a capsize ensues. These craft were sailed by large and agile crews who arranged themselves to windward along the connecting structure in order to keep the log flying just clear of the waves, hence the name flying proa.
It is clear that in adapting the proa for western yachtsmen, often sailing shorthanded, that some modification to the concept is necessary. The first step in this direction was taken in 1967 when Dick Newick launched Cheers. In Cheers, the accommodation, the schooner rig, and the rudders and lateral plane are all carried in the windward hull. The leeward hull is identical to the windward hull to the sheer; its displacement at rest is only about half that of the windward hull, however. This craft was highly successful and took a third place in the 1968 OSTAR against much larger and more costly competition.
The Cheers configuration (hull and rig to windward and a buoyant outrigger to leeward) was termed an 'Atlantic proa*. Since then Kelsall has built two Atlantic proas, Sidewinder and Lillian% however neither has enjoyed any racing success to speak of. In the case of Sidewinder (he iluggcrhoards were placet) in the outrigger which led to a balance
problem which we shall examine presently. Another problem occurred proas in that the two masts carrying high aspect fully battened sails were placed too close together and the foresail had a marked tendency to backwind the main at speed.
Proas, being the laterally asymmetrical craft that they are, are a natural for the application of leeward Bruce foils. Realizing this, Cdr. George Chapman, R.N. designed and built the 18 foot hydrofoil Atlantic proa Tiger in 1972. In this craft the long main hull together with the sail and crew are all to windward. The hydrofoils (two in number) are mounted on the ends of the tubular crossbeams by 3-foot struts and have a dihedral angle of 41 degrees. Between the leeward beam ends is a foam and plywood outrigger of very low buoyancy to stabilize the boat at low speeds. The sail was a single mainsail mounted on a rotating mast placed amidships. This craft is shown
have a major flaw concerning helm balance. With the foil angles of attack set at 5 degrees bow and 4 degrees stern, Tiger was balanced, that is, the yawing torque was zero with the rudder centred only for a sheeting position just free from close-hauled. Sheeting in produced weather helm and freeing gave rise to lee helm. The mechanism for this inbalance is shown in Fig. 11-3. We see from this figure that the line of action of the sail force FA only coincides with the line of action of the resultant hydrodynamic force FH for one sheeting position. If the foils are retracted and a leeboard mounted on the weather hull is used for leeway resistance, then good helm balance can be maintained over the entire range of sheeting positions. This discovery accounts for the fact that Cheers with its daggerboards in the windward hull had a much more satisfactory balance than Sidewinder with its leeway resistors in the lee hull. 109
Weather helm helm
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