In this chapter we shall be primarily concerned with the problem of multihulPs extreme initial stability and the consequent high com-at sea. Other grave situations may occur, of course. Multihulls may be dismasted in situations where a monohull would not owing to the multihulls extreme initial stability and the consequent high com-pressional loading on the mast owing to gusts. Multihull rudders and boards are similarly more prone to breakage than monohull rudders as a result of the considerably higher speeds attained. These factors must be anticipated in the design. It is, however, the fact that multihulls are stable in the inverted as well as the normal upright position that causes the most concern.
Capsize can be caused by wind, wave, or a combination of the two. It is obvious from our stability studies in Chapter 4 that a large beam, low centre of gravity, and a low aspect rig are all design virtues from the point of view of capsize avoidance. The question of trimaran outrigger (or proa leeward hull) buoyancy is not so straightforward. If the buoyancy is insufficient to support the entire weight of the craft, then the initial stability is reduced and the onset of capsize conditions under sail are more readily apparent as the lee outrigger begins to bury. Under extreme storm conditions the yacht should be relied on to behave as a stable raft and be capable of sliding sideways on a steep wave slope to avoid the breaking crest. Craft with low-buoyancy outriggers do not do this. As the wave heels the boat, the lee outrigger submerges and the boat becomes very resistant to sideways motion. Consequently the outrigger can act as a stationary axis about which the crest of the wave will capsize the craft. Any vertical hydrofoil such as a dagger board or leeboard in the outrigger can contribute markedly to the sideward resistance, consequently, these should all be retracted when lying ahull. The same does not necessarily hold true for leeward Bruce foils, however. If the dihedral angle is in the recommended 40-45 degree range, then the flat bottom surface of the foils will become parallel to the sea surface as the boat heels and will provide a sort of water ski on which the boat can move sideward. This will, of course, be more true for the low aspect foils favoured by Bruce and Morss than for high aspect ladder foils. About the only meaningful experience in this area belongs to Dave Keiper who has done many thousands of miles in the Pacific in his hydrofoil trimaran Williwaw safety and and has weathered some fairly severe conditions. seakeeping
The proa configuration optimizes its stability by keeping the heavier hull permanently to windward. In a seaway or under changing wind conditions the danger always exists of being caught aback with the wind on the wrong side. Under these conditions the lateral stability of the proa is then less than that of a catamaran of comparable beam. This must be compensated in the design. During an early trial cruise with Cheers, Tom Follett experienced such a windward capsize which was fortunately limited to 90 degrees by the buoyant masts located in the windward hull. Follett was unable to right the boat without outside assistance however. As a cure, Newick installed a sponson or bulge on the windward side as shown in Fig. 7-1. This sponson was designed
to halt a windward capsize at about 40 degrees where a release of the wind pressure could allow the craft to recover. In a cruising proa, such a bulge could add considerable accommodation space. In a racing machine where weight and windage are at a premium, the sponson could consist of an inflatable outrigger (two Benyon-Tinker 15-foot inflatable catamaran hulls installed transom to transom) connected by short beams to the hull, a sort of deformed trimaran. Another approach to the windward capsize problem for proas is to arrange a mechanism whereby the sheet or sheets are automatically released in the event of wind pressure from the wrong side. The details of such an arrangement will vary from one rig to another, however it should not be a difficult thing to engineer.
The only sort of sheet cleats that should be used are cam cleats which can be released manually by a quick upward tug on the sheet. Piver designed an automatic sheet release as shown in Fig. 7-2 for use on his trimarans. This consists of one or more cam cleats mounted on a base which is hinged on the sail side of the cleat. A length of shock cord is attached to the opposite end of the hinged base and led downward to another small cam cleat. There the tension is adjusted to affect a release at any predetermined sheet tension. This occurs owing to the couple formed between the sheet and the hinge axis. A commercial sheet release has been manufactured by Hepplewhite Marine Ltd. This gear operates by mercury switches at a preset heel angle in either direction and releases the cam cleats magnetically. Unfortunately Hepplewhitcs have recently discontinued this item. The effectiveness of any type of sheet release gear depends on the ability ol the sheet to run free. The use of multifold purchases and winches 79
gh speed Sheet cleats
obviously inhibits this ability severely. The pyramid rig offers a great advantage in this regard since only one sheet is involved and for all but the largest rigs, the sheet does not use winches or multifold purchases owing to the nearly balanced nature of the rig. As added insurance, a sharp hand axe should be clip-mounted in the cockpit in order to be able to chop the sheets should all else fail. Drills should be arranged and each crewman should have a predetermined task under such emergency conditions.
Some designers have favoured the use of permanent masthead buoyancy in the form of a fat disc or weathercocking teardrop form. I do not believe this to be a good idea. In the first place, the weight and windage of such a device at the masthead will promote the problem it is designed to cure. Secondly, if the yacht does capsize, the masthead float may well exert such an impulsive force as to break the mast. On balance, such measures probably do more harm than good.
In the event of a capsize, a multihull with a high-volume beam structure such as Three Cheers (see Fig. 2-9) will float quite high in the water. By fitting a capsize hatch under one of the bunks such that it is capable of being opened from either side, quick access to or from the hull is provided. Such a capsize hatch is permissable in a trimaran and possibly in a proa, however it is not suitable for a catamaran. A well-designed trimaran has two watertight outriggers to provide ample buoyancy when the hatch is opened and the pressurized bubble of air in the hull is released. Additionally, adequate handholds must be provided on the underside of the beams and on the hull above the waterline. The liferaft and all survival stores must be stowed so as to be readily available with the yacht in the inverted position.
Over the years the AYRS has published many schemes for righting capsized multihulls at sea, however none has worked out except on a 80 trial basis in harbour under ideal conditions. Most of these schemes
have involved slight variations on the idea shown in Fig. 7-3. Water is pumped into the leeward outrigger and the halyard is used to raise the mast to the 90 degree position from the life raft. The submerged outrigger is then pumped out and at some point wind and wave action are expected to right the craft. The problem with this scheme is that it is difficult to kill the buoyancy of the leeward outrigger and beam structure, especially in a foam sandwich boat. Only a trimaran with cold moulded wooden outriggers and tubular aluminium 81
cross members would have any hope of being righted in this way and then only if the mast had not been broken by the capsize as often happens. For catamarans and proas no righting over the side scheme seems to hold promise.
The first real breakthrough in multihull righting occurred at the World Multihull Symposium in Toronto in 1976 in the form of a discussion and model demonstration by Carlos Jim Ruiz of El Salvador, Central America. Ruiz's idea involves righting over the bows and is
Fig. 7-4. The Ruiz scheme for righting a capsized multihull.
shown in Fig. 7-4. Figure 7-4(a) shows the boat on its back with sufficient foam in the cabin top to cause the craft to float high and offer as dry as possible accommodation in the cabin until the seas subside sufficiently to make a righting attempt. The crosshatched areas represent foam flotation and the vertical dashed line indicates a watertight bulkhead separating the cabin from the bow section. Figure 7-4(b) shows an A-frame of alloy tubing hinged at the leading edge of the forward connecting beam. This A-frame is connected to a water bag at the vertex and pivoted forward over the bow and the bag allowed to fill. A small A-frame is positioned in sockets in the forward beam as shown in (c) and a line is run from the bag attachment point, over the block at the vertex of the small frame to a large winch mounted on the bottom of the rear crossbeam. Valves are opened as shown in the deck and bottom of the bow section(s) to allow water to enter and air to escape as the water bag is winched up. In this safety and way the boat is brought to a vertical position as shown in Fig. 7-4(d). seakeeping
An openable window is fitted in the cabin so as to be just above the waterline in this position in order to allow water to drain from the cabin. In this position the boat is quite stable (as spar buoys are)
and the cabin will be dry. Wave action and a little more winching will then put the craft in an upright position. By leaving the two valves in the bow open the foam floatation in the bows will then cause them to self-bail and you are back in business. Note that in this scheme we have not had to depend on the mast which may or may not be broken in the capsize. If the mast is intact, then an inflatable masthead float of small dimensions may be used to assist the righting from (c) to (d). If the mast is lost then the A-frame used to right the boat can subsequently be used to make a jury rig. At the time of this writing (December, 1977) this scheme has not been attempted to right an unplanned capsize, however, in my opinion, it stands a very good chance of working.
1 Follett, Tom, Dick Newick, and Jim Morris, Project Cheers. London: Adlard Coles Ltd., 1969.
2 Harris, Robert B., Racing and Cruising Trimarans. New York: Charles Scribner's Sons. 1970.
3 Henderson, Richard, Sea Sense, Camden, Maine: International Marine Publishing Company, 1972 and London: Adlard Coles Limited.
4 McMullen, Michael, Multihull Seamanship. New York: David McKay Company, Inc., 1976.
6 Myers, Hugo, Ocean Racing Multihull Design Considerations, SNAME New England Sailing Yacht Symposium, New London, Conn. 1976.
7 Various AYRS members, Multihull Safety Study, AYRS, 69 (1969).
8 PERFORMANCE PREDICTION
In chapter one we derived an equation for the ratio of boat speed to true wind speed
where y and /? are the course angles to the true and apparent wind vectors respectively. We want to be able to predict the speed of a boat on any course angle y subject to a given wind speed VT, thus we shall consider VT and y as known input variables.
The angle was found to be the sum of the aero- and hydrodynamic drag angles SA and dH, where
In order to find VB (or /?), we must therefore calculate the lift and drag of all parts of the boat above the waterline and that of all parts of the boat below the waterline. Let us see what this entails. The aerodynamic lift can be written as
The quantity \pA = 1.19 • 10"3 slug/ft3. The apparent wind speed VA can be expressed in terms of VB, VT and y,
by applying the law of cosines to the sailing triangle (Fig. 1-1) or, in terms of /?, VT and y
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