Contest 1 - 19 Ft. Cruising Tri

A trailable trimaran designed to be recoverable from capsize and to
provide simple two berth accommodation under a cockpit tent.

The great majority of trimarans have outrigger buoyancy well in excess of total craft weight so that when really hard pressed the craft can sail supported entirely on the lee outrigger while still retaining some freeboard and pitching stability. This is fine while it lasts but it only takes a slight mistake to end up capsized, probably with the trimaran completely inverted and the high buoyancy outriggers giving excellent upside down stability and little chance of recovery without assistance. By contrast the design presented here has outrigger buoyancy less than total weight so that when hard pressed the lee outrigger will be driven under. The boat will then be at a considerable heel reducing capsize risk by shedding wind loading from the rig in the same way as for a monohull sailing boat. If heel continues the craft will end up floating with the mast on the water and the lee float submerged by the depth of one cross beam. Although this is a slightly embarrassing position there should be a much better chance of recovery from this position than from the fully inverted position likely to be adopted by a capsized catamaran or trimaran with full buoyancy floats.

The above concept is not new but has only been tried with a small number of designs and can thus be considered to be experimental. This design is presented as a possibility for further development, it is not claimed that it is based on proven principles.

Immersible outriggers were tried on some larger trimarans in the early years of multihull development but were generally unsuccessful since the reduced righting moment limited performance and unacceptably increased the risk of capsize. For a cruising cabin boat a capsize of any kind is considered unacceptable, whether that capsize is to 180 degrees or to only 90 degrees might thus be considered irrelevant.

For this day-sailing and camping boat the loss of maximum righting moment due to use of low bouyancy floats may be more acceptable since with this type of boat it is possible to considerably boost righting moment by suitable positioning of crew weight. Also, at this size range it may be relatively straightforward to recover from a 90 degrees capsize and in this simple camping cruiser there is no electronic equipment or furnishings which might be vulnerable to damage in such a capsize. The method of capsize recovery chosen for this design is by use of adjustable tackles attached to the shrouds combined with a buoyant mast and a small masthead float. This can be a simple manually operated system, on a larger craft it would not be so easy to implement. 4:1 cascade tackles link each shroud to the chainplate and the tails for these are connected across the beam of the boat so that the shrouds only slightly slacken as the mast is inclined up to approximately 20 degrees.

The rig is generally similar to that used on a typical medium sized day racing catamaran and indeed it might be possible to adapt many rig components from such a catamaran, possibly from one with wrecked hulls obtained at scrap value.

For cruising use deep reef points are required on the mainsail, these could be retrofitted to an existing mainsail.

The mast may be a standard catamaran style rotating alloy wing section spar supported by a single pair of shrouds and diamond stays. The wide shroud base of a trimaran makes for rather lower rigging stresses than for a catamaran. To take advantage of the capsize recovery system discussed above the mast should ideally be sealed for buoyancy and fitted with a small masthead float. These floats are now available as a stock item for fitting to some day racing catamarans. If the mast used is of a type which cannot be effectively sealed then the masthead float should probably be slightly larger than shown on the drawings.

The jib is slightly larger than that for a comparable size catamaran and the clew is lower so as to close the gap between the foot of the sail and the foredeck. A window in the jib would be strongly recommended. Provision to take a single reef in the jib is suggested as an inexpensive alternative to a roller furling jib and avoids the need to stow a jib luff spar when trailing. For cruising it may also be necessary to carry a storm jib to balance the fully reefed mainsail.

The geneker set from a folding bowsprit is optional and most cruising sailors would not want it.

The outriggers are supported from the main hull by small decked wings aft and by tubular cross beams forward. Both the decked wings and the forward crossbeams are hinged from the main hull with hinges locked by stays which can be set to give two alternative outrigger heights. When sailing the outriggers would be in the high position so that in most conditions crew position can be adjusted to balance the boat mainly on the main hull minimising drag by reducing wetted surface and by making only a single main wave pattern. The lower outrigger position is for use when not underway and gives high initial stability to improve comfort for living aboard. The lower outrigger position might also be used when using the outboard motor.

For trailing the hinges between outriggers and the main hull are dismantled and the outriggers stowed on their sides with the aft cross beam wings vertical. This makes a trailing package of less width than most monohulls of similar length and much less width than a comparable unfolded catamaran. The low bouyancy floats are small and light enough to be easily handled ashore by the intended two person crew so a folding mechanism as used on larger road trailerable trimarans would be an unnecessary complication.

The decked wings aft provide a seating platform for the crew while sailing and are fitted with effective footrests/handholds. Portable waterproof cushions would improve crew comfort on long passages. For use on the decked wings of this boat it might well be worth trying the type of camping cushion which has an integral folding backrest.

The aft end of the cockpit is wide enough to provide seating positions each side of a small footwell. The forward part of the cockpit is narrower and would be used underway as a place to cook and navigate. At night the cockpit footwell can be boarded over to provide a sleeping platform. The main dry stowage area is aft of the cockpit accessible through a sliding hatchway. There is further stowage space forward in the cockpit and under the foredeck. A picnic stove could be mounted under a lifting cover alongside the centreboard case.

A single large centreboard is fitted in a case at the forward end of the main cockpit. This is simpler than the alternative possibility of having a centreboard in each outrigger. It is also more suitable for a trimaran which in light and medium wind strengths would be sailed with both outriggers mainly clear of the water. The centreboard is slightly offset from the centreline of the hull so as to raise the slot clear of the ground level when dried out on the foreshore, thus reducing the risk of stones jamming up the cenrtreboard.

A camping tent fits over the cockpit with small corner poles and guy ropes taken out to the extremity of the outriggers. These guy ropes and corner poles hold out the sides of the tent allowing sitting space at the aft end of the tent. The ridge of the tent has sewn on hooks which engage sail slides fitted into an extruded slot under the boom. The boom remains on the normal sailing gooseneck with the tent up but the aft end of the boom is raised using a halyard to give near standing headroom at the aft end of the tent.

The proposed construction is ply joined and sealed with epoxy. The joints between deck and topsides and between the main bulkheads and the hull skin are made by screwing and gluing onto wooden gunwhales/framing, other ply joints are generally made with epoxy fillets. The structure is well reinforced at the outrigger mounting points. The design avoids highly curved panels and is well suited to simple ply construction.

The structure is generally 5mm ply. All wood surfaces are epoxy coated and the external surfaces sheathed in glass cloth set in the epoxy coating.

When the boat is capsized to 90 degrees the outrigger skin panels are subjected to hydrostatic loading of approximately 20kPa (3 psi). This loading is supported by ply bulkheads spaced 200mm apart inside the outriggers.

The mainhull and the outriggers are vee bottomed and fitted with rubbing strakes amidships to protect the ply skin when dried out on the foreshore. These rubbing strakes are shod with strips of 16g stainless steel.

The various hinges and stays which support the outriggers are fabricated from 4mm aluminum alloy plate and large section 4mm thickness aluminum alloy angle. Screw fasteners are mainly cut from 8mm stainless steel studding and hinge pins are large stainless clevis pins locked in place with snap fit spring clips. The stays below the outrigger cross beams are telescoping sections of thick wall aluminim tube with cross pins to lock at two alternative lengths. If necessary all this hardware could be fabricated in a home workshop using a cheap drill press and hand tools. A coarse Sandvick file will quickly plane the edges of the plates smooth and all the aluminum work can be deep anodized as a single batch.

John Perry