Trimaran

Overview

A trimaran carries its accommodation and rig in a central main hull, the vaka, and is steadied by two slender outrigger floats, the amas, held out on either side by cross beams called akas. The single sailing hull keeps wetted area and displacement low, so the boat is light and fast, while the wide spread of the floats gives it stability that a narrow monohull cannot match. A cruising or performance trimaran in the 10 to 15 m range will sail upright at speeds well above its waterline length would suggest, and it does so without the lead ballast keel that a monohull needs to stay on its feet.

The boat divides into a Main Hull (Vaka) that holds the cockpit and living space, two Float (Ama) floats, the Cross Beam (Aka) beams that tie them together, the retractable Daggerboard & Rudder for grip and steering, the Rig and Sail Wardrobe that drive it, the Deck Hardware & Running Rigging that trims those sails, an auxiliary Auxiliary Propulsion package, the Steering System linkage, the Electrical System and Navigation Electronics systems, and the Accommodation fit-out.

Main hull and floats

The vaka is a foam-cored fibreglass hull, subdivided by bonded watertight bulkheads so that damage to one section does not flood the whole boat. Longitudinal stringers stiffen the shell against rig and rigging loads, and a moulded deck is bonded on top. The mast load is fed into the structure through a reinforced mast step. Most of the boat's weight, the crew, the engine, the tankage and the stores, sits in this central hull, low and on the centreline, which keeps the floats free to do their job.

Each ama is a much finer hull with a high reserve of buoyancy in the bow. In normal sailing the leeward float skims the surface or carries only part of the boat's weight. As the wind builds and the boat heels, that float is pressed down into the water, and its buoyancy rises sharply to push back. The float acts as a long lever arm out to leeward, and the further the boat tries to heel the more righting moment the float produces. This is the three-hull stability mechanism: rather than a heavy keel resisting heel by hanging weight to windward, the trimaran resists it by lifting buoyancy to leeward. The windward float lifts clear and adds nothing, so it is built light. Both floats carry their own watertight bulkheads and beam fittings.

Cross beams

The akas are the most heavily loaded parts of the boat. Each beam is a carbon-fibre tube or box section spanning from the vaka to a float, and it carries the full sailing load that the floats feed back to the hull. End fittings bond the beam into the main hull on one side and the float on the other. On a trailerable or marina-friendly trimaran the beams include a Beam Folding Mechanism, a hinge and lock arrangement that swings the floats inward against the main hull. Folded, the boat drops from over 8 m of beam to about 3.5 m, narrow enough for a road trailer or a standard marina berth. Locking pins and over-centre locks hold the beams rigid when deployed. Tensioned trampolines fill the gaps between the hull and the floats, giving crew a place to work and move forward.

Rig and sails

The rig is built around a rotating wing mast, an aerofoil-section spar that turns to present a clean leading edge to the mainsail and adds drive of its own. The mast is stayed by cap shrouds led to the float beams, a forestay and a backstay, all tensioned through turnbuckles at bonded chainplates, with spreaders pushing the shrouds outboard for support. A rotation arm and control tackle set how far the wing turns. The boom tensions the foot of the Square-Top Mainsail, a square-top, fully battened sail whose wide head gives power up high. Forward, a self-tacking jib crosses on its own track when tacking, and a large flat screecher flown off a bowsprit adds power for reaching and running. Both headsails set on furlers so they can be rolled away quickly.

Deck hardware

Sail trim runs through self-tailing winches, a mainsheet traveller on an athwartship track, rope clutches that hold loaded lines, and a set of blocks that lead the running rigging back to the cockpit. The Deck Hardware & Running Rigging is sized for the high loads a fast multihull generates, since a trimaran does not relieve sail pressure by heeling the way a monohull does. Halyards, sheets and control lines are low-stretch braid, and deck cleats handle mooring and anchoring.

Propulsion and systems

Auxiliary power comes from a retractable petrol outboard on a lifting bracket, raised clear of the water under sail to cut drag and lowered for manoeuvring and motoring in calms. The outboard drives a propeller through its lower unit and charges the house bank through a small alternator. Steering is by tiller: a lever on the rudder stock, with a tiller extension so the helm can steer from the windward side deck. The electrical system runs a lithium house bank charged by deck solar panels through an MPPT controller and by the outboard alternator. Navigation electronics include a chartplotter, masthead wind, speed and depth instruments feeding cockpit displays, a tiller Autopilot, and a VHF radio.

Accommodation

Living space is in the vaka, since the floats are too slender to use. A typical layout fits berths for five, a compact galley with a gimballed stove, a sink and a cool box, and a small heads compartment with a marine toilet plumbed to a holding tank. Fresh water is carried low in the bilge to keep weight central. The accommodation is lighter and simpler than a cruising catamaran of the same length, which is the trade the trimaran makes for its speed.