Separate the Hulls

The steering blade needed a support that could carry its reaction into the boat. A side float presents the same structural question on a larger scale. Water pushes upward beneath a body some distance from the main hull. Something must connect that support to the people, rig and structure it helps to carry.

The opening identified a catamaran’s two hulls, a trimaran’s main hull and two side floats, and the offset support of an outrigger. Their spacing changes the restoring geometry; their connections make that geometry usable under load.

Give the separated support a route home

Picture a trimaran viewed from ahead, with its main hull in the middle and a narrower float on each side. A crossbeam is a structural member extending across the intervening space and connecting the hulls. With forward and after crossbeams, the assembly also connects the hulls at separated positions along their length. We met an after crossbeam as a place to lead a sheet; its principal job is to transmit loads between the floating bodies.

When the leeward float is pushed farther into the water, its increased buoyancy acts upward away from the main hull. The float’s shell and supporting structure carry that load to its beam attachments. The crossbeams carry it across to their attachments and structure in the main hull. The separated upward support then contributes to the restoring moment of the whole craft. The beam needs adequate strength and stiffness, and so do the joints at both ends.

The route works in more than one direction. Mast compression, crew weight and other loads in the central structure can be shared with the outer supports. A wave can lift the forward end of one float while the other hulls are supported differently. The crossbeams and their connections then experience bending, shear and a tendency to twist the connected platform. That torsion follows from unequal loads and their positions, even if the boat’s total weight has not changed.

A catamaran distributes the same connecting job between two principal hulls. Its crossbeams can be visible separate members or part of a more extensive connected structure. A monohull carries its corresponding hull-wide loads within one continuous buoyant body. Counting hulls identifies the broad layout; it does not tell us the material, the joint detail or how much relative movement the designer allows.

The construction principles remain those already established. A bonded beam root needs a route from the beam into the surrounding structure. A bolted connection needs supported bearing surfaces and a way to spread its reactions. Where a connection passes through sandwich construction, its concentrated load needs the appropriate insert, solid laminate or other designed detail to reach the load-carrying skins. The presence of a sandwich panel alone does not reveal which detail a particular boat contains.

Read the names in their setting

Contemporary English multihull descriptions sometimes call the main hull the vaka, an outrigger hull or float an ama, and a connecting beam an aka. In that usage, a trimaran has one vaka and two amas, linked by its akas. The ama supplies offset buoyancy; the aka carries the connection loads; the vaka provides the central hull and its share of buoyancy, structure and carrying space.

These words need their setting. Te Papa’s record of a Niuean canoe made by Harry Nemaia, Piavale and Donald Evans in 1973–74 calls the craft a vaka and its float an ama. It names the connecting booms kiato and the sticks joining those booms to the float tutuki. The documented Niuean arrangement therefore identifies the connections more specifically than a modern English list of three parts would do.

The museum record also places that craft within a living canoe-building practice. Its date records this particular making, not the invention of outriggers. Likewise, using vaka for a modern trimaran’s main hull does not establish that every community used the word for precisely that component. The names can help us understand a description while the named craft and its actual connections tell us what is present.

For a contemporary example of the English usage, Dibley Marine’s Robinson 5000 design account calls the outer hulls amas and the main hull the vaka. Folding the outer hulls inward was part of the brief for trailer transport. That requirement changes the connection machinery while retaining the sailing job of holding buoyant supports out to the sides.

Hobie’s Adventure Island manual shows a small folding arrangement in useful detail. In the illustrated edition, the inboard end of an aka has a bracket that straddles a deck bar on the main hull and is retained by a separate pin. Its curved outboard end enters an opening in the ama, with a bungee retained over a post. The tube and its mating surfaces locate the float; the retaining parts keep the assembled connection together.

The rear aka also has a diagonal brace running back towards a ball beside the seat. A spring-loaded sleeve secures the brace’s socket over that ball. With the brace engaged, its triangular route restrains the aka from folding around its inboard pivot. Releasing the specified connection allows the geometry to fold for the intended handling or storage operation. This edition’s manual explicitly prohibits sailing with the amas folded. A hinge gives a useful motion only when the sailing loads also have their intended restraint.

Change the joint without changing the job

A connection can be designed to allow some movement while still carrying substantial load. Wharram describes earlier catamaran crossbeams mounted with rubber blocks and steel attachments, followed by rope-lashed beam connections in the Pahi range from 1980. In the designer’s account, the laminated timber beams and their attachments form a deliberately compliant structure.

The lashings hold the beam and hull together through tension and contact at the joint, while the specified assembly permits some relative movement. The beam still spans the gap and carries load. Its allowed deflection and the joint’s movement belong to the design; replacing a timber beam with a superficially similar rigid metal section would change the behaviour of the assembly. The comparison concerns designed compliance, not a general rule that every multihull should flex equally.

Corsair’s folding trimarans provide a different documented connection. In the C24 and C31 arrangements described by its 2014 sailing manual, a beam pad fills a gap between the beam and the main hull above the upper folding strut. When the float pushes upward under sailing load, the pad supplies a compression contact between the beam and hull. The folding mechanism and its deployed contacts work together to carry the reaction.

The manual’s beam-end drawing and explanation give the C28 pad a different qualification, so the C24/C31 account cannot be applied to every model merely because all fold. Folding describes the permitted movement; it does not specify every contact that supports the deployed boat.

The material choices also coexist. The wood-and-epoxy trimaran Adagio, completed in 1970 and discussed in the construction chapters, already combined a multihull arrangement with bonded timber construction. Later examples can use laminated beams, metal tubes or composite structures to satisfy particular load, manufacturing and transport requirements. Hull count does not itself select a material or establish a single sequence of replacement.

Put a working surface across the gap

The crew needs somewhere to stand or move between the supports. A multihull bridge deck, often written bridgedeck, is a platform spanning between hulls. A solid platform carries a person’s weight through its panels and supporting structure into the crossbeams and hulls. Its spanning surface and the beams that support it are related parts with different immediate jobs.

A trampoline provides a tensioned mesh, webbing or suitable fabric working surface between the multihull’s structures. The name also appears simply as net. A person’s downward load deflects the surface and produces tension towards its edges. The edge attachments carry that tension into their supporting hulls, beams or support lines. An open surface can let water pass through, but its ability to carry people still depends on the surface, attachments and supporting structure remaining sound.

Corsair’s manual describes nets that extend with the folding beams and acquire their working tension in the deployed arrangement. It also identifies places where lashing is omitted to leave clearance for the folding movement. The connection has to carry the working surface while accommodating the actual mechanism. Tightening every apparent gap without understanding that movement would change the assembly.

World Sailing’s multihull rules use net and trampoline interchangeably and explicitly connect their edges to supporting lines. That documented usage helps identify the surface and its load route; compliance with a particular rule requires more than recognising the name. Neither a trampoline nor a solid bridge deck removes the need for the cross-hull structural connection beneath or around it.

There is another nautical use of bridge deck for a raised barrier between a cockpit and its entrance below. We will meet that separate arrangement with deck openings. Here the word identifies the structure spanning between multihull hulls.

Leave space for moving water

Bridgedeck clearance is the gap between the water and the underside of the structure spanning between the hulls. For a stationary boat in calm water, the measured gap depends on its loading and the particular underside being measured. Add load without changing the hulls and the boat must displace more water; its waterline rises on the hull and the available clearance can decrease.

At sea, the gap changes as the yacht and the water move. A crest can rise between the hulls while the spanning structure moves down towards it. Contact with a solid underside can produce the sudden pressure and impact of slamming. The relevant shape includes low projections, the fore-and-aft extent of the bridge deck and the position of its forward edge, as well as one quoted height above a calm-water line.

Richard Woods illustrates the importance of that distinction with the footwells of his Gypsy and Romany designs. A lowered central footwell has a different underside extent from a low platform spanning the entire width. His bridgedeck discussion directs attention to the actual surface water can strike. A narrow projection and a broad flat underside cannot be treated as the same encounter just because their lowest points have the same clearance.

Now place the forward part of one hull on a crest while the other is in a trough. The changing buoyant reactions load the crossbeams and connected structure unevenly. A subsequent impact on the spanning underside adds another load at another location. Clearance and torsional strength therefore address different parts of the problem: one concerns the relative space and encounter with water, the other how the connected structure carries unequal reactions. Two hulls alone settle neither question.

Follow separated buoyancy through heel

Consider a symmetrical two-hull model with its hulls connected rigidly, its mass secured, and all openings closed. Hold total mass, its vertical centre of gravity and the individual hull shapes fixed. Compare different hull spacings in still water, with no sail force and a stated fore-and-aft trim condition. This isolates a geometric question before adding the movements of a sailing yacht.

As the model heels slightly, the leeward hull immerses more and the windward hull less. More of the buoyant support acts to leeward, moving the combined centre of buoyancy away from its upright position. The separated bodies can therefore provide a large initial righting arm. In the small-angle description, that geometry can put the transverse metacentre high above the fixed centre of gravity and give a large GM. Such a boat is stiff in the initial-stability sense.

Barry Deakin’s experimental work used an adjustable catamaran model to examine this relationship. Its standard, narrow and wide configurations retained the stated mass and vertical centre of gravity while changing hull spacing. The reported righting-arm curves show that changing spacing alters both the early restoring response and the curve through larger angles. The model’s roll inertia also changed, so its dynamic results cannot be treated as a test of spacing alone.

The still-water comparison already explains why the first few degrees are insufficient. Once the windward hull is clear of the water, further heel no longer transfers support from that hull to the leeward one. The position of the whole craft’s centre of gravity continues to change relative to the remaining immersed shape. Its righting arm can reach a maximum and then decrease towards the angle of vanishing stability. The large-angle curve need not preserve the ranking suggested by GM.

Positive stability over the early part of that curve means a restoring moment towards upright. Beyond a zero crossing, a negative righting arm gives an overturning tendency relative to the original upright position. An intact multihull can also have a stable inverted floating condition. Strong initial resistance to heel therefore does not establish self-righting after capsize. Its upside-down buoyancy and mass geometry must be considered as well.

A ballast ratio adds another piece of information without settling the comparison. A multihull with no dedicated ballast can obtain substantial initial restoring leverage from separated buoyancy. A ballasted monohull may have a lower initial GM yet retain positive righting arms over a different range of angles because its mass is low in relation to its changing buoyant support. To compare two actual yachts, we need their actual hulls, mass distribution, loading and opening conditions. Neither hull count nor one ratio supplies the missing curve.

A knockdown, capsize and recovery also happen to a boat that can move and take in water. The intact model’s downflooding angle has been excluded by keeping it closed; an actual yacht’s vulnerable opening may admit water before it reaches the intact curve’s limit. Flooding changes the state being analysed. A tender initial response is not by itself proof of poor ultimate recovery, and a stiff response is not protection against every overturning encounter. These descriptions answer different questions about a specified boat.

Crew position gives a familiar connection between the alternatives. Moving crew to windward moves part of the total mass and changes its restoring leverage; the wire-supported crew encountered earlier use that same principle. The leeward float contributes by changing where buoyancy acts. Both affect the total moment, but one moves mass while the other changes the distribution of buoyant support.

Let the two ends exchange jobs

A proa belongs to an asymmetric multihull family, commonly combining a main hull with an outrigger. Some forms change sailing direction by exchanging the jobs of their two ends. The specific arrangement matters as much as the broad name.

The CHamoru flying proa described in Guampedia keeps its smaller outrigger to windward. Its main hull has corresponding ends, and the triangular sail and steering arrangement can be worked from the other end when the direction reverses. In the manoeuvre commonly called shunting, the former stern becomes the bow and the former bow becomes the stern. Sail and steering functions move to suit the new direction while the float remains on the windward side.

That requires a different working arrangement from tacking a conventional yacht, whose bow remains its bow as it turns through the wind. The regional account describes the change of ends and the sail and paddle-like steering equipment. It does not make every Pacific outrigger a shunting craft, or establish one universal line-handling procedure for proas.

Keeping the float to windward also changes how to read its restoring role. As a windward float rises, its own weight and any crew placed outboard can contribute a restoring moment through the connecting structure; while immersed, its buoyancy is part of the boat’s support. A leeward float pushed deeper supplies increasing buoyant reaction instead. The useful force depends on which side the float occupies, its immersion and the placement of mass, as well as the fact that a float exists.

The Niuean making record, the CHamoru sailing account and the modern folding trimarans describe distinct practices and purposes. They share the physical problem of connecting separated support, without supplying evidence of a single direct line of descent. The same geometry can serve local fishing, a small sailing craft taken home on a trailer, or a larger yacht with a spanning working platform.

The next changes make the support adjustable in another way. Moving ballast changes mass leverage; moving an underwater surface changes its position or the water force it produces. Those jobs must remain distinct even when one yacht combines them.