Catamaran Structure – Bridge Decks and Cross Beams

Editor’s Note: Many thanks to Ted Clements of Antares Catamarans and Shane Grover of Seawind Catamarans for patiently answering our questions.

There are a lot of reasons why catamarans are more expensive than monohulls. It’s not just the two hulls. There are many more complicated calculations and structures needed to build the complex shapes.

Building a bridge deck and the structures around a pair of hulls is a lot more difficult to design and build than a single hull, and we’ll explore a little about why.

Part 1: Forces on the Hulls

Load and force calculations on a boat hull isn’t a simple calculation, and even monohulls take a lot of designing to build a shape which performs well and has the strength to hold together at sea. Land vehicles have fairly predictable forces and motion on them, but boats can take forces and stresses in any direction.

Waves slam from all directions, boats plunge off waves and get smacked around in chop. Wind forces stress masts and rigging, which applies bending moments and forces to chain plates and the hull. Hulls flex and bend with this motion, and even from tensions applied to the rig.

Elements of drag, hull shape, keel shape and rig design all factor in, whether it’s a heavy, stiff cruising boat or a light, high performance racing machine. Those forces have to be figured, and materials and constructions are made to suit the conditions and situations where the boat will sail.

And that’s just a single hull. When you add a second hull to mix, you add in a whole new set of loads.

Bananas and Pencils

To illustrate these additional loads, we’ll do a thought experiment with a couple of household items. You can try it for real if you want to – but you’ll need two bananas and a few pencils.

Start with the bananas laying parallel to each other, then run a single pencil through the midpoint of each banana (the hulls) to connect them. They’re connected, but when you pick it up, what happens? They don’t stay parallel, of course. We need a second pencil or even a third one, to keep them in place.

The pencils are the crossbeams you’ll hear about in catamaran construction. If you put two of them through the bananas to connect them about 1/3 of the way from the end of each banana, you’ll get a fairly stable platform (for something made from soft fruit and pencils).

Imagine picking up this banamaran with two crossbeams with one hull in each hand. How can we still move the hulls?

First, we can twist one half back and one hand forward, putting lots of force on the crossbeams. To stop this, we could use much stronger beams, and we could put more beams at the ends of the bananas.

If you rotate your hands and the bananas, you demonstrate another type of force on the crossbeams. Pushing the bows or stern together also can move the hulls.

Building the Boat

Now imagine putting weight on the pencils – you’re adding the bridge deck. The mast sits on the bridge deck and creates additional loads and stresses on the crossbeams and hulls.

Finally, we add a sailing rig on top of the weight on the pencils. The rig needs support to stay up. On a monohull, stays run to the bow and stern to support the rig. But a catamaran the mast is centered between the hulls. The tension on the rig will provide upward pull on the hull shapes and usually attach to bow crossbeams. So we’re now pushing down on the middle of the bananas while pulling up on the tips.

The challenge to the catamaran builders and designers is to account for all these forces and build a pair of hulls capable of absorbing these loads without breaking or separating the hulls.

Part 2: Crossbeam Design

Catamarans are not new concepts; double hulled sailing canoes were used in Polynesia and Melanesia long before European explorers arrived. One of the first recreational catamarans was designed and built in 1876 by Nathanael Herreshoff and sailed well enough that the New York Yacht Club banned multihulls from racing.

Most beams are hollow to save weight in increase strength for the amount of material used. A hollow cylinder or rectangular tube gives more resistance to bending per pound of material than a solid rod of the same weight. There are mathematical explanations beyond both the scope of this article (and my ability to explain), but it’s important to know where the loads on beams are supported to understand this.

Greatly simplified – bending a beam creates compressive loads. The further from the center of the beam, the higher the resistance to compression. A rod will have a narrow diameter and resistance is lower. But if you make a cylinder or square tube from the same amount of metal (or fiberglass) you will have the same cross-sectional area, but the compressive force is applied further from the center of the cylinder.

Think of an I-beam from building construction. The compression is on the sides of the I-Beam, but the part in the middle is mostly to keep the sides in place, not bear the load on the beam in high stress applications.

Original Beams

A close look at these boats shows clear and obvious crossbeams connecting the hulls. Duplex, the early Herreshoff boat, had three clear crossbeams and a cockpit on the aft two between the hulls.

Smaller beach and racing cats will have obvious crossbeams, since the decking is usually a stretched piece of canvas or webbing. Other open bridgedeck catamarans, including many home builds, may have actual beams across them holding the hulls in place. And most cats will have some sort of beam across the bows as well.

But when you look at modern bridgedeck catamarans, you notice something strange about the beams. There aren’t any actual “beams” built into the boats.

Modern Cruising Cats – There Are No Beams

It’s more you notice something missing about the beams. Modern bridgedeck catamarans don’t generally have actual crossbeams built into them, as if you were glassing a beam or post into the boat. Instead, the construction of the boat is built around a design the provides the mathematical equivalent of a “beam.”

Bear with me. It took a while for me to get my head around this, too.

Picture a box – even a simple shoebox has rigidity to its sides. Yes, you can crush it, but the hollow sided box offers a lot more stiffness than a piece of cardboard on its own. The structure of the beams is in essence a box built between the hulls, with super strong modern laminate materials providing stiffness to take the loads and stresses.

When modern cats are designed, the “crossbeam” is a combination of internal structures built and connected to the bulkheads that create the load bearing capabilities of a hollow beam section. So the “beam” exists mathematically in the designer’s wireframe drawing of the boat, but when it is built, it is not an external beam added to the structure, but rather a set of structures that act like a beam because of their physical design.

Developments in materials technology over the last few decades allows for shapes and strengths that couldn’t be built with traditional materials like wood or metal.

Part 3: Building a Bridgedeck

Building the bridgedeck is the key piece of fiberglass catamaran strength. To be able to build a boat which can handle all these twisting and torsion forces, creating that “box” to add the strength, catamaran builders take one of several approaches. All can be effective and meet the design requirements, but there may be other reasons a builder chooses a particular approach.

“Tooling” refers to all the molds needed to shape a fiberglass hull. Tooling can be made from a number of different materials and represents a significant investment for any new production catamaran. Costs can run to many millions for tooling durable enough to build a hundred or more boats without changing design and build tolerances.

Molding and tooling to build hulls is a major expense, no matter which approach a builder takes. Building tools and molds can run into millions of dollars in expenses for materials and labor, and molds built for production runs of boats are considerably more expensive than tooling for a one-off or unique design. All of these factors into the decision making behind a build process.

Building a multihull also presents unique challenges compared to building monohulls. Building a single hull can be a fairly linear process – the hull is laid on the mold and built, the inside is fitted out, and decks are attached. While that’s a simplification of the process, it is relatively straight-forward because there is only one hull.

For catamarans, the integral bridgedeck structure doesn’t lend itself to a step-wise assembly. The crossbeams and bridgedeck need to be built integrally before the interior and decks are completely finished. For the builder, this means some parts of the boat have to be finished with reduced access to interior sections of the boat. For the designer, the challenge to is to make the boat so the builder can build it efficiently. For production vessels with build runs in the hundreds, cost effectiveness and production efficiency is crucial.

Not only does the bridgedeck hold two hulls together against all the twisting and torsion forces we’ve discussed, it also has to carry cargo. It holds the living space in the main saloon, as well as passengers and equipment.

Think back on the shoebox – it has compressive strength from the ends, but any individual side is fairly weak. You can deflect the sides easily. While this is fine on a shoebox, it would be disconcerting if the deck flexed and bounced when you walked across it. The bridgedeck also needs strength from the top and bottom to take this load in the central parts of the boat, away from the “box.” You can’t just make the decks and floors massive, that sacrifices headroom and internal volume. Instead, internal structures and stiff construction materials have to take up the load.

Two Piece Molding

Some builders build two hulls individually. One mold can be used if the hulls are identical, and the hulls joined later in the build process. Like any design and build decision, there are pros and cons for the builder which can affect the cost of the final product. If built correctly, there are no compromises in strength from a one piece mold.

For the builder, a one piece mold is much easier to handle. Any hull built on a mold will have to be removed from the mold once the hull is laid, and a single mold is smaller, lighter, and narrower. Breaking a hull out of a mold is a complex process, and may involve cranes and heavy equipment to support the hull as it comes of the mold and to protect the tooling from damage. To remove a hull, you need space to lift the mold and to get heavy equipment around the tooling. And you’ve got to put the molds some place when it’s done.

Connecting two hulls precisely once they’re molded is a more complex task. Unidirectional fibers bond key structures to bulkheads to build the support “box” making up the crossbeams. Although a one piece mold will give an inherently stronger single-material connection between the hulls, more than sufficient strength can be built in with advanced fiber and resin choices (such as carbon fiber and epoxy) when then build the deck connection.

One Piece Molding

Most production catamaran builders have moved to one piece molding. A single tool is built to lay up both hulls and the bottom of the bridgedeck connection in a single large piece. The fundamental strength of the build is higher, allowing for less expensive materials to get the same strength.

“One piece” is a slight misnomer, as the top of the hulls and decking is built in a second mold which is laid over the hulls and bonded along the joint near the top of the hull molds. The loads on a join between the bottom hull and top deck are considerably lower than those on the bridgedeck. There’s no real twisting and torsion on that part of the boat relative to the join between the hulls, so it can be laminated without the same concerns as building between the hulls. (Editor’s Note: Some builders mold the bridgedeck and inboard half of the hulls together and then mold on the outboard halves of the hulls and deck on top. There is a seam running stem to stern centerline along the bottom of each hull with this technique.)

There are a number of production and build advantages to this approach. Though the tooling is larger, more expensive and awkward, the lay up process incorporates the core hull joins in the initial build. You don’t have to line anything up and glass it in place when your hulls and deck are built connected. There isn’t need for as much material buildup to shore up the “box” since it’s part of the integral build.

Fitting the interior joinery and finish is more challenging, since the crossbeams and related components must be built early in the process. Extra care and planning in the design process can make this more efficient, but access to internal areas of the hull can be difficult during the build.

Alternative Materials – Wood and Metal

Composite construction – fibers and resin – lets builders make nearly any shape. Fiberglass has allowed for the wide range of affordable to high end production catamarans available today. Stronger fibers and better resins have only expanded the possibilities for light, fast, and strong boats.

Wood is rarely used for structural elements in modern production catamarans. It can be heavy, and it doesn’t lend itself to the complex molded shapes designers demand for optimal strength and seaworthiness. Plywood may be used for stiffening in places, and balsa cored decks may still be found. But mostly in older boats, home builds, and kit boats.

Metal construction has its own problems, and very few catamarans are built from metals. Curved shapes are difficult with metals; bending a smooth radius into a flat sheet without bending it, then welding it into place requires time and skill. And weight will be a problem. On the whole, fiberglass is a better solution for a light, strong catamaran.

Conclusion: Tying it Together

This overview only touches on some of the challenges multihull builders face, which monohull builders do not. But what does it mean for you when you’re looking at catamarans to buy? All the fiberglass build techniques will result in a strong boat if built properly. There may be differences in the amount of materials used, and choices for resins and fibers, but the boat should be evaluated as a whole.

It’s good to understand how your boat is built, and to be aware of some of the strengths and limitations of each build technique. But no build technique is inherently better or worse than the other – no matter how your next boat is built, the designer and builder will ensure the build is up to the task.