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Verrrrry interesting…. but STUPID.
— Arte Johnson (I’m half kidding)
Considering Multihull TriloBoats
Judging by the amount of correspondence I field on the subject, there’s a fair amount of interest in multihull TriloBoats – catamarans (cats), trimarans (tris) and proas.
Here, I’ll lay out the modest benefits and considerable pitfalls I see in this approach at cruising sizes, along with an impressive example of a smaller scale project that I consider highly successful.
I warn you… this is rather dull going. Unless you’re particularly interested, I’d bail.
A bit of jargon: Amas are the longitudinal ‘hulls’ composing a multihull. There’s some variation in use about the net, but it seems a pretty generally accepted term.
NOTE: The T40x20 CATAMARAN, shown above, was a ‘cartoon’ made on request for relatives who’d fallen in love with Phil Bolger’s DOUBLE EAGLE. It’s way beyond my engineering skills to even put it out as a design, but gives a taste of a square boat approach.
I should emphasize that this approach is not our taste, in general. Anke and I would be off on a Wharram TIKI, or at least a dory-ish trimaran if we ever succumbed to the allure of polyhullery and warm waters.
Multihull Principles
The use of multiple hulls is primarily to greatly improve stability. They may follow one or a mix of two strategies:
Float the leeward ama – Its reserve displacement resists heeling moment, tending the whole to low angles of heel. Its wetted surface and resulting drag, however, slow the vessel and induce lee helm.
Hike the windward ama – Its weight, lifted at the long end of a lever arm, resists heeling moment, tending the whole to low angles of heel. Lifting up and clear of the water’s surface reduces wetted surface and resulting drag, as well as hull-dynamic weather helm, optimizing speed.
Cruising catamarans favor the first strategy, while proas favor the second, as do most racing or speed maximizing craft. Trimarans mix the strategy, and tend to work with shorter lever arms.
The longer and leaner the hull(s), the faster the vessel can be. Wharrams suggest a minimum of 11:1 length to waterline beam ratio. Such hulls are easily driven to higher-than-displacement speeds. Extremely narrow hulls with very high freeboard (over low draft) are enabled by the amas, which keep them from flopping over onto their sides.
Circular hull sections are fastest, but have low lateral resistance and can’t be easily built of sheet materials. Rectangular sections carry the most load (assuming equal beam and draft), and have high lateral resistance. V sections have good lateral resistance and speed, but low displacement on a given draft.
Long and narrow has ergonomic consequences. In order to have a wide enough interior to fit even a snug double berth within a fast hull, the hull quickly gets long. Flare above the waterline can help, but a platform must be placed high to take advantage, reducing its headroom. The interior is linear, in shorter hulls, with little opportunity for circular social settings below-decks.
A common solution is to live largely on top of the hull/ama(s)/deck. But this generates considerable superstructure which adds weight and windage.
A mixed strategy is often employed… low superstructure to provide headroom over the narrow hull, into which, perhaps, only feet may dangle. It may overhang the sides to help provide ‘elbow-room’, storage or even low headroom bunks.
Wharrams recommend a 3ft sea-riding height for bluewater cats (I take that as applying generally to multis), measured from the waterline to the underside of decks between amas.
Caveat to Multihulls vs Monohulls
Here are some for DIY builders to consider when comparing types:
For a given footprint, a multihull adds cost, complexity and engineering challenges.
You are building two to three monohulls, cross beams to join them, with one or more decks and/or superstructures over. Each of these components represents a fair chunk of the effort required by a monohull. Special challenges – like mast steps, Ackermann steering, high leverage forces throughout – make design daunting to dangerous for the amateur. Everything multiplied x multi.
In terms of square sections, things get scarier… the initial and reserve buoyancy of a slab is much higher than usual sections. Larger forces develop faster than the usual rules-of-thumb were evolved to handle.
If one joins the very few pioneers in this field, I recommend a cautious approach, backed by modelling, certified expert advice and incremental sea-trials.
The payoffs are speed(!) under sail, humongous decks and undeniably cool. For the pure of heart, able to run lightly through the world, multihulls can more than pay their way.
So, with that caveat, let’s continue…
Simple Conversions: Adding Outriggers to Square Boats
Adding amas to a full-width TriloBoat designed as a cruising hull, I feel, is a losing proposition.
Triloboats and other box barges already have the highest monohull form stability possible on a given footprint (length x beam x draft).
As cruisers, box barges skim upright downwind, benefitting from shoal draft relative to their displacement (don’t have to push much water aside). On the wind, they benefit from heeling by presenting a V section to the water.
A simple, multihull conversion detracts from both. The outrigger(s) add displacement and drag off the wind, and force the hull more upright on the wind. Plus, they clutter the deck, increase the beam with high windage, vulnerable bits, and – starting from the high mono-deck – don’t offer usable sidedecks.
Critiquing the T40x20 CATAMARAN
Let’s take for granted that my T40x20 CATAMARAN can be well-engineered to be affordably and soundly built.
Even so, the two amas together total only 8ft hull beam. Displacement is roughly equivalent, then, to a T40x8 square monohull. But the monohull wouldn’t require the two, inboard ama sidewalls. We’d be able to live within the T40x8, rather than on top of it, eliminating much of the superstructure with its weight and windage.
If a large living platform were the goal, converting to a barge by hulling over between amas would increase displacement by roughly 250%, and likely halve construction effort.
Box Section Approach to Multihulls
So let’s look at it from the other direction. Could a ‘square’ multhull be worthwhile, designed from the ground up? This, to my mind, shows much more promise.
Triloboat approaches that might apply:
  • Square sections
  • Constant section
  • Whole and even fractions of sheet materials
These are independent, and can be considered throughout the design.
We’d likely want to start with a long, slender hull, as per multihull normal, for an easily driven hull.
Square sections carry the same weight on less draft than all others. That can be useful. Their right angle chines provide good lateral resistance. They’re easy to build, and interiors are easier to fit. Hull mid-bodies benefit most.
But flat bottoms forward – especially when held upright by amas – pound when slapped by the water’s surface. What to do?
Fining down the bow – and maybe adding a cutwater (a sharp, faux hull bonded under the bow) – helps a lot with this. I’d consider matching the forward curves in plan and profile (TAB) for least turbulence.
I doubt the stern needs to narrow, and full width preserves precious deck space at the transom. I’d consider a very easy exit, with the bottom of the transom at or slightly above the waterline. Being a multihull, we’re not going to heel much (so won’t drag the transom corners), while the release wave depresses to help match the exit angle. Never actually seen this in square hull action, so needs experiment.
Constant sections – of whatever shape – naturally develop parallel longitudinal lines (such as the sheer) along the constant section. This is especially useful amidships, with superstructure planned and fit along this stretch.
Constant section amas , with shaped foam ends (where shapes get complex) and glassed over, show promise, I think, for easy construction and good performance. I’d personally favor assymetrical V sections (especially Newick plow-style), with the leeward face(s) oriented vertically for lateral resistance.
Whole and even fractions of sheet materials can be applied to hull, superstructure and decks, for economies of effort an material.
Another approach to square sections might be to rotate the square sectioned hull 45deg, for an upright V section. We’d end up with a diamond-style section resembling Superman’s chest logo (variations are possible). The right-angle keel would run the entire centerline, eliminating any need for a cutwater at the bow. Downside is increased draft for the same displacement.
A Square Trimaran: (Mostly) A Success Story
Mark Meyer designed and built an upright box section trimaran at 27 feet. It is wicked fast, fun, good looking (I think) and carried his family safely across the waters of northern Southeast Alaska. He used it extensively to fish Tenakee Inlet for at least a decade.
Notably, he used square section, aluminum girders, rigidly mounted, for cross-beams. These worked very well.
His only disappointment were the flat-bottomed amas – skimming across the water they pounded hard. He added a V section cutwater toward the bow, which improved the situation, but not much. Apparently, flat-bottomed, flying amas proved to be a poor choice.
NOTE: Wharram Designs abandoned flat-bottoms for their catamarans for the same reason… the windward ama raised enough to pound badly. Nevertheless, they safely travelled far and wide.
Mark definitively proved the concept for the main hull, and new amas were in the works last I checked in.
My personal opinion is that  square sections are a viable choice for hulls that stay immersed, especially when building quick and dirty or on a small scale.
I, myself would opt  for other shapes in any project large enough to represent a significant investment. It seems to me that, beyond a certain point, the savings in construction effort cut into long-term multihull values.
But that’s me.
So there you are, dear Readers. For those of you interested in these exotic pursuits, I wish you happy doodling.
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