JF: Okay, but why are these folding systems heavier than demountable ones?
MW: The lightest solution will always theoretically be with fixed crossbeams and amas. With such designs, there are less hard points with concentrated stresses, so more of the structure can share the loads.
Rigidly attached (generally bolted) demountable systems are a compromise between a fixed built-in system, and a folding one, and bring with them less of a weight penalty than a pure folding system. The interconnection with the main hull can be spread, as for a built-in. But mating surfaces will need to be doubled (one on each part) and the actual fixations themselves (bolts etc) will be extra too. A demountable design will not always have a very deep connection at the hull, so in such a case, additional waterstays will be required, and whether they are a simple S/S strap or a steel wire with fittings and attachment plates etc. they will weigh slightly more than a built-in.
One of the simplest demountable solutions was developed by former aeronautical engineer Lock Crowther for his early Buccaneer designs. In effect, the aka was simply a tubular strut with a short sleeve joint that acted more like a ball joint than anything more rigid. The amas were kept in place and position by waterstay straps passing right through the main hull to the other side, and then, by the outer mast shrouds that held the amas up.
It was superbly simple, though if anything broke, the whole rig AND boat just came apart. I once owned two of these and made sure this never happened. Lock himself used to ocean cruise the larger Buccaneer without failure. (The design did show some lower shrouds though, in order to support the mast while lowering, just as on the Farrier boats).
Folding systems ultimately attach to the cross beam (aka) or ama, at one or two points, and that point will carry pretty high loads. Extra reinforcement will be required to spread that load into the whole arm or ama itself and that will add unavoidable weight.
The closer the supporting brace or bracket is to the interconnection of the cross beam with the main hull, the higher the load it will carry (unless waterstays are used) and the more load there will be at the connection points. This will require heavier reinforcement, and hence more weight… although with a composite fabricated aka, the use of carbon graphite can help a little to reduce this weight.
The same point loading will apply to the attachment on each side of the main hull too. The lightest weight systems can theoretically be made by using wire waterstays to take most of the load. But there's a trade-off here with reliability, as sometimes the fittings or attachments can fail without being able to anticipate this happening. The only safe thing here is to over-design this part. But again, this adds more weight.
Using aluminum alloys can help reduce weight, but this material is often not as tough and reliable as stainless steel, and is more susceptible to corrosion in salt water. But stainless steel adds more weight, and so on it goes. The Farrier boats for example, chose a good aluminum‑alloy for their folding arms, while Dragonfly choose to stay with tough, corrosion resisting stainless steel… not surprising considering their location near the Baltic Sea.
JF: While on the subject of demountable systems—are there any other advantages… and what are the main disadvantages?
MW: Perhaps the main advantages other than saving weight are simplicity, lower cost and potentially higher performance. Simplicity is easy to understand, but why the latter?
Well, demountable systems permit designs with a greater beam than folding systems can tolerate. Without special road permits or expensive escort vehicles, folded trailerable boats must not exceed 8'-6" in overall beam for most state highways, and this challenges the designer and ultimately limits the maximum beam that can be designed in.
Even most swing-arm models require that the amas be unbolted and stored on their trailer closer to the main hull, in order to stay within the trailing width limits. Additionally, greater beam would add significantly higher loads to the folding parts, and only systems using added waterstays could readily handle that.
One of the first designers to show that greater sailing beam added significantly to the performance, was the Australian Lock Crowther. Over 40 years ago his designs stood out for what then seemed like an extreme beam… on both his trimarans and his catamarans. Today, this higher beam is far more common, and is now used on all performance multihulls. Higher beam means a greater ability to carry sail, and that translates into acceleration on a wind puff and more overall speed. (Of course, it can also become a major issue navigating and mooring within a marina)!
The first Dragonfly (Magic Hempel) was a fine example of this format, with 22' of beam on a length of 25.5'. Anyone who has sailed on her will vouch for the amazing acceleration this gave. Instead of losing a wind squall to heeling and the associated resistance, the boat just accelerated with enough zest to almost take one off their feet if not braced for it. The boat also sailed with less heel of course. This was not only drier and more comfortable, but also added efficiencies of its own, with less hull drag and the more vertical presentation of the sail square to the wind.
The later swing arm version was tamed down somewhat and the beam reduced to 19.5'. This was still relatively high for a folding system, and was made possible by the retained use of waterstays, but not as extreme as was possible with the demountable one.
In the case of Magic Hempel, the cross beams were made from a full width mast section, passed through the hull. Although the section sagged a fair bit with the amas slipped on, these one piece beams permitted the boat to be assembled and launched without it's mast. As long as the water was calm, the boat could be motored to another site for mast installation. Only after the mast was up and the shrouds tensioned, would the amas be lifted up, the cross tube straightened and the waterstays take their correct tension.
The disadvantages are mostly in the set-up time and the extra help required to do this. If the boat can be kept all season on a mooring, then I consider this a minor issue and the extra performance well worth it. Assembly for Magic Hempel was only about 30 mins for the amas. But lashing up the tramps and installing the 40 ft mast, took at least another 2 hours with 2–3 persons—not such a practical solution for a sailor who wanted to travel from area to area, or who had no permanent mooring or dockspace.
The demountable system gets even easier to erect for smaller trimarans (say under 7 m) and I am surprised and dismayed that there are not more trimarans using this system. One other disadvantage for the smaller boats is that a straight cross beam can end up too low in relation to the water level. For this reason, most small tris need to use cross beams that are arched up on each side. Lock Crowther partially solved that with a two-piece system, and some dihedral between them. But then, having the mast in place at all times was essential to holding the boat together. As mentioned earlier, every design is a compromise, and once the owner has input his needs, it's up to the designer to make the difficult choices.
Note: Because boat design is a constantly evolving art, the writer recommends that interested readers contact the designer or builder of preference to get the latest specs on his or her design, as some details may have changed since this information was first written up. Also, other than the designers already mentioned, the writer also recommends noted designers such as Kurt Hughes, Chris White, Derek Kelsall and Richard Woods (among others that might also include the Searunner Constant Camber boats) who have some interesting designs that reflect years of experience.
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