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October 2008

Comparing Trimaran Folding Systems — Part 1

The following questions were put to me by Joe Farinaccio (from SmallTrimarans.com) about folding systems…

JF: Can you offer some insight into the various trimaran folding systems in use by current models of production trimarans?

MW: Sure, I'll give it to you as I see it… as an engineer but also as a user — a sailor ;-)

First a little history and description…

There are two basic systems in current production… what I term the ladder-frame folding system that Ian Farrier developed, and the swing arm system used by Dragonfly Boats Other manufacturers have generally worked from one of these basic types with their own models.

The newer Telstar 28, for example, uses a modified swing arm system, which is quite different from the now obsolete hinged drop-down system of the earlier, heavy, solid deck Telstar 26. But as I've not personally seen the Telstar 28 system, I cannot comment on it directly, though certainly some of its features must parallel those of the Dragonfly.

Although designer Jim Brown used an early swing arm system with truss arms, Ian seems to be the first to incorporate a really good working system into a production trimaran. It was initially developed for his plywood Trailertri 680 and 720 series while in Australia, but then became known and appreciated in the USA with his fiberglass production F27.

For those early models (pre '86), Ian had the aka's (arms) extend into the hull deck almost to the centerline. This required large wells and recesses to accommodate them. They were securely bolted down, and gave great rigidity in all directions, and ultimate security.

I remember when I first met Ian, on board his first F27, at an East Coast boat show. His son, then about 12, would demonstrate how easy it was to fold the amas in. Virtually every one coming on board would see a demo to get the picture of how easy it was.
Ian soon had F27s crossing both the Atlantic and the Pacific, and his system became proven without reservation. As I noted though, his folding system back them did create some big deck recesses, which took away significant interior space, and the long akas stood up very tall when the system was folded, adding nearly 2 feet to the height of the folding rig.

As time went by, Ian must have reviewed the acting forces, and worked on creating shorter arms (which also reduced the recesses in the main hull). The result was an improved system, without significantly weakening the essential parts of it. A light diagonal line under the tramps provided horizontal stability to the amas, and the revised design became somewhat lighter.

So the Farrier system evolved a bit, and it's clearly proven itself with the number of trimarans now successfully using it. Ian Farrier used high-strength alloy for his ladder frames, struts, mounting pivots and backing plates etc. And the arms (akas) are reinforced with carbon graphite fibers, and constructed to strict controls governed by Ian's well detailed layup plans.

By contrast, the Swing Arm system is a horizontally folding demountable system. The arms are basically formed from stainless-steel fabrications, with a streamlined cover of fiberglass, all pivoting on substantial vertical S/S bolts or bearings. Cables are used to crank the amas in and out. Additional vertical support is commonly provided through steel wire waterstays.

All folding systems will be heavier than a simple demountable one. But because it's not easy to get a good attachment of the amas to the akas with a pivot, the swing system tends to add somewhat more weight than even the Farrier concept.

I was once told that the difference in weight of a folding system over a demountable one was in the order of 500-600 lbs. Part of that is also influenced by the choice to use stainless steel instead of alloy (see later).

The swing-arm system moves the hulls rearward when folded, so it increases the folded length of the whole boat somewhat. When the system is open for sailing, the inherent weakness of pivots requires that waterstays still be fitted, just as used for demountable types. Although the system can be trailed for short distances as folded, the amas need to be removed from the akas and stowed lower on the trailer, in order to fit within the permitted trailing width. This is not so different from the work required for a demountable tri, and if frequent trailing is an issue, it's something to be considered.

JF: Can you offer an opinion and compare the pros and cons between folding systems?

MW: At first glance, there seems no competition. The Farrier system appears both stronger and potentially lighter. Plus, the fact that the folded boat can more quickly be on the road within the permitted trailing width. I also like the flat upper surfaces of his akas, that are great to walk on. But there are disadvantages to consider also.

First, the folding Farrier-type amas turn sideways down into the water, so that their exterior surfaces lie in the water all the time the boat is folded. So if you need to fold the trimaran in order to fit in a standard marina on a regular basis, you will have to pay attention to cleaning those amas. Another solution might be some form of bag enclosure, with some added chemical, to prevent marine growth. Further, the amas are not in such a friendly position to either moor the boat against a dock, or to use for walking on, as with a system that leaves them horizontal.

I have a sailing friend who finally chose to buy a Dragonfly 800, and one of his reasons was because of this issue. He chose to buy his boat in Europe, and spent two summers passing through the miles of inland waterways to visit many inland cities that they link. The swing arm system suited his needs perfectly. The amas stayed horizontal in place and gave him extra walking surface, easy access to their storage space, were easy to moor against in confined areas and they did not lie in the water with the opportunity to get dirty.

The Farrier-type system, however, is tough, easy to use and ultimately very strong. So if one is contemplating a rough trip, an ocean voyage, or can moor the boat with the arms extended, then this could be the better system for your needs. Also, if you plan on doing a lot of trailing from one place to another, the set-up time is shorter than for the swing arm system.

On the Telstars, geometry plays a part. Apparently, the pivots are set so that the amas actually drop lower as they are cranked in. (Also true for the Farrier system). Although this will add stability to the hull when folded up, it will take a little more effort to get the amas there. By contrast, the amas will be easier to crank out and will tend to default to their outer position, having to provide less static buoyancy.

Either way, the two folding systems are tried and proven, and one needs to look beyond just the folding system to make ones choice of the best boat for your personal needs. The Farrier-Corsair boats, Dragonfly's and now the Telstars… are the three major current production folding trimaran designs. They have different hulls shapes and both feel and react somewhat differently when sailed. So it's worth trying them before making any purchase decision based on the folding system alone. For example, a local fleet of one type or the other, may also be part of your decision. Or you may need space over performance. So this is just a look at the folding systems, nothing more.

Continued in Part 2…


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