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December 2019                             .

Comparing Trimaran Folding Systems — Part 3

Hinge and Latch system

This system works well on small trimarans and is employed on the Cross 18, Discovery 20 and the W17.    

On the older Cross 18, this hinge system is created with vertical metal plates bolted to the sides of relatively small beams, and mating plates are sandwiched together with a pivot bolt.   Once unfolded, a lock bolt is added to keep it open.    This system means that the plates do project out above the beam.

The Discovery 20 has large vertical reinforced plywood palms, the full height of its deep beams, held in place with large bolts.   Very strong, because the beams are large and deep.   But as initially designed, the amas rose high in the air when folded, and as this created considerable windage on the highway, some Discovery owner/builders have dispensed with the original hinging and they now remove the amas completely at the joint.   This permits them to be stowed much lower on a custom trailer and while this solves the windage issue, the craft would now more correctly be designated as a ‘demountable trimaran’, rather than folding (see photo). 

The W17 has a somewhat neater system using horizontally flat upper hinges and lower latches, that allow the beams to be flat to walk on with nothing projecting much below the beam either.  The wood beams are unique in that they are engineered with glass-reinforced palms on their ends, specifically to take and transmit the tension loads to and through the hinges and latches.    On most boats, these hinges and latches are now made of fiberglass which is not only lighter and corrosion proof, but also bonds very well to the wood beam ends, creating a strong continuous structure. See here for FG latches.

But due to rising ama weight on larger boats and the resulting difficulty to lift them ‘up and over’, the system is mainly limited to trimarans of 18ft or less, unless the amas are proportionally small.

Telescoping Beams

A few designs use telescoping beams, such as the French Astus, the L7 by Mike Leneman (no longer being offered). some boats designed by Kurt Hughes .... plus a few others.

While theoretically quick and neat to use, common characteristics and issues are that:

1) the two sides cannot be symmetric and identical.  As one telescoping set must be behind the other, the beams are not in line Port and Starboard.   This add complexity to the building of both the main hull and amas, and arguably, a less attractive boat. 

2) the main hull will always be encumbered with tubes or boxes that must pass through the main hull from one side to the other.   Particularly if large, these not only take up much space but also break the structural continuity of the upper sides, often allowing cracks to develop after the craft have been in service a while.   

3) many (to different degrees) historically have a tendency to jamb in operation, as keeping them perfectly in line and free of sand, is a constant issue.   Giving the tubes (or boxes) added clearance is sometimes the solution used, but then there can be constant movement while sailing, which, while generally acceptable technically, can be annoying and disconcerting.  Teflon sleeves can help.   

4) Overall expanded beam is geometrically limited by the arrangement, as each aka tube length cannot exceed the permissible road width without a complete dismantling.

For these reasons, this system is generally limited to relatively inexpensive small trimarans under 7m.


POSTSCRIPT for the Farrier System.

Having a good friend with a folding Farrier, I was encouraged to look at whether anything could be done to solve the main negative issue of the otherwise brilliant design …  the issue of the side-turning ama when folded.  This not only removes the relatively flat deck of the ama that is so useful and safer for docking and boarding on the Swing-Arm boats, but also there's the issue of side fouling when left afloat for long periods while folded – as in a marina (see Header photo above).

A review of the geometry shows that IF the ama could be left free to pivot while the arm is folded, it would take up a fairly level position and could readily be held there with a simple retaining line.  (A possible solution is now entry #126 in an Ideas & Inventions Record Book I started back in 1952 ... with this one being 'just an idea'.   Even a documented 'idea' needs to have a prototype built and proven to become an 'invention' ;)

To achieve this, several modifications would be required.  While much easier to build-in during initial construction, it could be possible to retrofit an existing Farrier with some effort and ama surgery.

Here is what I am suggesting, though test experience could expect to bring about a few tweaks.  

  1. Rectangular holes cut in the deck in line with the aka beams
  2. The installation of two heavily reinforced bulkheads or webs under the deck, both forward and after of each hole and added longitudinal reinforcements to compensate for the hole 
  3. The attachment of a solid, strong foot-shaped part at the outer end of each aka .. solidly made of aluminum, fiberglass or carbon fiber and well integrated into the aka beam end.   This foot would fit snuggly between the two new bulkheads and be drilled to take strong bolts through the heel and toe of the end piece.  (see photo)
  4. The heel would have a semi-permanently installed pivot bolt, while the toe would take a locking bolt that would only be installed after the aka was lowered to the horizontal sailing position
  5. Naturally, hinged covers would cover the deck holes once securely bolted down, but if the area was created as a watertight well (like an anchor locker) with a drainage hole to the inboard side, that deck cover would not need to be 100% watertight.
  6. Detailed design would need to take care of bolt access and also of all loads typically expected on the ama.

It could be expected that initially, it would take about 20 mins. to attach each ama for sailing, but that this time could be cut in half with practice and planning, making the whole operation one of about 10 mins. for 2 persons.    Probably worth it for the convenience of a flat deck in most cases and if fouling is not an issue, one may decide to turn only one side, purely for easier boarding.    Just some thoughts to chew on ;).

Click here for Part 1 of this article

Click here for Part 2 of this article


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