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5m folding tri
Design by Horst Werner werner@cad.uni-sb.de

LOA 5.5 m perspthumb.gif
LOD 5 m
LWL 4.70 m
Beam (max) 3.50 m
Beam (on trailer) 1.90 m
Mast height over WL 5.77 m
Sail area 14 m2 (main 10.4 m2, jib 3.6 m2)
Weight empty 250 kg
Load capacity 480 kg
Cabin headroom 0.95 m over berth
Draught (leeboards down) 0.80 m
Draught (leeboards & rudder up) 0.18


Design Philosophy:

This boat is designed consequently for trailering, which means that it is easy to handle by two persons, can be towed even with a small car and should take not more than 20 minutes to be ready to sail.

It will carry 6 adults for daysailing and two adults (and maybe 2 kids) for cruising. Accomodation is very basic, but sitting headroom is provided over the whole cabin length. Cooking can be done ashore or in the cockpit (under a boom tent when the weather is bad).

Despite of its small size the boat offers relatively much living space: The cockpit is 1.80 m long and 1.70 m wide, further extended by two platforms of 60 cm width. The aft cabin is only 1.60 m long but offers two 2 m berths extending under the rear bench of the cockpit.


As for sailing performance, safety is stressed rather than speed. Each of the amas provides 350 kg of buoyancy, yet the sail area is chosen relatively small. But even though speed was not a primary objective, I think the tri should plane if there is enough wind, since it is light and has a flat bottom which is about 1.25 m wide over almost the whole length. Due to the center cockpit, trim will not be affected by a large crew. The transom will stay clear from the water surface till the nominal payload of 480 kg is reached (which is almost two times the boat's own weight!). A boom jib makes tacking easy and also contributes to safety


Even if the outer appearance reminds of ancient sailing ships, almost every detail is caused by functional aspects: The aft cabin is needed to put the crew weight towards the center of displacement, i.e. the middle of the boat. The raised foredeck supports the ama together with the aft deck when the tri is folded for trailering. It also provides a small "cabin" for the kids, a dog or just lots of stowage. The raised foredeck should also protect the crew from spray to a certain degree.

The gaff rig is needed because the mast had to be placed far to the bow. The bowsprit allows for a reasonable size of the jib. The ribs on the outside are needed to stiffen the thin plywood (5 mm) without interfering with the interior space, especially in the cabin.


Safety Aspects

Apart from the modest sail area, there are several features emphasizing safety. First of all the double bottom, wich contains 20 cm of styrofoam over almost the whole length. Thus a self-bailing cockpit is provided as well as unsinkability. Even serious grounding, or maybe firing a shotgun through the bottom will barely cause wet feet.

To prevent pitchpoling, the amas can freely rotate around their center of bouyancy (or a hub a bit more to the aft). This leads to a decoupling of lateral stability which is provided by the amas, and longitudinal stability, which is entirely provided by the main hull. In other words: the bows of the amas can not be forced under the surface by the rest of the boat. Even if they cut into a wave, the enormous difference between buoyancy before and aft of the hub should cause a self-balancing effect, since the rotation of the ama is unhindered. In the worst case, it will rotate a full 180° and continue upside down, but this probably can't cause the whole boat to pitchpole.

Sufficient strength is provided in spite of the lightweight construction by the way the mast is stayed: Shrouds and forestay are replaced by aluminium tubes (40 mm x 2 mm) which lead the sail forces directly to the amas, which means that the main hull has very low stresses to bear. In order to prevent bending of the "shroud" tubes under pressure, they are linked in the middle by a traverse tube passing in front of the mast, thus forming an "A".

The outside of the mainhull is protected by one layer of fibreglass (200 g/m2), the bottom by two layers and two wooden "keel" strips (20 x 30 mm) running over the whole length. The following cross sections show the inner structure. The first one is directly aft of the mast, looking to the bow. The second is a cut behind the front wall of the cabin, also looking forward. Shades of grey are used to indicate the distance to the viewpoint: the more distant a surface is, the darker it appears in the image.




The main hull is mainly stitch-and-glue, some wood strips are needed for the assembly of the seat benchs and decks. Material is 5 mm Okume for the bottom and sides and 10 mm Okume for the bulkheads (Of course, pine or fir will do as well). The whole outside and the cockpit floor and benches are glassed. The amas are glued together from styrofoam plates (there is a somewhat harder styrofoam called "Styrodur" in germany) and formed with handsaw and power grinder. Then a plywood deck is glued on top and a wooden strip on the floor. Two layers of fibreglass are laminated onto the styrofoam to protect it.

The estimated building costs are about US$ 3000 including a used trailer. (I spent about US$ 600 for the plywood and about US$ 400 for fibreglass and epoxy, but don't know which unexpected costs will appear during the building process).


The following images taken from a cardboard model show how the boat is stored on the trailer and how it is unfolded. Note that the amas are rotated 180° in order to point to the inner side during transport.

Rudder and leeboards stay mounted during transport. In order to reduce setup time to a minimum, a stump of the mast stays rigidly mounted on the hull to keep the lowered mainsail in place (an idea I stole from the Weekender).

fold1.jpg fold2.jpg
fold3.jpg fold4.jpg


  • One problem of the aft cabin concept is that a normal tiller can not be used. This issue is solved (I hope) by using a vertical tiller in combination with an offset rudder and a bar with two cardanic joints between them (which has the beneficial side-effect that no room in the cockpit is lost).
  • The second issue is the placement of an outboard motor. A good place might be the rear end of the left platform, so that the helmsman can sit on the platform to control outboard and tiller at the same time.
  • The concept of rotating amas may turn out not to work in rough sea, since once the ama is out of the horizontal position, lateral forces can not be lead into the platform any more, thus the axle may be loaded with serious bending momentum. Whether this is really critical can only be decided by testing. Maybe there should be a possibility to fix the amas completely to the platform if necessary.
  • The double bottom and the ribs make the building process more time-consuming, even if they don't add much to the material costs.


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My name is Horst Werner, I'm a mechanical engineer and have been reading about boat design as a hobby for three years. I'm currently working at the University of the Saarland in Saarbrücken/Germany and have built one small sailboat (LOA=2.30 m) in stitch and glue with some of my students. I have just begun building the design I submit, so I hope I can report about the practical tests next summer or autumn. 

Greetings from Germany

Horst Werner

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