Folding mechanisms for the new catamaran

Unless I get filthy rich, I never plan to own another boat that can’t be put on a trailer and hauled out for maintenance and safety. This was partly a lesson learned after Hurricane Opal, in which our 27 foot keelboat was slightly damaged. I sailed Dilvermoon over to the Shalimar Yacht Basin, where she was hauled out and set in an adjoining gravel parking lot on jackstands. I was assured that the fee for storage while waiting for the insurance adjustor would be reasonable. After a month I decided to have a guy with a big hydraulic trailer set the boat in our side yard where I could do the repairs myself, and was presented with a storage bill for over a thousand dollars, not counting the haul-out, for which I was charged another few hundred. For a month of sitting on a gravel parking lot. Their excuse was that the low rate they’d quoted me was on the assumption that they would be doing the repair work. They didn’t mention this when they hauled the boat.

Now we have laws against hurricane gouging, but we didn’t then.

Anyway, now that I have returned to the multihull fold and fallen in love with catamarans again, I have a dilemma. Trimarans can be designed to fold fairly painlessly, and Ian Farrier has become the designer with the greatest number of cruising multihulls afloat as a consequence. But my opinion is that trimarans are inferior to catamarans as cruisers, particularly in the smaller sizes. And catamarans that can fold quickly and easily for narrow slips and highway-legal trailering are not as easy to design as folding trimarans are.

There are many clever cat designs that allow folding or sliding to fit on trailers, but most of them have drawbacks, from my point of view. Most require that the mast be unstepped before folding. Some fold by rotating the hulls under a central pod. My particular situation is that I have a very generous neighbor with a slip that he allows me to use. But the slip is not wide enough for a 23 foot cat of reasonably modern beam. So my boat must fold in such a way that keeps the antifouling in the water and the mast up.

One of my Slider builders, Renato Scolaro in Italy, sent me pictures of a very clever sliding beam system that he worked out.

I believe he will be able to get 10 feet of sailing beam for his Slider in this manner. Slider’s hulls are about 33 inches at the widest, so the webs are extended inward and slide out on bolts set through the bottomless main beam, as I understand it.

This is a terrific solution for a small cat, but at 23 feet, and with substantially beamier hulls, Slinger would not have enough travel with this system to extend the beams to the minimally acceptable sailing beam of 12 feet, and still have enough beam overlap to be sufficiently strong for offshore conditions.

So I’ve taken a different approach. In one of Thomas Firth Jones’ excellent books, he described a 23 foot cat of his design called Brine Shrimp. This boat folds along the centerline and the hulls rotate inward as the centerline rises. He described the way the idea came to him as the realization that the hinges in his system did not have to carry the sailing loads. They only had to allow the boat to fold, and the beams could be reinforced for sailing with bolt-on butt blocks that fit inside the I-beams he favored for his multihull designs. What I took from that idea was the separation of functions– folding and sailing.

Our old Wharram Tane was sort of trailerable, but it was so painful that we rarely did it. It involved removing the beams and replacing them with temporary transport beams. Because the beams were fastened together in pairs with cedar slat decks, the units were heavy and awkward to wrangle around, and temporary beams had to be devised to hold the hulls in place before they could be shoved together. If a mistake was made, the hulls were so top-heavy they would fall over and lie in the water on their sides, and then you really had a problem, because getting a 27 foot long heavy deep-V hull back upright was a tormenting task. When my wife and I had to do it by ourselves, the dread specter of divorce was always present. If we had 15 friends to help, someone always got drunk and had to be rescued from a near-drowning. I’m not saying it wasn’t fun, but the next time the boat had to go on the trailer we usually had to find 15 different friends, because the first batch knew better than to get involved again.

I like to build simple models to try out various engineering concepts, and so I built this one, which is designed to test the geometry of the Nacra 5.2 rig with the deck layout and folding beams of the new boat. Here’s a series of photos of the model. In the first one, the boat is extended to sailing beam with little spring clamps taking the place of bolts and washers:

Halfway folded:

Notice the shrouds are still tight. And completely folded:

Shrouds are still tight.

Now, this system as shown would work, but it has a couple of drawbacks for my purposes. For one thing there must be enough space behind the cabin for the main beam to sweep forward as the boat folds, and this eats up interior volume that would be very pleasant to have. Another problem is that like the sliding beam system, there must be adequate overlap between beam webs and beams to insure strength. As my new cat will be taken offshore, this is a very big concern for me, because I’m scared of the ocean and its casual strength. The need for sufficient overlap limits beam to a fairly modest amount, less than I feel is optimal in a cat of this size.

However, the main point I’m making here is that the rig geometry supports a mast-up folding system based on this idea. I think I may have figured out a sturdy way to do this. It involves removable beams, but the beams are reasonably light and not awkward to handle. The boat is always under control when folding, and the folding system does not have to overcome the friction problems that sliding beam systems have.