High performance outrigger part 1
GALLERY | Click images to enlarge
Sam Frosh reports on the design and construction of an outrigger Moth!
After more than two decades sailing, designing and building sailboards including a six metre long tandem I decided to go back to my sailing roots, that is a Moth dinghy. However I needed a craft for two as my son has accompanied me on my sailing journey for the last 20 years. The other problem was that when I last sailed Australian Moths they were 1.3 metre wide scows, and now they are ultra narrow 30cm wide skiffs that are more stable lying on their side, capsized, than upright, sailing. However the modern Moth is the speed machine of the monohull sailing world.
The design brief then was a two person Moth, with equal or better performance, and much better stability. This required in my view an ama that would skim the water surface and a construction that would be extremely light. Another facet of the design was that in Western Australia we have strong winds throughout summer so that I wondered whether a hull design that would be efficient in displacement mode, and also planing mode was possible. It is generally accepted that the International Moth with its canoe stern as well as almost all catamarans achieve high speeds, but always sail in displacement mode.
I decided that for a crew weight of 150kg and boat weight of 80kg it was possible to plane in 15 knots of breeze with a hull of 5.6 metres in length and 40cm. width. I did not use any complex math for this; it was purely experience and intuition. The hull would not be canoe sterned, but have a transom, as this was deemed essential for planing.
My passion in boat construction has always been wood materials and the challenge was to use older methods and materials, rather than carbon and foam cores to achieve the lightness required. Cold molded plywood has always fascinated me, and I saw this as an answer. I believe that this method was first used to build the English Spitfire fighter planes in WWII.
The ama was previously built for my paddling outrigger canoe using two layers of 1.5mm marine plywood, cold molded in a rounded V shape. It is covered in carbon woven cloth making it practically bulletproof. However it has no internal framing, except a keel and inner gunwales. The main hull was to be slab sided with a partially rounded bottom with distinct chines to allow planing at various angles of heel. The skin was to be 2mm marine plywood with 4mm. ply bulkheads, and red cedar stringers. The sides were simply made with large flat pieces of ply butt joined inside. The bottom was to be cold molded; however I wondered if I could use one layer only instead of two layers laid diagonally, each layer at 90 degrees to the other. I went with the one layer and with care during construction, any gaps between ply strips were no more than half a millimeter, easy to fill with thickened epoxy. One layer of lightweight woven glass was then applied over the bottom, and then the hull finished with polyurethane paint.
As this was to be a tacking outrigger (not a shunting proa) I decided that the akas would not be used for the crew to hike out on, and a large trampoline would be constructed in aluminium tubing 50mm diameter with a fabric infill laced to the frame. The trampoline would be fixed to the main hull centrally, as if it was a monohull.
Rig and Other Details
In previous issues of Proafile I had seen James Wharram’s Tiki design using his "wingsail". This short battened high aspect gaff sail with a luff sock fascinated me, as I saw it as being light, simple, and more efficient reaching and running, than conventional fully battened sails, and with no disadvantage when sailing on the wind. A genoa foresail was also included to give the crew a job to do, and to keep the centre of effort low. Sail areas are mainsail 10sq.m. and genoa 4.5 sq.m.
A timber centerboard was also included and conventional stern mounted rudder. Other fun accessories are a trapeze for the crew, and a choice of small or large asymmetric spinnakers. I haven’t had the opportunity to use any of these accessories yet but this is the next step.
Sailing Performance
This boat has two personalities, mild and wild. If the crew weight is positioned so that the ama is also partially immersed then stability is fairly good and capsizes are infrequent. Speed is very good in this mode especially off the wind probably due to the great efficiency of the wing mainsail.
By using a fair amount of agility to fly the ama, but keeping the main hull fairly level, the whole contraption moves up a level in performance, and on a reach the hull planes on the rear half of the hull only and matches beach catamaran speeds.
Capsizes occur mostly when the crew is well hiked out with the ama to leeward and the wind pressure suddenly drops. This is a very wet experience but both hulls are fully decked and so do not fill with water.
