Designing and Building Our Own Catamaran
The design brief we set ourselves was for a catamaran of about 11m that would be comfortable for a couple and a few occasional guests, highly practical to sail and live aboard and not difficult or expensive to build. I wanted a boat that performed well in all normal sailing conditions, good reliable engines, simple foolproof systems, easy access to the water for snorkelling, and not too radical looking for eventual resale.
Since the happy summers of my teenage years, spent sailing an open boat in Moreton Bay, I had always wanted to sail the Great Barrier Reef waters in a decent sized boat with a good turn of speed. At the age of 60 I realised the time had come to do something about it.
In 2009 my wife and I had made the decision to sell up in Auckland and move to the Noosa region to be closer to my father in his last years. As soon as we arrived in Noosa we looked thoroughly at the second-hand catamaran market, only to find that prices were just a bit beyond our means.
It didn’t take long to convince myself (and my long-suffering wife) that the best way forward was to build our own boat. I read all the articles I could find in magazines and online and talked to people who were building and to designers, all of whom were very helpful and encouraging.
Cedar strip bottom planking and 9mm ply sides.
I had been an architect for 20 years, but my spare time had been spent (or misspent) sketching boats of all kinds. In the 80’s this impulse had resulted in me designing and building a traditional 27’ keelboat, an exercise which was immensely rewarding and gave me 15 years of great sailing, before selling her for not much more than the cost of materials. Before leaving Auckland I’d spent a year studying small craft design and lofting at Unitech, which gave me the confidence to tackle the design stage, knowing the result was unlikely to be a complete failure.
As a starting point I set up a spreadsheet with all the vital statistics of nine similar catamarans (length, beam, displacement, hull shape, sail area). I then calculated the important ratios for each: hull waterline length/beam (how skinny the hulls are) displacement/LWL (how heavy the boat is) and sail area/displacement (the power/weight ratio). This gave me a starting point for my own design, which I entered in the spreadsheet as ‘ideal’. As the design developed I entered the actual figures as ‘current design’. This process gave me a good understanding of how my boat compared with other similar craft. Of course there is plenty of debate about what these figures really mean and how important they are, but at least it gave me a starting point.
Maxsurf screenshot: hydrostatics calculated in less than a second.
Now to the fun part, designing the hulls. I was prepared to spend a bit of time getting a really nice underwater shape to keep the performance sharp, so the hull bottoms would be built in strip-planked cedar. Above that I would switch to 9mm gaboon ply glassed both sides, with a soft chine above the waterline to avoid excessive flare. Above that the sides would slope back in to a rounded deck edge. This would keep the apparent height lower and the panel sizes smaller, with all the interior cabinetry acting as stiffeners for the hull panels.
To design the hulls I used Maxsurf, a simple but powerful surface design tool. After setting up the hull length and beam and the desired displacement I could modify the lines endlessly, trying a flatter run aft or more fullness in the bow sections, then with a single click I could display the new hydrostatics to check if it was improved. Magic! The other thing Maxsurf is very good at is refining the fairness of the lines. It’s lots of fun to use and very addictive.
However, having got to that point (and being on the wrong side of 60) I had to resort to my old-school drawing board to assemble the two hulls into a catamaran, complete with deck, cabin, cockpit and rig, not to mention interior fitout drawings, rudder and daggerboard details, and rig drawings. In all it was about 20 sheets and kept growing as work progressed.
Underwing built, faired and painted in the shed.
By this time we’d found a place to live, with a shed big enough to at least build the hulls and other components. Assembly of the whole boat would need a tarpaulin shelter alongside the shed, but we would deal with that when the need arose.
The first step was to build the part-bulkheads and set them up with temporary frames between so the maximum spacing was about 1.2m but mostly less than a metre. I found I could print the bulkhead shapes straight off Maxsurf and transfer them to plywood. They would be 6mm ply each side of a timber framework to give a stiff bulkhead and room for wiring. I used hoop pine for the framing but should have used cedar, as it would have been plenty strong enough and a lot lighter.
Once the bulkheads and frames were set up on a strongback and a couple of stringers run, the cedar strip bottoms progressed quickly. It’s not as difficult as it looks and gives a beautiful shape. If I was doing it again I’d probably take the cedar strip right up to the chine. Then the ply sheets went on, cut to shape first, then glassed on the inside on the table before fixing them in place. The sheet joins were heavily ground back then joined with several layers of glass to avoid having ugly butt blocks inside. Skegs to protect the saildrives and rudders were built up with foam and ground back to a sleek shape. The outside was glassed (with rebates for the overlaps) then faired with a minimum of fairing compound. The rebates allowed me to use the beautiful smooth flat surface of the plywood to get a fair surface with a minimum of work.
The day came for moving the first hull out of the shed and a motley crew of friends and boatbuilders turned up to help. I’d built the hull on a strongback with a double base so I could slide one on top of the other with the help of a little candle wax between the two surfaces. After removing half the end wall of the shed we all gave it a heave and it took off down the garden on the waxed rails! Luckily it stopped in the right place and we turned it over with a couple of big half-round frames and a bit of manpower. Then it was slid into place beside the shed and preparations made for the second hull (after a few beers of course).
The second hull progressed a bit more quickly but it was hard doing the same things all over again. I was yet to realise that building a catamaran meant doing the same things over and over and over again! Once I had two hulls ready to tie together with bulkheads the need for a covered outdoor space for assembly and finishing became urgent. The budget didn’t allow for a permanent structure but there was a huge clump of bamboo just down the road and plenty of poles from forestry thinning ...
Laminated ply sides and front, foam roof, cedar/carbon beams.
The 12m bamboo ‘rafters’ looked pretty silly on top of the Landcruiser but it was a country road so we got them home safely. The poles were transported just after dark for ‘safety’ reasons, and with a 12m x 9m tarpaulin and some clear plastic we soon had a very respectable shelter beside the shed, protected down the sides but open at the ends for good airflow, essential in the Queensland summer.
It took a full day to line the two hulls up perfectly, to the millimetre, before I could start tying the partial bulkheads in the hulls together with the framed-up section that spanned between them. Once all four bulkheads were completed I could glass them and reinforce them with unidirectional carbon fibre. I used this sparingly, only in the high stress points, where I felt the extra strength warranted the higher cost. In hindsight I probably overbuilt some parts of the hulls and connecting structure, but the extra weight penalty was not high and the peace of mind in rough conditions is worth every extra kilo.
So now I had something that looked a lot like a catamaran (from a distance) and it was week 52, a year into the project. I knew I was nowhere near halfway, but if you’d told me I was only a quarter of the way I wouldn’t have believed you. Just as well I didn’t know.
The next step was to make the underwing in 20mm foam and glass, with fore and aft stringers to carry the floor loads between bulkheads. I used foam for this, the decks and the cabin/cockpit roof for its stiffness, light weight and thermal insulation. It was easy to work and glass and perfect for these big flat areas.
Once that was installed the decks came next then the cabin sides and roof, which extends right out over the cockpit and beyond, providing shade, a place for the mainsheet traveller and support for the dinghy davits. The side profile of the cabin top, its structure and materials, and the way it fitted around the mast and track for the self-tacking jib had been redesigned several times. Eventually I had to make a decision and go with it, and I couldn’t be happier with the result. The fore and aft structural beams on top provide good strength and a recess for the solar panels, two forward-facing opening hatches give us excellent ventilation, the mast base is well aft and the jib track stops within the side extensions, which strengthen the deck and make a great dorade box for passive ventilation, so important when the boat is left closed up on a mooring.
Just a bit of finishing and painting left to do.
An endless list of smaller jobs had to be tackled before and after the big jobs. Hull floors, daggerboards and cases, rudders and bearings, aft steps, forward curve, deck hatches and many more, each presenting a unique challenge which had to be solved in the best AND quickest way. Well, we all know that’s impossible. My good friend Ross Blair, a veteran catamaran builder of vast experience, had warned me about falling into the ‘perfection trap’. I had to be constantly on the alert for the symptoms and I must admit I didn’t always manage to avoid doing things better than they really needed to be done. However, at this end of the process I don’t have any regrets about the quality of the finished product.
One component where I could have saved time was the forebeam. I’d had a quote for the aluminium version which required only that I build the attachment points. However, I really wanted to try building a curved cedar/carbon beam which I thought would look better and tie into the hulls better. The attachment points for the forestay and beam would be tricky and I’d have to build the A-frame. Nothing like a challenge, I thought, and kept a careful record of the weight, cost of materials and time taken to build it. It came out at the same weight as the aluminium version but vastly stronger, cost considerably less (excluding my labour) and took exactly a month to build. For the cost of an extra month I was more than happy with the outcome.
Finished and installed, looking better than aluminium.
So now we had a closed-in catamaran with almost all the deck details finished and an elegant curved forebeam, primed ready for painting and only needing the interior furniture, engines, plumbing, electrics, and electronics. Then we just needed to paint the whole thing inside and out, install the deck gear and get it to the water. We were now two and a half years into a three-year project. What could possibly go wrong?
Part two next issue