In the last issue of MR I was struck by two starkly contrasting articles, one about the Tin Can Boat relentlessly pursuing originality using modern materials and the other, James Wharram’s latest venture, exploring the gradual evolution of prehistoric double canoes. Reading these articles from the angle of design and construction triggered some questions about the choices I have made, and how we get to make construction decisions in the first place.
I have long admired James Wharram’s work. To my eye his boats have a simple, timeless beauty and look like they have been shaped by the sea. His advocacy of ‘appropriate technology’ and trust in the efficiency of evolved designs are important aspects of his design philosophy. For example, rope lashings for beams are simple, practical and soften stresses – I well remember my Pahi creaking and moving in a seaway!
At the opposite end of the scale David Vann’s homebuilt trimaran seems to have come from the notion that metal is strong and predictable, aluminium is strong and light, therefore it must be the best material for the job, never mind stress concentrations or poor welding, let alone flat hull bottoms. Unlike rope lashings his hull connections appear to concentrate stresses onto small areas of welding. It is possible to replace a lashing at sea, but has anyone tried aluminium welding while afloat?
‘If it looks right, then it probably is right’ runs the maxim. And we non-experts mostly go through life like this, trusting to bridges, bicylces, aeroplanes and boats mostly on the gut feeling of rightness. But the maxim is a false friend. ‘Rightness’ comes from the inside out: proper design and construction look right because they work. Something made of the wrong materials might look superficially ‘right’ but fail in its functional purpose.
Observing evolved design in nature provides a lesson in seeing how the environment dictates efficient shape. A dolphin’s back for example is very similar to a boat’s hull upside-down. Convergent design is seen in the streamlining of cars, and sometimes in the superstructure of boats, especially the faster ones. Racing boats of course are always happy to sacrifice headroom and comfort for lower air resistance.
I think it was Dick Newick or one of his contemporaries who came up with the notion that in a multihull you can have only two of the following three things: Speed, Comfort and Safety. Thus a safe comfortable boat will not be fast, a fast safe boat will not be comfortable, and a fast comfortable boat will not be safe. But these are not absolutes. It does not follow that all fast comfortable multihulls are unsafe. In boat design compromises are always being made. Searching for the right compromise is the constant design challenge.
We can often spot dodgy design. Many people shook their heads when they saw the huge bows of Team Philips with no forward crossbeam. The port bow break-off was eventually blamed on poor construction, which I think just avoids some important design questions. I keep reminding myself that a single cubic metre of water weighs a tonne. How much was slapping up against that bow? And it didn’t take an expert to imagine that the huge twin rigs might work to bend the boat in different directions, and sure enough, it came to grief. It would have been great to see Team Philips II, modified to learn all the design lessons from the prototype, because it was still an amazing boat.
Looking at the essential elements of design and engineering, I wondered how close aspects of my own construction project were to David Vann’s approach. Let me be the first to hold up my hand: I am technically incompetent in engineering terms – my school maths allows me to understand what they are driving at in those complex equations, but I come over a bit fuzzy with the figures. And yet I have had the audacity to modify my set of plans and add a substantial bridgedeck saloon, not to mention composite front beam, wingmast and glass chainplates. So what steps have I taken to ensure that my boat is not a failure?
- Be unoriginal
Much of my early thinking on the boat I wanted to construct was based on the aesthetics. A sensible well-tried design with modifications that I have seen on other boats. A well-rounded cabinroof, not too high, nothing too extreme. Let’s be honest – if it were a good idea to build a boat out of unusual materials, someone would have done it already.
- Employ a designer
In the first instance I bought plans from a recognised designer with a reputation for sturdy boats. I liked the approach he took towards fibre orientation, which suggested he knew a thing or two about stresses and had done some finite element analysis. The designer may also have some responsibility for ensuring that the design is capable of what he claims.
- Know the materials
I also have confidence in the chosen materials – ply, epoxy, glass – and my experience with woodworking gives me an understanding of the way wood performs. In particular I reasoned that the underwater hull shape was efficient without making too many compromises for the limitations of the chosen materials.
- Think about loads
When I was designing the modifications I thought long and hard about why a particular structure had been included, before changing it. For example the original plans showed a box arrangement of lockers at the base of the mast behind the main beam. This must have been intended to provide stiffness for the beam. So my modifications include a similar box arrangement, in fact a slight structural improvement.
- Test it out
The raised cabin roof is of similar construction to the original deck, with some vertical supports at the top of the companionway steps, so that it is not a continuous unsupported structure. I have taken care to make sure that it is well-curved, not just for aesthetics but because this shape is inherently much more rigid than flat panels. Two of us have climbed all over it, so it works fine.
- Pay an expert to do the sums
In many ways the hulls and bridgedeck are the easy stuff. The mast is much more demanding in engineering terms, and I did not hesitate in spending money for some sound advice from Nick Barlow, an expert at wingmasts. He was able to do all the calculations that put my head in a spin, advise on precise lay-ups by email, and give me the reassurance of his experience.
Why then did I make a mess of the spreaders?
Simple. I fell into the trap laid by that maxim. I didn’t bother to ask Nick about them, but just shaped them up from foam and put 5 layers of uni carbon around the outside. Classic bodge engineering – I reasoned that the mast had 7 layers, so 5 should do for the spreaders. They looked wonderful – sleek and light but ..er, strong? It turns out that the compression strength of composites is not easy to predict. It depends on the resin matrix holding the fibres in column, and of course on the fibres being straight, and presumably also on what the fibres are pressed against at each end. Would my pretty spreaders be man enough for the job? The mode of failure would be bursting outwards as the spreader compressed, probably on a dark night in a sudden squall miles from land. Then the mast would bend and collapse in quick succession, and….well, you can fill in the nightmare details.
My worries were compounded by the fact that the fibres of the spreaders did not all finish directly under the ferrule holding the rigging wire, so some would be taking very little load, meaning that I would have to deduct a proportion from the already marginal cross-sectional area for calculations. And the trailing edge relied only on resin to hold it together, so this would be the obvious place for failure to start. Nick suggested that the spreaders would be ‘probably OK’ – meaning that it is my call (He was too polite to call me an ignorant fool!)… ‘Probably’ isn’t really good enough for an offshore boat.
I turned to an old friend, a brilliant book called ‘Structures: Or why things don’t fall down’ by J.E. Gordon, originally published in 1979, with the latest edition being 2003. This book is a great read, pitched at a non-technical level but with formulae in the appendix, and some amazing stories of engineering discoveries. Plenty about ships and masts, aeroplanes and bridges. If David Vann had read this book maybe he might have done things a little differently. I cannot recommend it highly enough.
Prof Gordon tells me that aluminium is several times stronger in compression than in tension. And a tube shape is the most efficient. So that is why aluminium is such a favourite for masts and spreaders! I immediately cut a slot and excavated some of the foam from my spreaders and inserted a tubular aluminium strut with the rigging wire ferrule bearing directly onto it on the outboard end and touching the mast side inboard. Faired in it looks no different from the original shape, but I think I can now be confident that the spreaders at least will not cause the mast to come crashing down.
But the reasoning that led me to ally tube for the spreaders does not necessarily translate into its suitability for crossbeams. Much of the discussion on beams relates to end fixing – the mast tubes I have seen used as crossbeams are generally big ones sleeved into substantial glass fittings or lashed down, spreading the loads and taking care of torsion and bending in all planes. Mr Vann’s truss construction does not appear to have taken this into consideration.
I am more confident about the glass chainplates, another off-design innovation. These are not new, and have the advantage of spreading rigging loads more evenly across the hull surface. They also avoid the point-loading that comes with bolts. Research suggested that a cross-sectional area about three times that of the stainless chainplate would be needed, so an area of around 300 sq mm. This was achieved using 8 pieces of 100mm wide 600gsm unidirectional glass tape folded to go round the ends of the ‘dumbell’ and through the holes into the hull. The glass tails are up to 900mm long, unpicked and spread out across the inner hull surface. Several layers of biaxial glass were also used to help spread loads. Some foam took up the wedge-shaped gap between the outer and inner tails before the glass reached the inward-sloping hull below gunwale level. For good measure I bonded the chainplates to the outer hull skin as well by laying more uni over the top. This now seems like a fitting substantial enough to bear the 3500kg loads it might be subjected to.
I have added some bumper keels, short solid keel sections with oak bases, for when the boat takes the ground. These are my own idea, given a streamlined shape and placed at the centre of gravity – actually slightly aft so that the boat would tend to fall forward rather than back (on the reasoning that one would nose forward into shallow water and that protecting the rudders is important). They are about 180mm deep and will inevitably create some interference with the daggerboards, but by coving them in carefully they should not be too draggy in the water.
I hope that I can admit my shortcomings and correct them, as with the spreaders. I feel that I have to be reasonably cautious for a cruising cat. Adopting a cautious approach probably means overbuilding the boat somewhat. In a home-build this is almost inevitable unless the designer is able to specify every detail, and some are much better than others in this.
But beefing it up does not necessarily improve a boat. It can lead to other problems. Overweight multihulls are not good sailers, and rigging loads increase because more energy is required to drag that extra weight through the water.
Nothing is easy! Sometimes it feels like a tightrope walk.
‘Structures; or why things don’t fall down’ Prof. J.E.Gordon. Penguin Science paperback £7.69 from Amazon books. Read this book!
‘The new science of strong materials: or why you don’t fall through the floor’ Prof J.E.Gordon Penguin Science paperback £7.69 from Amazon books. Another brilliant book by the same author. Lots about composite construction.