Following Swedish small boat designer/buider/sailor Sven Yrvind’s blog has given me confidence to consider building my boat of a foam core glass fibre sandwich composite. I had initially thought it would be beyond my capabilities. It’s essentially a glass fibre boat but with a thick layer of plastic sandwiched inside the glass that’s full of air bubbles called cells.
The only example I had seen was a large curvy hour-glass-shaped hull being built at the Boat Building Academy in Lyme Regis, Dorset by a chap called Gary from Northern Ireland. He had to loft it out carefully, build moulds at many stations then strip-plank it with thin strips of the closed-cell plastic foam core. There followed a few days of planing and fairing, then glass fibre cloth and epoxy sheathing inside and out.
However since then I had decided I should build a flat-bottomed boat with chine runners from a sheet material like plywood – this is much easier for an amateur. What I didn’t realise is that the foam core glass fibre composite is much easier to use as a sheet material in a flat-bottomed boat with hard chines. Here are some photos from Yrvind’s building blog. He uses poly-vinyl-chloride closed-cell foam made by one of the leading manufacturers, Divinycell who are in his home country of Sweden. It’s 4cm thick.
Butt-joining three sheets of Divinycell.
No joiner’s scarf joints needed – just a straight edge:
On with the hull sides:
He wanted a V-shape at the bow which necessitated all the extra cramps and weights.
I might just settle for a flat bottom instead:
He says he could then shape the foam core easily with a sharp kitchen knife. When he was happy with the shape, he covered it in woven glass fibre cloth and epoxy:
He would later do the same on the inside, completing a strong but lightweight composite hull. He has kindly shared a detailed illustrated log of his build on his website at yrvind.com/present_project – I highly recommend you read his website. He also shares the logic and principles behind his unconventional ideas, and some fun stories from the adventures of his youth.
All the above took him less than one month. The rest of the boat, including fitting out and finishing, took him a further three years. He then sailed this 15 ft boat across the Atlantic ocean last December – another great triumph for small boat enthusiasts. Here it is:
If I could design a boat with a less complex shape I might get away with less moulds, maybe even just the bulkheads, and have less hassle balancing cramps and weights. I would only really need a gentle curve on the sides and some rocker on the bottom.
What’s the advantage of foam core glass fibre composite?
The effect of the foam core and the two laminate skins is the same in principle as an I-beam for increasing strength and stiffness. It’s much stronger and stiffer than a single skin of the same weight, but much much lighter than a single skin of the same thickness. There’s a nice article on sandwich construction at boatdesign.net/articles/foam-core, here’s one diagram from that article:
The strength of the hull increases up to a certain thickness of foam core (I’d like to know what that critical thickness is). The stiffness continues to increase with increased thickness. There’s an equation for stiffness on the aforementioned Boat Design website, but it’s all Chinese to me so I won’t pretend to understand it.
There are other advantages of foam core construction that I find very appealing. It’s an excellent thermal and acoustic insulator. Some of the designs on the internet for plywood micro-cruisers are not insulated, because they’re used for leisure purposes in warm climates. But insulation is paramount for me because I want to live aboard in temperate climates like in Britain.
If I build a boat out of plywood, I’ll have to cover the inside in insulation. This is standard for Matt Layden’s design the Paradox. However I’d then have to worry about rot spreading on the unventilated surface of the plywood behind the insulation, where I can’t see it to keep check. The foam adds buoyancy in the event of the cabin flooding, but it does not contribute anything to the boat’s strength or rigidity.
In a foam core sandwich hull the insulation is an integral part of the structure. It adds strength and rigidity, and being completely inert it’ll never rot. The whole hull and possibly some bulkheads made of foam core will contribute even more buoyancy in the event of the cabin flooding – with a thick enough foam core the boat will be unsinkable. I want thick insulation to make my boat very thermally efficient because I’ll live aboard all year round and I like the dramatic scenery of the high latitudes like the Norwegian and Patagonian Fjords.
A typical plywood cruising boat like the Paradox is effectively complete and sailable when the plywood is all planked up and glued. The plywood hull with its frames and bulkheads must be strong and stiff enough when naked to handle the strains and stresses of its intended use. However it needs glass fibre/epoxy sheathing, at least on the outside for rot and abrasion resistance, which does increase its strength a little more, and then a thick layer of insulation on the inside to keep it warm and stave off condensation problems. Many people would also glass the inside or at least seal it with epoxy. Each layer of the hull has a different purpose, but none of them seem to have multiple purposes, and the sum of them all is a more complex and heavier structure.
In contrast the foam core glass fibre sandwich all works together as one structure with many purposes – strength, stiffness, insulation, abrasion resistance, compression absorption – it seems the minimum possible way to acheive all these things and of course the end result is very light weight. I imagine feeling very cosy and secure inside this unbroken, continuous monocoque structure where ever I end up cruising. I can fully understand why after fifty years of designing, building and sailing small boats in cold rough seas it has become Sven Yrvind’s favoured method of construction.
In a small boat without standing headroom the sandwich construction will give me the strength and stiffness I need to be able to sit directly on the inside of the hull, doing away with the complexity of adding floors and a cabin sole. I had a discussion about this on the Boat Design Forum here: http://www.boatdesign.net/forums/boat-design/do-some-boats-have-cabin-but-no-sole-floors-39759.html. This is possible in a very small plywood boat (see Matt Layden’s Enigma), but perhaps not quite as solid.
Unfortunately there’s a big disadvantage to the composite construction when it comes to fitting out – you can’t just put screws in like plywood planking. The foam core on it’s own is useless at taking the point loads of bolts and screws. It becomes necessary to involve a larger area of the core and both skins to support any one point load. This could be done with a backing plate or by hollowing out some of the foam core, filling the gap with epoxy then redrilling the hole. You’d better get it right the first time – I can only imagine the mess it would make if you kept shifting the fittings around over the years…
This is very important to me. A good quality marine plywood boat, well constructed and protected will be immensely strong and last a lifetime. That’s good enough for me, so if I’m to consider the alternative of a foam core composite construction perhaps the environmental implications will be the decider.
Being completely inert, boat builder’s PVC foam core, glass fibre and (when cured) epoxy have no effect on the natural world, but of course this means they’re definitely not biodegradable. Some argue that at least they trap carbon in an item like a boat, but wait a minute, wasn’t the carbon already trapped in the oil that was used to create these plastics and epoxies in the first place? There’s a good side to this, that if well built, the boat could last several lifetimes so there’s no need to expend the energy and resources required to replace it any time soon. But realistically, in our modern consumer culture, how many plastic goods actually do get passed down to great-grandchildren? Anyway I would hope to build my boat well enough to see me out. I’m not even thirty years old.
Marine plywood may look more natural, but cut through it and of course it’s actually several veneers of wood glued together with a weatherproof glue like epoxy. These glues are not biodegradable and give off toxic smoke when burned. But at least the veneers are natural, that’s good, isn’t it?
Not really. Those veneers in the best quality marine plywood are almost invariably mahogany from tropical rainforests in Brazil, Central Africa or South-East Asia. There’s not much rainforest left, and a mahogany tree takes five hundred years to grow to any decent size suitable for harvesting.
But there’s hope…or is there? Two famed suppliers of the world’s highest quality marine plywood, Robbins of Bristol and Bryunzeel of Holland are now offering marine mahogany plywood with Forestry Stewardship Council (FSC) certification. That said, Bryunzeel says FSC “according to available credit” and Robbins doesn’t even state the type of FSC certificate. I think I’ll treat any mahogany branded as FSC as suspicious – many types of mahogany are critically endangered, and with the little rainforest we have left I don’t exactly feel reassured by the FSC stamp that says: “don’t worry we’ll just plant another tree to replace each one of the thousands that we’ve cut down; they’ll only take five hundred years to grow to the same size and support the rainforest wildlife. No harm done.” Still, if I am to build with marine mahogany plywood I would definitely support these suppliers that have made the effort towards sustainable logging.
One alternative suggested on the UK Home Boat Builder’s Rally Forum was to use Finnish birch plywood instead. It’s touted to have many of the desirable qualities of mahogany plywood, but is grown in sustainably managed forests in Scandinavia and North-Eastern Europe, where these vast forests of the stuff are not at any risk from over-logging. Several high quality boats have been built to prove this, and the builders report the top end stuff to be completely devoid of voids. However from what I’ve read its only recommended for boats that spend most of their time out of the water because of its propensity to mould and rot. Not good for my live-aboard cruiser. It can be sheathed in glass and epoxy, but still…
I had been lured by the mahogany grain – it had persuaded me subconsciously that plywood was the more ‘natural’ option, whereas foam core glass fibre would be a very synthetic plastic creation too similar to the white GRP bathtubs that litter my local river for comfort. I didn’t want to be guilty of bringing more eternal plastic into this world. But hang on a minute, what’s really going on here? It’s time to take a step back and look at what these methods really involve in a finished live-aboard cruising boat:
- Plywood hull – hardwood from tropical rainforest
- Glass fibre/epoxy skins
- Closed-cell foam insulation
Foam core composite boat:
- Closed-cell foam core
- Glass fibre/epoxy skins
A plywood boat is not natural at all, nor any better for the environment. It employs all the plastic and glass of the composite boat, plus tropical rainforest. Using a good marine plywood ie. tropical mahogany is actually very bad for the environment and will probably remain so for at least a thousand years to come. So the two boats are equally as bad, but the plywood one also involves cutting down the rainforest!
We might temper this argument by considering that not all plywood boats get glass fibre and epoxy sheathing both inside and out, and because the insulation is not structural, and the plywood offers a little insulation, less dense and less quantity of plastic foam core need be used. However a live-aboard plywood cruiser like the Paradox does use some heavy glass/epoxy sheathing on the outside for rot and abrasion resistance, and some builders also glass bits inside the boat. And don’t forget the many layers of epoxy-like glue in the plywood itself.
I’m going to have to admit that I still can’t let go of my heartfelt longing for a wooden boat. But this is a reality check for me – a plywood boat sheathed in glass/epoxy and covered in plastic insulation is not a wooden boat! A real wooden boat that has zero impact on the environment is made from solid timber – not mahogany but a wood from sustainably managed forests. If I’m not going to go to the effort of traditional solid timber construction and maintenance, I might as well build a plastic boat.
The other revelation has been that a foam core composite boat is not necessarily much more difficult to build than a plywood one. Sven Yrvind’s hard-chined build detailed above is a great example. Yes it’s more difficult to put screws and fittings in afterwards, but a lot of them can be replaced with more epoxy and foam – it’s easy to whittle any shape out of the foam and just glass/epoxy over it. There’s no need for so many beams and frames because of the extra rigidity of the sandwich structure.
Then it struck me – what if I could make my boat of recycled plastic? I asked an instructor at the Boat Building Academy about recycled foam cores because he had built foam cored ocean rowing boats and won the transatlantic race. He just looked at me like I was from another planet.
Undeterred, I spent quite a while scouring the internet, and lo and behold, I found marine-grade closed-cell foam core made from recycled plastic bottles! The construction foam giant Armacell, based in MÃ¼nster in Germany, are pioneering marine-grade foam cores made purely from polyethylene terephthalate (PET) which is a 100% recyclable plastic. Most drinks bottle are made from the stuff. I was quick to email them and quick to receive this reply:
PET as such is a foam which can be recycled …………if it’s made from a “virgin” PET raw material or a recycled raw material.
Only for very big project we can discuss about producing foam made from second generation PET.
Of course my project is very small so I was disappointed, but perhaps it’s such a new thing that they still don’t have the industries of scale to mass produce it. So it might become available to me in the near future.
If so this would be the deciding blow against building in plywood. I think it’s brilliant that Armacell are pioneering this technology. Just imagine, a warm and sturdy composite boat made of recycled plastic drinks bottles! Now if only I could find a source of glass fibre cloth made from recycled glass bottles…