Foam Core Glass Fibre Sandwich vs Marine Plywood

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:
Three sheets of Divinycell foam core simply joined end to end on the floor with weights and clamps to hold them in place


The moulds to define the shape of the hull are stood up all carefully aligned ready for planking. It looks like they're made of MDF.

On with the hull sides:

Two long sheets of Divinycell are planked onto the sides of the moulds

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:

Long sheets of Divinycell are planked onto the moulds, an array of cramps and weights ease the core material into the desired curve - the difficulty was the V-shape he wanted towards the bow.

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:

The upside-down hull covered in a layer of woven glass cloth and epoxy. Not yet trimmed to the sheer.

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 – 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: - the finished boat. He's finished her in very bright yellow, with blue below the waterline after the Swedish flag. He's sailing her on a close haul in the Stockholm archipelago.

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, here’s one diagram from that article:

Three cross sections of a hull to show the advantages of foam core laminate sandwich construction. One single skin, one with a a medium thickness core, another with a thick core. There are figures in the image showing that as the thickness of the core increases, so does the strength and the stiffness.

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.

A half-complete Paradox build just after the insulation has been put in but before the deck installation.

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: 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.

Mahogany trees in India - they're huge!

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 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 Münster 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:

We only have “virgin” quality which means…………..not recycled.
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.

A piece of Armacell foam core on a background of green leaves.

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…

14 thoughts on “Foam Core Glass Fibre Sandwich vs Marine Plywood

  1. Hi Richard
    There may be another way out, or at least a reasonable compromise, to build a hard chined boat with a long life expectancy and a relatively small ecological footprint: light metal composite. Roughly a welded seawater resistant aluminum alloy outer shell with a sprayed-in foam core covered by a light inner sheeting of whatever material you like. There is no real need to paint the alloy on the outside, except for some kind of antifouling, nor the inside. A small boat like Paradox would also need very little in the way of inside hull stiffening.
    The alloy goes at roughly 3 times the weight or 1/3 the thickness of marine plywood. That gives 6mm or more for the bottom, 4mm for the sides and 3mm for the deck: not a big deal for an experimented alloy welder, almost indestructible, as maintenance free as a GRP-boat. The sprayed-in foam takes care of thermal insulation and makes her also unsinkable! But you would most probably need professional help to redesign an build her.
    Best regards,

    • That is a good idea for a strong and light boat. I’m not capable of building and maintaining an aluminium boat so I’d have to employ an expert as you say, although I’d rather build in something I can do myself.

      The other issue I have with aluminium is that it is an excellent thermal conductor, exactly the opposite of what I want in my hull. I appreciate your suggestion of spraying foam insulation on the inside but I think that foam core glass sandwich construction is a much more efficient use of resources, where the hull is both the strength and the insulation. Plus I can easily build it myself.

      I’ve used heavy scantlings in my workings, I think if I made better calculations the foam core glass sandwich would have a better strength to weight ratio than the aluminium-foam combination. I also think that for a small boat I could build the sandwich with much thinner skins than I’ve been proposing, but need to verify that.

  2. I have helped my father build a sailboat hull with basicaly the same method that Yrvind uses. I agree that it´s a good combination. It´s a lot of polishing, but the paradox has easy polisheable surfaces, so that will probably be fine. Don´t use too little epoxy on the last layer of glass fibre, because you should avoid polishing all the way down to the Glass fibre cloth when you finish up the surface. We did 8 layers of polyester on the outside, it took about 50 hours of work with not much rest, and we we´re 4 people. The paradox is smaller and glass fibre is a bit easier to work with, but be prepared that it will be a marathon, and use your friends to help you. I meet Yrvind at a boat show in februari 2013. I asked him what he thought about building a Paradox in aluminium? He wasn´t very exited about that. He thought it would be cold and heavy. But he saw the benefit of building it fast. Building with foam core and epoxi is not very fun =( I personaly think that aluminium would be a great material for the Paradox, beacause the Paradox cant be too lightweight and have too much boyancy. And it´s a recycleable material =) So maybe I will do that, depending on the cost 😉 Ohh and another thing about epoxi… use plenty of protection! Epoxi is dangerous before it has hardened. I would recommend not only a mask with carbon filter that covers the mouth, but rather something that covers the whole face, because the epoxi can also get into your body through the eyes. Good luck with your project. You can write to me if you want to juggle ideas( Maybe we can have a Paradox gathering in the future ;D

    /Charlie, sweden

  3. Richard,

    Great blog, I followed most of what you wrote. But, I got hung up on your decision to use foam over ply. Because wood is not terribly green, you will chose to use foam? Would you explain that more for me?


    • When I first dreamt of cruising it was a solid wooden boat. I like wood, it’s lovely to work, in appearance, and with enough care and attention it can be sustainably sourced – if this is the case the most sustainable boat build could be of solid wood, for example clinker/lapstrake or carvel planked.

      However I would not want to live in a solid wooden boat because it’s high maintenance, prone to rot and leakage, which in the long run can require more energy and less sustainable materials in upkeep.

      I live in colder climes where insulation would have to be added to the inside of the hull. So it would probably end up being synthetic antifouling on the outside, wood in the middle, and closed cell foam on the inside for insulation and buoyancy in the event of flooding. Not so natural, not so beautiful, and still high maintenance.

      Then I thought the compromise would be plywood. Unfortunately the best plywood is made of mahogany from the tropical rainforest, which is sometimes possible to source with a sustainable forestry label, like FSC, but I question whether these ancient hardwood trees should be harvested anywhere in the next few centuries. These forests are already under huge pressure.

      Quality mahogany marine plywood, well treated, has a high resistance to rot, but would still need some care and attention to antifouling. What’s more when the boat comes to the end of its life it’s not biodegradable like a solid wood boat because of all the epoxy lamina.

      The plywood boat would also need to be insulated on the inside.

      I concede that a plywood boat could be the easiest build, especially the stich and glue type, and I would get to work with wood.

      I realised that any kind of wooden boat will require paint, varnish and antifouling, and with the abuse I’m likely to give it on my beach invasion adventures would probably benefit from a thick layer of glass fibre and epoxy on the bottom. It would also need plastic foam insulation all over the inside. The insulation would not be structural.

      That’s when I realised why experienced boat builders like Sven Yrvind choose a foam core fibreglass sandwich construction technique. The plastic closed-cell foam core not only provides a complete surround of insulation with no gaps, and enough buoyancy to make the boat unsinkable, but it also forms part of the boat’s structure giving a light, stiff but strong hull.

      It’s sandwiched between layers of fibreglass sheet saturated in epoxy, more layers in areas of expected abrasion or stress.

      The result is a boat that’s warm, light, strong, stiff, will not rot, is low maintenance, and all parts of the hull are multifunctional. It’s not biodegradable but this could be a good thing – there’s virtually no end to its life, unlike wood where more trees need to be chopped and other resources consumed when the boat dies to build a new one.

      When I discovered that I could source marine grade boat builder’s foam made from recycled plastic bottles from Armacell in Germany, that was just the icing on the cake.

  4. Hi Richard, I’ve been following your dilemma and can’t really give you guidance to the right path as I am in the same situation. I’ve spent 6 years designing my 10ft microyacht PUFFIN and making 1/4 scale radio controlled models for testing on our local model boating lake. I’m now in the position to build and set sail.

    It has always been my plan to use 6mm and 9mm BS1088 Marine Plywood and since moving home have found a supplier right on my doorstep. Literally just about to order 5 sheets each 6mm and 9mm from Farmwood Products:!products/mainPage They also sell on ebay and it was here that I saw a note about their 6mm only being made up from 3 plys! I rather assumed that 6mm genuine BS1088 has been 5 plys and 9mm 7 plys. With all the low quality plywood coming in from the far east, some with a central softwood core and two out veneers yet still stamped BS1088 Marine Plywood, designers have been quick to point out that 6mm in 3 plys or with the thicker middle core are totally unsuitable for boat construction. The only really genuine material I’ve found and shy’d away from because of the price is available from Robbin’s Timber. But this multiplies my sheet materials bill times four!

    Having followed Sven’s build’s for his past two boats using foam core construction, like you, I’ve been considering this as an alternative. I have experience in GRP boat construction having built a mould for a 16ft boat and then gel coated, laminated etc etc to produce a one-off boat. I’ve also used Stitch and tape with West Epoxy resin, in fact that’s how I’ve made my scale models of PUFFIN.

    So, I either have to find a source for foam or an alternative Plywood supplier ?

    Mmmmm dilema


  5. FSC IS not the gurantee you might think FSC has proved to be a clever PR job but it is by no means foolproof and there have been many documented instances of abuse the only true way forward is to buy from an importer who does his due diligence correctly from whichever supplying country he buys from.Indonesia once the pariah and victim of sustained Greenpeace attack has built up a true certification process SVLKwhere the plywood and the raw material can be keruing spcies one of the most durable of hardwoods harvested sustainably a mulilaminate 100 percent one piece core a proper pehonlic glue that passes the BS-088-2003 boil test and crucially 1mm face and back veneers after sanding this together with a durability gurantee means that you can buy quality and build with a clear concience and peace of mind.I have been involved in the plywood industry for some time and have been sailing for 40 years and this represents a quality product at an affordable price

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  7. I lived for three years in a 18.5 by 5.5 foot powerboat with an enclosed cuddy cabin. In winter you have some condensation but the plywood itself provides good isolation. More important seems to me proper watertight ventilation. Just in the colder days I burnt alcohol in a small tuna can and it provides heat for all the morning.
    Not problem in a boat this size to walk over the bottom planks, and it lowers de CG.
    Nice blog mate. I come here reading about foam core construction. You bring me an idea, I wish not to sound silly but what do you think about a whole boat of PET bottles (the small ones), then filled with sprayed foam, sanded and fiberglassed over?

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