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Dave and Anke
triloboats swirly gmail daughter com

Saturday, January 7, 2012

Sea-going SIPs: Toward Creating a Market

Pros spreading LPU(?) Glue in a DIYish Step
(Photos from ProWall Factory)

Finished SIP... Note Gluing Press in Background, Right

I been thinkin'.

Ply/foam/ply composite construction has a LOT of advantages in boats. Foam insulates and floats, two valuable characteristics in WaterWorld. Separating skins of ply creates a girder very much more rigid, and stronger than the same two pieces bonded directly together.

So let's get some acronyms aboard:
  • SIP (Structural Insulated Panel) - A pre-fabbed ply/foam/ply composite. Unfortunately, the more commonly available SIPs are made with nautically worthless OSB (Oriented Strand Board).
  • XPS (Extruded PolyStyrene) - Available in boards of several useful dimensions from local building supply yards. Commonly referred to as blue- or pink board. It has high R-Value (insulates well), high structural strength (as foam goes), is completely water-proof, and stable (doesn't disintegrate into beads).
  • AC plywood - Has one 'good' side (knots and gaps filled, sanded) and one not-so-good (holes not filled, usually sanded.
  • DIY (Do It Yourself) - Mostly a great way to go. But I'm not the kind of guy who likes to knit, believing that machines do it faster AND cheaper.

For square boaters, especially, SIPs are an excellent way to shorten build time and dramatically improve insulation and positive buoyancy.

Square Boats of the Bolger Brick, Puddle Duck Racer, TriloBoat, Shantyboat and so on types stand to benefit most from SIP panels. Triloboats, for example, use 85% to 100% flat-panel construction. Time savings would be huge if building a whole boat from SIP, rather than DIY composites. But even Curvy Dogs use flat panels for bulkheads, sloped decks and superstructure construction.

SIPs can be DIYed. One can build them in place (generally the way to go whenever curves are present). Or one can pre-fab them, flat on the bench, before assembling them into the hull. In either case even clamping is the tricky part, to ensure good glue up and leave no voids. Vacuum bagging is ideal and within reach of amateurs, but requires a good set-up and tools you won't be taking with you.

Manufactured SIPs use high quality processes to ensure perfect adhesion. The downside is that they're mass producing for the housing market, which has different needs. XPS is more exotic, in this market, and the low end of the standard is 3 inches. The 'ply' used is most often OSB (Oriented Strand Board), a nautically worthless material.

The trick will be to come up with a standard or two, whose many customers make manufacture worth a company's while.

For starters, I'd suggest a SIP standard of 1.5 inch XPS sandwiched between two layers of 1/4 inch AC ply of a good, marine wood (red cedar, fir or equivalent, yellow pine).

1.5 inch foam provides reasonable thickness and allows framing with standard 2x lumber. If more thickness is required it can be laminated up without too much trouble, as 1/4 inch ply accepts staples for even, DIY clamping. AC plywood is much cheaper than marine, while SIPs' girder strength makes up for any structural advantages derived from engineered marine ply.

So, Square Boaters... do we represent enough of a market to start reaping economies of scale?


  1. Perhaps incorporate this into a grant proposal for a coastal cargo fleet that stands ready to fill the gap if peak oil or a outright sudden disruption of fuel occurs. There's your economy of scale. With 3 year school for youngsters (oldsters too though): year one basics, year 2 building in yard, year three as first mate. Well trained crews, full time production. Some idealistic visionary financier might go for it.

  2. The Sail Transport Company ( is up and running in Puget Sound. They're doing all wind/human power delivery (hand-truck and cycles, ashore).

    An apprentice program, as you suggest, is an excellent idea in so many ways! Sea Scouts would be a great group to integrate.

    Let's bootstrap... waiting for idealism in the 1% is long odds!

  3. This isn't exactly SIP, but concerns foam.

    has an excellent article on the promising-but-not-yet-fully-realized state of soy based expansion foam.

  4. And PS:

    I forgot to mention Tiny House folks among our numbers... my apologies!!!

  5. Is Dotmar's Uniboard a possilbe alternative to SIP?
    It is documented here as an alternative to marine ply:

  6. That Uniboard looks great, though a first pass through their site doesn't include hulls in their application lists. Don't see any reason why not, if it's strength numbers are high enough.

    Uniboards are SIPs in every sense of the word.

    I see that it's 'weldable'. Probably not glueable. But plastic welding is no big hurdle, and makes low profile, waterproof joins.

    Costs, even if high, may stack up favorably against double ply, foam, glue and labor, plus more involved joining procedures. It sounds as if it might not need a 'finish' for further savings.

    Of course, it WOULD be on the opposite corner of the globe from Anke and I! :(

    Definitely worth investigating! Thanks for pointing it out.

  7. I intend to use cool room panels for my houseboat.
    Its mde from colorbond steel with a foam core.

    It would not be suitable for serious ocean going , but for my use it`s ideal.

    As for DIY , there are cheap , low tech ways to get that clamping pressure to yield a sound , void free panel.
    Can be as simple as a roll of poly plastic , and damp sand on top. Or , again poly plastic placed in a frame of 2x4 filled with water. Good article Dave !

    1. Those panels look great. I see they can even can be rolled.

      The damp sand is a good idea, but I've personally not had much luck with weighted approaches. By the time there's enough weight, set up and take-down really get slow.

      But the water idea, now! That can be pumped full and dry very efficiently. I'll add that to our bag of tricks!

      Thanks for the good ideas!


    2. Dave and cohorts, hello!
      I know this is an old thread, but I'm excited to find it! I'm just trying to find out how much clamping pressure is applied in the professional manufacture of SIPs, and here I find a blog right up my alley in a bunch of different ways! Junk rigs, liveaboards, sail freight projects, yahoo!

      Anyway, to my comment on this particular post - I have some 4"x4'x8-12' EPS foam (often incorrectly called styrofoam) boards and am trying to decide what to do with them. I had just decided definitely not to use them in any kind of boatbuilding project after learning that EPS absorbs water. I decided instead to either insulate my attic or manufacture some SIPs to roof over my housebus project. Has anyone built a boat of SIPs with good results?

      Second comment, regarding methods of clamping large areas: a large vacuum bag isn't hard to assemble, and can generate a surprising amount of clamping force; OR, some strawbale home builders have been pre-loading their walls before plastering by inflating an air bladder with a wooden frame built over the top of it and chained to the foundation. The bladder is a length of large diameter vinylized fabric hose with boards clamped on the ends and a tire stem glued in one end, and when the plaster is cured the bladder is deflated and replaced with the roof structure. Seems like a 4x8 or 4x12 panel could be clamped up similary, except you'd need several bladders and a stronger timber frame over the top.

      As for making your own SIPs for boatbuilding, you need closed cell foam and a waterproof moisture-cured glue. Liquid polyurethane meets these criteria, and a post on an alternative building forum recommended Excel One. From the manufacturer's website I see it has an open working time of 25-35 minutes and a clamping time of 2-5 hours. I read elsewhere that a humid environment decreases open working time, so have clamping equipment and jigs set up and everything ready to close it up before you start spreading glue. They also make an Excel Xpress if you're in a hurry, clamping time is only 30-40 minutes, but open working time only 8-10 minutes. Might be a good solution if you're in a dry cold environment and trying to save heating fuel in your workshop - full cure time is 5 hours instead of 12. I am not affiliated in any way with Excel Glue, just wanting to pass on good info I found elsewhere. I have read about its use for many years on wooden aircraft building mailing lists, so that might be another place to look if you're needing reliable info. Aircraft Spruce and Specialty sells it, so they might sell other liquid polyurethane glues. For the environmentally conscientious, moisture-cured glues have no solvents and do not off-gas petroleum distillates, which is why they can be used on foam.

      Closed-cell versus EPS (expanded polystyrene) - according to a sailboard building book on my shelf, EPS will soak up water through the tiniest pinholes in the fiberglass laminate, and nothing short of major excavation will get it out. True Styrofoam (Dow brand name) is an _extruded_ expanded polystyrene foam, and has somewhat better resistance to water penetration than regular EPS, but can still absorb some water between the beads. The same book recommends most strongly a white closed cell polyurethane foam made in the US under the brand name Clark, and as Bennett in Australia.

      So, if I was considering making SIPs for boatbuilding, I'd go down to Home Depot and get the smallest sheet of closed cell polyurethane foam and a bottle of Gorilla Glue (moisture-cured liquid PU like Excel One, higher priced, but available locally in small bottles), and conduct a few experiments.

      Looking forward to catching up on this blog!
      David Field

    3. Hi David,

      Thanks for all the good info! Especially, I'd never heard of Excel products, and LPUs are hard to find.

      We built SLACKTIDE (six years old, now) with XPS bonded with a latex, cove adhesive. Samples were great, large scale not so great... viscosity too high for consistent clamping over large areas.

      On the boat a'building, we're using XPS with Titebond, and so far so good (see

      We've not heard of blueboard absorbing moisture, though I HAVE heard that the insulating gases can leak out, and therefore it should be sealed. Another candidate is Simalfa 123.

      All very hopeful, and even a poor job seems to be a big improvement over NO insulation.

      Vacuum bagging is VERY attractive to us, but, alas, we've never managed to try it. Dick Newick designed a series of constant camber,, fishing trimarans for third world construction using local woods, epoxy and vacuum bagging ( Trials demonstrated that it worked well in third world conditions, i.e., cheap and practical.

      Glad you found us!


    4. PS. Oh, and since the latest comments, has sprung into being.

  8. This comment has been removed by a blog administrator.

  9. Me and my wife are looking at building a house boat on pontoons out of galvanized steel sips

    What do you guys think about it?

  10. Joseph, I've been looking at doing exactly the same as well for quite some time now. There is much health-related issues to learn about that the Corps ignore or at times even lie about the off-gassing issues.

    I'm lucky my supplier of the metal clad, foam panels is pretty close and specializes in the foam my research indicated was less efficient but healthier to live in close proximity to.