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Fair winds!

Dave and Anke
triloboats swirly gmail daughter com

Sunday, June 28, 2020

SIP RIP: Deciding Against a Ply / Foam / Ply Hull

Adding inboard layer of 1/4in ply
Next space to my right has fitted foam ready and waiting
Two areas to my right, the inboard face of the hull is exposed

This guy oughtta be wearing his ear protectors!

I think I'd better think it out again!
-- Fagin, from Oliver! lyrics by Lionel Bart

SIP RIP: Deciding Against a Ply/Foam/Ply Hull

A friend of ours built a shop with reclaimed Structural Insulated Panels (SIPs). It flew together, was obviously superbly insulated and was strong and stiff.

But commercial SIPs aren't typically geared for marine environments (though I wish they were). Custom orders are possible but too spendy for us. With tape n' glue construction, these would be the bomb. Sigh.

DIY ply/foam/ply adaptations seemed within our reach, and they were. We've now built two boats with two approaches, and both worked out (WAYWARD's example). But, looking back, we've (pretty much) decided not to do it again.

Insulation (R-value) is awesome and totally stopped condensation. Positive buoyancy is a benefit we hope never to call upon. But...

Here's a list of cons:
  • Framing is necessary, finicky and expensive.
  • Through-hulls need blocking (in advance or retro-fitted).
  • Installing foam and inner ply is time consuming and adds cost.
  • Voids (which can rot or mildew) are difficult to reliably avoid.
  • They can't be disassembled for inspection.
  • Foam takes away from interior volume.
  • Foam adds fire hazard with toxic smoke.
  • Foam is a pile of plastic waste that can never be "disposed of properly".
Framing deserves special note, and is what truly tips our scale.

Its purpose is to take fasteners for furnishings (vs. epoxy welding or tape n' glue which we prefer to avoid), tie the inner skin mechanically to the outer, join panels and seal edges (as around window cut-outs). It must be well-planned, precisely installed and devil take those who change their minds. Corners are particularly aggravating, and may need doubling up.

So what's our alternative?

We now lean toward two layers of ply, laminated (we like an LPC such as Gorilla Glue for this).

Condensation is much reduced by 1in of ply, and 1 1/4in (using one layer of 3/4in for accepting fasteners) takes it down to near none. While insulation is relatively low compared to foam, it's still adequate. Strength is high. Buoyancy is still positive, though reduced. It weighs a bit more. Cost is oddly about even (pound for pound, thinner ply is generally more expensive).

Best of all, interior framing is eliminated... you can fasten anywhere into the wall or drill right through it without blocking.

TriloBoats are intended to be quick and simple as possible. We've found that ply/foam/ply in the hull sides run against that grain. While the results are good (at least in the shorter term), we feel that the net benefit doesn't pay for the effort.

Should we build again (please O Great Spirit, NO!), we'll continue to use a SIP approach for the main deck but go to a solid wall hull.

So R.I.P., S.I.P.s.


  1. Never would have thought this refinement necessary but makes total sense and only could have been realized from experience. Simpler too!!! If building a trilo I would have followed your lead and gone with foam. Makes even more sense the farther into temperate or tropical waters the vessel goes. Plus more and more reassuring the thicker the skin gets!! Witness the super thick ply skins on Chris Morejohns sea sharpies.

    Re the "O Great Spirit, NO!" call to wood butcher anew Colvin scratched his build itch by having his own small shipyard. Older shipwrights seem to gravitate to model building, fine joinery, etc.. Some perhaps build a microcruiser or daysailer. The sad sack contingent fall into the neck of a rum bottle and never surface. Hard denying the itchy insistence once you have built (and designed) a few boats yourself though.

    1. Hi Roberto,

      One of the advantages I didn't mention is that ordinary 4-ply sheathing (relatively cheap) can be used... the thick walls make up for the weaker ply.

      RE "the boat-building itch"... As a friend says: I felt the urge to build something, but I lay down and it went away! 8)

      Dave Z

  2. One of your reasons, if I recall correctly, was needing less firewood, a big plus in the elder years. Did that pan out? And if you were using 3/4 for both outer and inner, you wouldn't need to remember where frames were located for hanging pretty much anything. And if you used framing material that is 1/16 thinner than the insulation and used screws to attach the inner face, you could remove to inspect once a year. With some kind of wax, you could make a "resist" that doesn't allow GG or spray foam to adhere to the plywood/frame, so you could remove the foam as well, and still be able to fill voids. Still lots of extra work & materials, but insulation is probably worth the investment imo.

    1. HI UWB,

      It turns out that I can't really tell the diff for firewood... our stoves tend to produce more than enough heat for warmth. Complicated by the fact that our newer plate steel (vs cast iron) stove works with salt wood, which is generally easier to gather. So I'd say that if there is an advantage, it's not striking.

      Great idea using wax! Johnny rings are a synthetic (cheaper) bees' wax that can be potioned up to make good sealers. Only issue is that they and paint are hard to work together...

      The inner 3/4in layer is a good thought, though doubled is pretty heavy... better in larger vessels? It's the main drawback to double walls thick enough to insulate. The framing adds up in weight, too. Generally, I figure 25lbs per sheet per 1/4in of thickness.

      I agree that insulation is worthwhile, even to the point of necessary (LUNA's 1/2in was an ongoing headache). But I've slowly come to the conclusion that (for us), double wall is sufficient insulation, with other advantages tipping the scale.

      But I sure wouldn't argue the point!

      Dave Z