|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".
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.