Plywood Construction for Less

 

Popeye Character by E.C. Segar

A vessel should be built as cheaply as possible. 
But no cheaper.

-- Adapted from Albert Einstein

Plywood Construction for Less

If we choose to build in plywood, before long you'll hear the statement: You'll be building with marine plywood and epoxy, of course. Both are fine materials and considered to be state-of-the-art.

But... here's where I find myself after three decades of boatbuilding for full-tie living aboard / cruising in the Pacific NorthWest (rainforest).

Marine Ply vs ACX

Marine plywood (MP) is AA (meaning both sides are nearly flawless), should be free of voids, have plies of equal thickness and relative more plies for any given thickness. It's a available in a number of species, each of which have a suite of virtues. The result is a stronger panel for its thickness when compared to plywoods of lower standard along any of these vectors.

The curious thing is that MP has mostly enabled boats to be built which are adequately strong, but lighter - using thinner marine plywood than if using lesser plywoods. Lighter means easier to drive toward a vessel's hull speed for a 'faster' boat. Except when using extreme methods the weight savings are slight for most cruisers.

In other words, the high price tag for marine ply buys 'speed' within a very narrow range (let's say from 0 to 4 - 8 knots). This is important for racers, but not nearly as much for the rest of us. 

ACX (one near flawless side, one OK side and eXterior glues) of decent species (fir, various pines, spruce and cedars) should last about as well as all but the most specialized MPs).

From ACX, we can build a strong, potentially long-lived vessel for far less cash outlay.

If we can personally pick through piles of ACX and have located a vendor with generally decent stacks, we can:

• Check for voids. Use a wire to assess edge gap depths... shallow knots are easy to fill, deep voids may be injected with glue.
  
  
• Check for an odd number of plies. Even numbers double interior veneers... in case of outer veneer failure, these are transverse and weak. Most 1/2in house sheathing plywood is 4 ply.
  
  
• Check for even ply thickness. Reject outer veneers which have been over-sanded.
  
  
• Check for general damage and irregularities. Reject as needed. If you know how much of a sheet you'll be using, damage can be allowed in offcut areas.

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NOTE: When going through stacks, consider being scrupulous about re-stacking. You'll not only be welcome back, but you'll get a lot more help from appreciative, unpissed-off staff.

Epoxy vs PolyUrethane

Epoxy Resin is amazing stuff. Water-proof and solvent resistant once cured. Small molecules penetrate well and make the most of micro-surface areas. Various additives alter its properties for a wide range of uses. Its application is well understood and documented... although winging it is not recommended, with a little research and discipline, even beginners can get good results.

But it's expensive, toxic (mainly skin contact while wet), generates a small mountain of waste, requires special, hazmat disposal. 

While plastic (bendy), epoxy is not elastomeric (stretchy). 

Here's the rub: while most modern construction adhesives exceed wood fiber strength by a large margin, that wood fiber strength is the limiting factor. Merely plastic adhesives can point- and edge-load wood fiber until it gives, loading the next fiber in line. Elastomeric adhesives spread the load over a (small) region, allowing wood fibers to act together for much elevated failure thresholds.

We do continue to use epoxy in small amounts, mostly for minor repair. But it's down to a trickle.

PolyUrethane (PU) and Liquid PolyUrethane (LPU) are moisture activated, waterproof and solvent resistant when cured. 

LPU expands as foam to fill gaps. This is handy so long as we recall that expanded foam is considerably weaker than the unexpanded, non-elastomeric glue film. LPUs have a quick turn-over time that can be freaky, but helps move the project along.

PU is gap filling but does not expand but is highly elastomeric. Not all are created equal, however, so check the specifications of your candidates! Before cure, it can be thinned with mineral spirits, turpentine and various oils. This makes it more compatible than epoxy with oil / pine tar finishes (we've found that it has considerable adhesion over oiled wood, especially when thinned a bit... fasteners are primary in these cases, however, on a schedule to take the full, expected load). PUs tend to have long working times (varies with brand, temperature and humidity)... this is a mixed blessing, depending on the task.

Encapsulation vs Breathable

Encapsulation means sealing the hull completely with a waterproof, usually composite barrier (fabric, resin, primer, paint). Great system. But waterproof isn't proof against hard knocks. Dings let water in, and it can't easily get out. A season of haul-out for thorough drying is advised, with solid springtime maintenance on its heels. For full-time liveaboards in a wet environment, it's a long-shot.

The old, reliable method is oil and pine tar. It wipes on in the minutes of dry between days of rain and can be done piecemeal. It's water resistant from the moment of application. It breathes and moves with the wood.

It's not only cheap, but can be made DIY.

Conclusions

Here's our current thinking for less expensive construction. We've tried most of this on one scale or another, and find ourselves returning and doubling down.

• Select ACX plywood - Fir if available. If laminating, A sides out.
  
  
• LPU Glue for lamination - LPU has good adhesion in close contact and foams up with less of adhesion to fill voids, but even this lesser bond is considerable. We don't use LPU for small area jobs, however, as it has frequently failed in these cases (non-elastomeric). Consider its use for some bulkhead framing (which can be wider area), backed up by fasteners.
  
  Brands include Gorilla Glue and less expensive AkFix.
  
  
• PU Glue for high-stress bonds - Chines, bulkhead / transom edges, deck-hull-joins can be made with chine-log construction and/or tape-and-glue methods (similar to epoxy, with PU thinned for tape saturation).
  
  Brands include 3M5200 (the gold standard, but expensive) and DAP PU Construction Adhesive (much cheaper and seems indistinguishable in practical performance on wood).
  
  NOTE: One advantage of PU is that it acts as a gasket, even with near-zero adhesion (contact using fasteners, say). Being elastomeric, it is compressible to create a water barrier. In these cases, any adhesion is gravy.
  
  NOTE: We haven't tried it, but think that PU in tape-and-glue should be a good match for copper bottoms with mechanical fasteners. Epoxy and tape doesn't seem like it would be as resistant to water penetration as it wouldn't likely form a happy gasket around fastenings for copper. Could be wrong on this, but it's an expensive experiment.
  
  
• TiteBond III for sheathing - Especially with a fabric matrix, this is relatively inexpensive and easy to apply while producing a durable, waterproof barrier that is easy to maintain and repair. Topcoat with primer and paint.
  
  NOTE: We're about to try concrete slurry with a fabric matrix for deck sheathing. Hoping for low cost, longevity and good footing while avoiding paint topcoat. Will keep you posted with results! Might even work for the hull above and below the waterline (though we won't be trying that).
  
  
• Oil / Pine Tar for sealing - Without a fabric matrix, these (in various Boat Soup proportions and recipes) are inexpensive, can be applied in a wide range of conditions and results in a breathable finish (moisture can come and, importantly, go!).
  
  We're trying a primer coat of tung oil (we've read that it's more resistant to mildew than linseed oil... so far so good) with  10-20% pine tar (can be purchase inexpensively at agricultural supply stores as treatment for animal wounds) and thinned by turpentine as conditions require (We're building in winter. Again.).
  
  The top coat can range from 50-100% pine tar. In the interior, we'll try furniture wax over the primer coat for the interior for a wipe-down finish.
  
  

Last Thoughts

One conjecture from George Beuhler that rolls around my head is the use of asphalt roofing tar for lamination. He noted that it's very adhesive once set (most volatiles evaporated). It would be far less messy between sheets of ply!

We've used it with success for various small jobs around the boat. Once set, it can be painted with latex paint without bleeding. White paint helps keep it cool and solid in (at least our PNW) sunny weather. On decks it can be topcoated with aluminamized trailer paint with a slight stipple for good footing.

A last possibility we toy with is using trunnels (wood 'nails') rather than metal. They're time / labor intensive, but superior in almost every other respect. Maybe in our next youth.

Lots of savings possible if building out-of-the-box!