Marine plywood — choices and options

As mentioned in the article 'How materials have affected boat design', laminated wood of various forms has been around for 1000's of years. The concept of bonding criss-crossed veneers was first patented in the USA in 1865 by a now almost unknown John Mayo from NYC and another 40 years past before even a fledgling industry got started. But it was the formation of the Douglas Fir Plywood Assoc in 1933 that started a construction boom in the USA and although WWII took most of the output up till 1945, plywood became a major success. Just 10 years after the war ended, the number of plywood mills had tripled and plywood is still in high demand today. If you're interested in more history, you might enjoy this site: www.apawood.org/plywoodpioneers/history.htm

After the war, a marine grade of plywood became available with important differences over construction ply. First, the bonding resin became a phenolic-based waterproof product and secondly, a higher standard was specified for the veneers with tight limits on hole sizes even if plugged. In Britain this was covered by a British Standard (BS 1088)* and many manufacturers overseas still manufacture to that high standard. For many years now, even construction ply has adopted a waterproof quality bonding glue but that still does not make the ply a marine product. (Another exterior quasi-marine ply standard was BS 6566 (now obsolete) but the outer veneer is about 30% thinner and of slightly lower grade. However, if to be epoxy coated or glassed, it might still be a cost effective option where available, if the specific application has been well considered.)

While Douglas Fir is a tough wood, it is also relatively heavy, has many small knots, a surface grain that is difficult to keep flat and has a tendancy to split - with small surface cracks being hard to seal against moisture infiltration.  Sheathing with fiberglass is almost mandatory.   It is also so high in natural resin that even epoxies do not bond as well to it as they do to other woods, such as the more expensive Okoume or Gaboon that are more porous, even grained and lighter. Actually, it's rather bizarre how Okoume has become so popular in recent years as the boat-builders 'marine ply of choice'. It was once considered somewhat of a 'trash wood' compared to the more durable alternatives, but the increasing rarity of good mahogany-family options plus the effectiveness of epoxy coating, has changed all that. One now pays a premium for Okoume, as its lightness and resistance to warp offer ideal benefits to the marine builder.    When used with epoxy, it's porous grain is a real plus, as the relative weak wood needs such protection yet the open grain accepts it well.   Unless you are really looking for more strength and inherent resistance to moisture, options like Khaya, Meranti and particularly Sapele, are generally too heavy in my view if large panels are also to be epoxy coated, though Red Louro might be another marine ply of potential interest. See: www.fpl.fs.fed.us/documnts/TechSheets/Chudnoff/TropAmerican/html_files/ocotea2new.html

Finnish Birch could also be an option, though typically, birch tends to rot faster than the African woods of the mahogany family.  Birch has a nice finish though and is tougher, stronger and harder than the cheaper luan-type plywoods (see later for Luan).   The strength and smooth finish of Birch plywood has made it a popular choice for light planes and Aircraft quality plywood is generally even higher quality than marine ply.

Despite the flaws noted above, waterproof-bonded Douglas fir plywood is still used for some rugged commercial motorboats or utility dinghies etc.. but plan on sheathing with fiberglass and expect a heavy result.   For more info on North American plywood, go to:
www.glen-l.com/wood-plywood/wp-index.html#index and
continuouswave.com/ubb/Forum3/HTML/003543.html and also
www.noahsboatbuilding.com/noahusa/items.asp?cc=33&bc

As far as availability in North America, Bruynzeel (Holland), Joubert (France) and Shelman (Greece) appear to be the top manufacturers of marine ply to BSS 1088, although less expensive options may be available locally. For prices, one has to continually shop around though expect to pay close to $100/sheet for a high quality 8' x 4' x 6mm marine plywood (2010).  If trying an option to the 3 brands indicated here, always do at least a 1 hour boil test before cutting more than 1 sheet and return anything that cannot pass that simple test.

A few words about the British Standard BS-1088 are in order.

Having said that, there could still be a few other plywoods available to you that are worth considering. But you'll need to be very thorough in examining the panels—both face and core for cavities or unglued cracks—and I would suggest doing a boil test on a piece of each sheet that might be used for a main hull skin or other critical part.   In Spring 2009, I was quoted just US$13 for an 8' x 4' sheet of ¼" luan* ply with a claimed 'waterproof bonding'. The surface of both sides was excellent, though the core was soft and I noted a couple of open cracks. Personally, I'd not use if for a boat over 18' but to try out some new design or for a quick-build skiff or canoe, I'd give it a go, making sure that any core cracks were not part of the plywood being used. I would also sheath the inside with 1 layer of 4-oz cloth though, and optionally the outside too—certainly the bottom.   (For an inexpensive small boat project, one can even get by filling small core gaps with shish-ka-bob sticks covered in epoxy ;)

Because a higher price can be obtained, there appears to be many plywoods out there 'claiming' to be 'of marine quality', including some potentially interesting ones from Russia and South America, but without the full certification, one needs to be prudent and do your own testing and examination. One hour in boiling water is the traditional 'tough test'.

Also see www.woodenboat.com for additional knowledgeable information on marine plywood by John Harris, owner of CLC Boats (2017).

Luan veneer
Luan plywood (sometimes called Lauan) is made from trees of the "Shorea" family—an even grained wood similar to mahogany, though less dense. There's a white and a red luan, depending on which Shorea it comes from.

The main advantages of this wood for the small boatbuilder, are its lightness, low cost, and porous surface that takes coatings well. The downside is its relatively low and variable strength plus its uncertain bonding and core.
For the right application, it can be a great buy though.

Options

As high quality plywood prices continue to climb, it's quite possible we will see other marine panels appear that are cleverly engineered to offer similar qualities, so keep your eyes and ears open.   Always do your own testing though as early claims may prove to be exaggerated just to get enough sales to pay off the development costs.   If the panel is more expensive than plywood, look for advantages to offset this .. such as less required finishing work or lighter weight.  But also make sure it will flex enough to work for the design you want to build - or otherwise, consider a design that will work better with the material.                                                                                mike-2010/2017

 

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