Plywood types for stitch and glue kayak building

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1. Plywood types for Stitch and Glue Kayak building

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About "Stitch & Glue" Plywood

The vast majority of stitch and glue kayaks is built from 'Marine plywood' which is overall an excellent choice but marine plywood is generally limited to only a few unimpressive looking wood species and predictably, kayaks built entirely from such material do look accordingly. After all, many S&G kayaks are painted to hide the wood. Marine plywood is made to certain specifications so it is of predictable quality and as a result, has been adopted by and large as the only 'acceptable' and 'safe' material for stitch and glue kayaks. This is certainly understandable since confusion and mystery surrounding marine plywood abound and 'kit biz' interests are quite happy to perpetuate this orthodoxy.

So, what is this stuff ?!
Marine plywood has actually less to do with the wood itself as it has with the specifications to which it is manufactured. Let's look at the 'wood' first. The vast majority of marine plywood in the world is made from the Okoumé (Acuomea Kleinea) tree which is harvested in the African state of Gabon in the tropical forests of the Congo Basin. This wood is also often referred to (correctly or not) as Gabon, Gaboon, Okume, Ocume, Ocoume, BS1088 etc. and the plywood core as well as the skins are made of the same wood. Virtually all Okoume plywood is manufactured outside of the US and Canada so it is a good bet that the plywood in your kayak will likely come from some factory in greater Asia, Israel, France or Greece.

Another less common variety of marine plywood is 'Meranti'. It is made from many subspecies of the 'Shorea' tree species which also happens to be the source of wood for the common Lauan (luan) plywood. Both Lauan and Meranti plywood are also referred to interchangeably as Philippine Mahogany and the wood is harvested in Indonesia and the South Pacific rim forests.
Now, there is also White Lauan made from Shorea almon, Red Lauan made from Shorea negrosensis which happens to be the Red Meranti. Other 'Meranti' plywood is made from Shorea curtisii, S. hypochra, S. leprosula, S. multiflora, S. platiclados, S. pauciflora to name a few. Confusing enough ?

Marine plywood is also made from Fir, Ash, African Mahogany (Kaya), Honduras Mahogany and/or combinations of Okoume core with Sapele, Teak as well as other wood skins but those are largely used on plywood thicker than 4mm which is unfortunate - for kayak builders.

The look of marine plywood is not only determined by the wood species but it can also vary according to the ply cutting methods. The most common is rotary cutting which give the plywood surface a look of 'contours' on a topographical map (same look as common plywood in a lumber yard). The other method of cutting is 'quarter sawn' slicing which produces nice 'ribbon' look with the tree growth rings appearing as parallel lines (like wood strip kayaks). The immediate implication of this is that building your kayak with the 'same' type of marine plywood as your friend built a year ago may make your kayak look quite different (color and texture wise). The plywood will have likely come from a different lot of logs and/or subspecies. Also, Meranti plywood is less available in thickness less than 1/4" (6.2mm) and is largely heavier due to the wood density itself as well as additional glue since it comes in 5 ply. Okoume plywood under 4mm is always made in 3 plys. A 4' x 8' sheet of 4mm Okoume can range from 12 to 16lb ($30-50 price range) and Meranti about 19 to 24lb. All in all, you will be hard pressed to find anything lighter than Okoume.

Now, the main thing that entitles plywood to be classified as 'marine' relates to the manufacture under a set of standards, specifically to the quality of the wood as in the number and size of permissible knots and cracks, glue type, moisture content and thickness tolerances of the plys etc... Marine plywood is sold under BS (British Standard) designation such as BS1088 or BS6566. BS1088 is made from Okoume and is of the highest quality with a defect free core and both faces of solid A surface while BS6566 standard is less species specific and is more permissive so the surface of this plywood may show a few skin defects on one or both faces. Here is a short excerpt of the BS1088 standard.

What about other plywood? The nice thing about building your One Ocean stitch and glue kayak like the Cirrus is that we can ignore most of the BS1088 standard for it is quite irrelevant with respect to this building method, with one exception - the requirement of a 'void free' core. This suddenly opens up a whole new world of plywood choice. Remember, that the sole purpose of the plywood, (like the strips in a strip kayak), is to serve as a solid sandwich core, reinforced on both sides with continuous and water proof skin of epoxy and structural fiberglass. Consequently, the 'BS' rules for plywood thickness variation, individual ply variation, wood type, rot resistance, small skin defects, and most importantly the glue type - make NO difference whatsoever. As a point of interest, the BS standard recognizes four types of glue bonds. All BS1088 and BS6566 marine plywood use phenol formaldehyde (Weather & Boil Proof) glue.The other non-marine types are "Cyclic Boil Resistant", "Moisture Resistant" and lastly "Interior" glue which is very strong in the dry state but has no resistance to moisture.

Now, the perspective to keep in mind is that the plywood in your kayak will always stay as dry as it was when you covered it first in epoxy and it will remain 100% weather, moisture, air and UV proof. It will certainly not be boiled (perhaps only cycle boiled in sprint as you lean into the paddle ;)). Let me also point out that strip kayaks are made of many wood types (balsa included) and the glue used is common yellow carpenter's glue (not water resistant or 'proof'). I still wait for reports of kayaks dissolving in action.

The last and most important point that deserves consideration is the requirement for a void free core. This is a structural issue and large voids or cracks in the core essentially reduce the thickness of the plywood in that area to the sum of 'thickness' of the skins and the fiberglass. So if you have 6mm Lauan on the hull with a huge crack in the core, the actual thickness of the plywood becomes about 3/32" (2.5mm) which is very bad! BS1088 and BS6566 plywood will guarantee a completely solid core but it doesn't mean that it is the only plywood in existence that has it. If you look around, you will find a vast amount of excellent quality plywood on the market which uses the same glue as BS1088, and even if it doesn't, the core and skin will be as solid, perhaps better than any BS 6566 plywood. If you decide to build from non marine plywood, make sure to examine the sheets from the side (perpendicular to the grain orientation of the skins) to verify the quality. In my experience, top quality exotic veneers (which is what we like in our kayaks) are not bonded to crappy cores so the plywood sheets will generally be consistent with the overall quality and price of the veneer. So far, this gives us a decent selection but if you want something really special you can also make your own plywood by bonding veneer (of which there is even larger variety) dierctly to 3mm or 4mm plywood. To do this right, however, will require vacuum bagging or veneer press since the gluing will need to be done with slower setting adhesives such as Titebond II or epoxy. Some plywood merchants also provide this customized service. So, if you are building the Cirrus, Okoume and Meranti plywood are not your only choice and that's no 'BS'.

The picture on the left shows a mere 5 candidates for your kayak. From the left top corner down, there is Lauan (Meranti), Baltic Birch, Okoume, Poplar and ribbon strip Sapele (quarter sawn sliced).

There are also 3 and 4mm and 1/4" plywood types with skins of Teak, Mahogany, Walnut, Cherry, Maple and quite a few exotics.

Side view of the same. From left to right: 4mm Okoume, 1/4" Mahogany, 3mm Baltic Birch, 1/8" Sapele, 3mm Okoume, 6mm Lauan. Please ignore the metric scale in the picture since it was distorted by the lens. Note that both Okoume samples have plys of virtually same thickness. This plywood profile is also common on 'bendable' types of plywood where the mid core is as thin or thinner than the skin plys. In general the opposite is also true in that the larger the thickness of the core as a percentage of the total thickness, the stiffer the plywood will feel.

	Scarf gluing and sanding veneer plywood is always more difficult since the veneer is very thin and will not tolerate sloppiness in sanding or aligning and gluing of the scarfed sheets. As you can see from the Sapele sample on the left, the margin on the veneer is quite small. If you are going to build One Ocean Kayaks Stitch & Glue Kit, the scarfing is avoided with the special ScarfLOCK joint. The Okoume (center and right) as well as any thicker skinned plywood will be far more tolerant of sanding or misalignment. Veneer thickness is about 0.5mm and less while the skins on Okoume are about 1mm and more depending on the overall plywood thickness.
	Ouch! Nice core voids in the 6mm Luan (Lauan). This is definitely not going into my kayak! It is possible to find very good Luan but it's more a matter of luck than anything else. Since the grain orientation of the skin will likely go along the length of the kayak plates, the grain direction of the core (and any voids) will run perpendicular, which will be fortunately visible on the sides of the narrow kayak plates. It is doubtful that a core void will start and terminate within the 'width' of the plates which are at the largest span about 10" wide. When you do a quality check on your kayak plates, these flaws will be unmistakable and such plates cannot be used. That is the compromise of using cheap material.
	Cheap 1/8" Lauan on the right shows grain runout that required patches on the entire sheet. Nothing like the right piece should ever end up in your kayak - not that you would put it in. The left piece is luan too but significantly better.

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