plywood for boats
In reading some specific postings on the Plastic Classic forum I’ve collected some notes for my own future reference. This forum is so helpful to anyone who would like information on boat restoration and rebuilding.
On plywood choices for interior construction, both Tim Lackey and Charlie J have contributed.
The terms “9mm” and “3/8″” are used interchangeably, as they refer to basically the same thickness (9mm is actually 0.35433″). Meranti plywood, however, is sold in metric thicknesses, so 9mm is the correct term.
13mm plywood is approximately 1/2″. One sheet of this was used for the cabin sole. The other interior structures are 9mm meranti; the three main structural bulkheads are 18mm (3/4″). One should choose a plywood thickness that is appropriate to the strength requirements of the job, and taking into account the unsupported spans involved, if applicable.
Excerpt from the One Ocean Kayaks site:
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.
Meranti is a type of Phillipine mahogany (CrossCut Hardwoods states that it is a type of Asian mahogany). The meranti plywood, which meets the stringent requirements of BS 1088 (the benchmark for the highest quality plywoods), is made from this type of mahogany. The facing surfaces are clear and free of those annoying “footballs” that you see on lesser plywoods. Some sheets look almost too nice to hide in a structural application, but in general it looks like plywood and lacks any real character that you might want to show off. But the surface is smooth and much nicer than the best A-face standard plywood that you could find. Plus, since it is in fact a mahogany species, it looks more like real wood and less like trash.
The king of marine plywoods may be okoume plywood, which is manufactured to the highest construction standards in the industry. Meranti as a wood is actually more durable than okoume, but plywood made from okoume (hence “okoume plywood”) is technically preferred because of its high manufacturing and quality standards. It also meets standard BS 1088.
Interestingly, while okoume is often regarded as the “granddaddy” of marine structural plywoods, meranti is actually a more durable species, but is somewhat heavier in weight. Both marine meranti and okoume meet the same BS1088 standard for construction and void-free certification.
The One Ocean Kayaks site poses the following:
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.
That said, marine meranti plywood is constructed to the same regulated standard for the end result, and this means that it is a high quality, void-free plywood with more plies per thickness than the plywood you might find at a lumberyard. This makes it extremely strong and stable, and you know that when you cut into it you shall never find a void.
Lauan plywood is junk! It is most certainly not marine grade.
Lauan and meranti are both species of mahogany, and the species themselves may be similar, but the products made from them could hardly be more different.
This is verified by the One Ocean Kayaks site which states:
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.
The key to the quality of the meranti plywood is in the number of laminations, and the fact that all laminations are certified void-free. This makes for a stable, solid product. In fact, it’s the laminations and the void-free certification that make a marine ply a marine ply; the glue is rarely any different than that found in exterior grade.
Charlie J writes:
Luan is not a form of mahogany- it’s related to the cedar family instead. It’s just CALLED mahogany by many people (I cannot find support for this statement made by Charlie J). Same way with Okume- it ISN’T actually a mahogany- different family completely. Okume is an african hardwood, named Okume.
On the fir plywoods, standard and marine, be VERY careful with the stuff should you decide to use it. I’ve been seeing stuff labeled as “Marine” with only 4 plys in 1/2 thickness. Run, don’t walk, from that kind of junk.
Actually, American made plywood has been going downhill for a number of years, and has now gotten to the point where I don’t even consider it as a viable material for boat building. It’s pretty much trash. I did use some good grade fir marine in building the interior of Tehani, but I was quite careful in what I got. I’ll probably not use fir again. Okume, ( or Meranti) just isn’t that much more expensive, considering the time and effort put into a boat. A boat like my Rhodes Meridian or the Triton’s should serve well for YEARS once restored- no sense in shortchanging yourself or the boat, by cutting corners on quality woods, just to save a few pennies.
Note that the adhesives used in a true marine plywood are not necessarily different than those used in standard exterior grade plywood. However, true “marine” plywood refers to the construction of the interior laminations of the product, and is certified void-free, and with no flimsy interior laminations. Interior lams in true marine plywood are, to my knowledge, the same as the exterior, other than in appearance and not accounting for any attractive veneers used in some types.
Meranti is not inexpensive, but is a good value when you want to do the job right. Its price tends to be only a small percentage higher than something like fir exterior plywood, but for a much higher quality product. Meranti is significantely less expensive than okoume.
I believe that in this day in age, only the best quality plywood should be used in boat construction, at least for structural applications. To me, this means any of the marine plywoods meeting BB 1088. You can cut price, but only at the expense of ultimate quality. Note that I have no problem using interior grades of veneer plywood (i.e. cherry), but only in surfacing applications, and never for structural.
9mm was used for the entire v-berth in the daysailor, including the top. You definitely need to build in some additional supports for the top (or in any wide horizontal span), as the 9mm is a little lightweight to avoid bending on its own in this application. It won’t break, but it will bend if not supported adequately.
The cherry plywood used in the interior is just standard interior-grade plywood–nothing special. At the time, anyway, there was no “marine” grade cherry plywood available, at least locally. I don’t know if such a product exists even today; given the wide use of cherry in boat interiors now, perhaps there is.
None of the cherry ply in the interior is used in a structural way, but nonetheless there have been no problems to report as a result of the construction of the plywood. I see no problem with using it for this application. I would not choose it for anything structural, however.
For interior partitions and “furniture”, you can use just about whatever you want. If you’re going for a nice wood look, then choose a good grade (as good as you can find) of veneer plywood in the species of your choice for these non-structural members.
The tabbing on these pieces is designed to hold them in place, and to each other; they don’t provide necessary structure. In the A-30, just the aftermost bulkhead (aft of the galley under the cockpit) and the forward bulkheads provide structural strength to the hull. Most of the rest of the interior is “furniture” only. Every boat is different, so take one case at a time.
This veneer, cabinet-grade plywood is of lower overall quality, at least as compared to a better quality marine product like Meranti, but will suffice for non-structural applications, like the galley area. Just stay far, far away from the fir plywood junk they sell at your local big box store.
“Make all attempts to seal the end grain, in all cases, as well as to finish the back sides of pieces that might be susceptible to moisture.
For general interior work, structural and non-structural, I recommend marine meranti plywood, though it should be used mostly in hidden areas, or else covered with some other product for a final finish. Some sheets of meranti look almost finish quality, but it’s not really intended as a finish product.