A note on Plywood

[Geminidawn]

Most of this most of you know already. I've just copied and pasted this from the notes I prepare for my students. It may help someone here.

A note on Plywood

Plywood comes in many different types from all over the world. From Domestic ply for Furniture to Exterior WBP (Water & Boil Proof) as used in the construction trade and on to Marine Ply for the Marine Industry.

Domestic Ply is usually bonded with just PVA Glue which is water soluble it has no business anywhere near a boat so we’ll quickly move on.

 A lot of amateur Boat Builders are convinced that WPB will do if you coat it in plenty of epoxy, well it is good enough if you get a good quality WPB to begin with.

Personally I never use anything but Marine Ply and the minimum standard I look for is BS1088 or its equivalent. All there is, is a few dollars in the price difference, if I wish to be using the boat in years to come or if I want to sell it on with confidence I stick with Marine Grade and it is usually the first question the prospective buyer will ask.

The problem with ply is that one veneer is about the same as the next, the real expense of ply that will last you any length of time is in the glue that is used in its manufacture and the amount of veneers per thickness. If you go to your local timber merchant and look at the edge of a 6mm/ 1/4” sheet of Marine ply the chances are you’ll count more veneers in the marine grade than you will in the WPB, this is a mark of higher quality.

Another quality to look for is voids, look down the edge of the plywood there should be no gaps or pieces of veneer missing. Voids are fine for flat panelling in houses but in boat building ply is often bent or twisted into shape, these voids then become moisture traps and weak spots.  

But what about the Glue?

The Glue that holds the plywood veneers together is what’s all important. But how do you know you’re getting good stuff?

In my practice as a Lloyd’s Qualified Surveyor I test products all the time and Plywood more than most. But the extent of the test depends on the extent of the project.

Preferably I like a long slow test. No better than to test it in the environment it will be working in. I’ll see what’s available locally and put a one or more test pieces hanging off of the pier wall at the half tide mark so it spends 6 hours in the water and 6 hours baking in the sun, over a period of months I’ll monitor how or if it degrades. But that is only reserved for pre-production or big expensive projects where there is a lot of money at stake.

But you want to build a boat now, not in a few months’ time.

Alternatively there is the fast test. I take 50mm/2” x 25mm/1” samples of what’s available locally all of the same thickness and I boil them all for at least 4 hours, dry them out thoroughly in a warm place and then boil them again, the last one to fall apart is the one I use. Any ply labelled Marine or a good WBP should survive boiling for at least the first 4 hours.

A note of warning: Not all ply is what it is labelled! I once got a piece of Ply labelled “Marine Grade” that fell apart within the first hour. This is a fraudulent but common practice. It’s easy to put a “Marine Grade” stamp on anything and charge more. Even if it’s from a reputable Ply merchant with a BS1088 stamp on it, I still test it, because Ply merchants can get conned too!

Ask your local merchant for  some test pieces, tell them what it’s for and what you plan to do, they’re usually quiet obliging and as eager to know the results as you are.

All timber merchants I have ever dealt with gave special consideration to boat builders for they knew the implications involved for selling them substandard products. An impromptu meeting for someone with a certain Davy Jones.

For aesthetic reasons you also may want to consider the surface veneers as surface veneers are often of better quality than the core veneers. Some Plywood comes in an A/A veneer which means good quality veneer on both sides but it also means it comes at a price. You can also get Marine Plywood with high quality Teak or Honduran Mahogany face veneers for yacht finishing joinery but to use this in small craft is just overkill. The Marine Plywood I usually use has an A/B veneer, one good side and the other looks like a patchwork quilt. This I don’t mind so much as I’m more concerned about the quality of the glue that holds it together. I just have a think what areas of the boat are going to be painted and what areas are going to get varnished, of course all varnished areas get the “A” side up, the “B” sides get a coat of epoxy to seal them in.

So before you parting with you hard earned cash put the kettle on sit down and have a think as to how you want your boat to look when it’s finished, paint or varnish, and where? If you’re going for paint what colour?  Keep in mind light colours attract too much dirt and can be a nightmare to keep clean, dark colours attract too much heat, the plywood expands which in turn can put stress on your taped joints.

The more thought, testing and homework you do now the more time and money you save in the long run.

[Peter Brook]

Hi Geminidawn

Much appreciate the information.  Please feel free to PM me with any further Philippines specific information.  I have contact details for Santa Clara ply in North Luzon.  I am sure Twin Aces will be fine, but I will investigate prices when I get started (should be July/August 2014).  Again, any contact details will be a big help.

Is EMCO the company producing Tuffply?  I understood it was unavailable for a while, but is now available again within Metro Manila (although we are Cagayan Valley, we travel toManila/Subic Bay area quite frequently).

Thanks of the advice re importing sailing gear.  Anything small enough to travel via luggage is no problem as I will travel to and from the UK for a good few years yet.  Balikbayan boxes will be a possibility for larger ties.  One of the reasons for deciding on a Wharram was to reduce the number of higher tech items/materials as I know they will be difficult to source in the Philippines.  That said, elements of the design and overall philosophy also appeal!

Many thanks again, and stay in touch.  It looks like I will really appreciate continued local advice!!

Best regards

Peter      drpeter.brook     at    gmail.com  for PM