View Full Version : plywood double planking

patrice Hubert
12-27-2011, 03:54 AM
Hi every one!
I'm bound to gather opinion about using a double planking (2 x8mm marine grade plyw. instead of a single 15mm. The single looks almost impossible to bend and the job seems easyer with this technic...
Has some one ever try this way? what kind of glue to employ (epoxy?) how to secure an efficient sqeezing ( straples, clamps?).
I'll consider every point of vue, thanks.

Peerie Maa
12-27-2011, 05:34 AM
Little Toot was built with layers of ply.
There are several pages here: http://skeltonengineering.com/index_files/BuildingLittleToot1.htm there are other methods, depending on form as well as this one.

12-27-2011, 08:54 AM
When doing this some folks drill holes...1or 2 per square foot... to let any trapped air escape and let a bit of glue squeeze out (slightly thickened epoxy). This is done on the inside layer, then filled with additional epoxy if the squeeze out isn't completely filling the hole. The usual method of fastening is to use sacrificial strips and screws that won't penetrate all the way through all the layers but do make it into the last one. Screw through the strip, through the inner layer and then into the outer layer without penetrating the outer veneer if possible. You can use pads instead of strips if you wish, but pre-drill the pads so they don't spin on you. You can pre-drill the strips too if you want. You want the strip thick enough so that the screw head seats into it, pulling everything together.

12-27-2011, 09:19 AM
It's very common to use multiple layers of sheet ply for the curviest sections up front. Often the after sections are thicker where the shape is relatively flat.

Use epoxy spread with a notched trowel to insure a uniform and adequate glue coverage.

I clamp the inner layer to the framework with drywall screws and fender washers. After the glue cures remove the screws, fill the holes with epoxy paste, let cure, sand. Same process for subsequent layers but you can use more screws as they can go in anywhere.

If the boat is designed for sheet plywood you can use thinner but still full size sheets. If not you can use diagonal strips (called cold molding).

12-27-2011, 12:22 PM
Hi Patrice,is this for the snipa planking? You could steam the plywood if you wish,if its top quality marine grade stuff it will be alright,and a damn site cheaper than smearing epoxy everywhere. I would not object to using a polyurethane glue like Balcaton for these laminations if you decide to go the two plank route. Cheers

Tom Lathrop
12-27-2011, 12:57 PM
I'm not aware of plywood available in either 8 or 15mm. You are right that 15mm would not bend well and I have not heard of anyone steaming plywood successfully that thick. The most usual method of laminating plywood is to drill holes in the plywood on the outside of the arc and screw through the whole assembly, using either wooden washers or wood strips to pull the layers tight. The reason you need to drill the outer layer is that full length threaded dry wall screws are commonly used and the hole is necessary to allow the screw to pull the plywood together. Pro builders of cold molded boats commonly use commercially available screw/washer fasteners to save labor time. In thinner layers, staples are commonly used but often require another worker to buck the inside to get sufficient force to set the staple.

David G
12-27-2011, 01:33 PM
Many boats have been built with layers of thinner plywood. Read up on Ruell Parker, for instance.

12-28-2011, 01:48 AM
My Pied Piper has exactly that construction for the forward section of the hull; two thicknesses of 4mm ply, the rest of the hull 9mm ply. This is whole sheet construction on a hard chine hull, not diagonal planking or whatever. Back in the 60s when she was built, they used resorcinol glue, these days I'd use epoxy.

patrice Hubert
12-28-2011, 04:13 AM
Thanks indeed every one. Skara, Yes it is for the Snipa. My plywood supplier disadvice me to try bending 15mm pw, too stiff for that purpose and yet i ve been told this could be done, i must admit i share his opinion for i suppose he knows his stuff.
Diagonal planking... Not for this shape of hull. The Snipa is a clinker lapstrake,each plan runing stem to stern along side the hull in a narrow widht 4 to 6 feet wide, so if using the route of double planking i need to sawn two layer of 8mm pw of that width, scarfed for a lenght of about 7 meter.
I take the idea of making tiny holes in the outer layer to allow the air to escape but i dont see how to screw the both layer (less than 16mm).
is polyuretane glue better than epoxy?

Peerie Maa
12-28-2011, 07:05 AM
If it could be built with 15mm solid wood planking without steaming you can bend 15mm ply round the same bends.
Is the shape of your snipa going to be about the same as this one, or more rounded?

patrice Hubert
12-29-2011, 02:48 AM
Yes peerie, abit like that, here a pict i took at flensburg (germany):http://a6.sphotos.ak.fbcdn.net/hphotos-ak-ash4/399498_206272892794224_100002345455769_445858_4554 81483_n.jpg

patrice Hubert
12-29-2011, 02:59 AM
The Fisher one is 570cm lenght 200 beam. her hull is built with 10 strakes clinker planks, medium width 4/6 inches.
I don't believe if using solid fir wood these planks could easyly be bent without steaming.
I thought using 4x8mm plywood would be a more easy job yet i am aware of course this route will give me twice the work...It means thant sawing the panel i have to cut 4 boards, 2 for eatch side of the hull.

12-29-2011, 11:11 PM
15mm ply is way over spec for a boat less than 18ft long. 12mm is heavy, 9mm will do fine. My Pied Piper is 9mm ply & she's 22ft. Rick (RFNK)'s ply lapstrake Folkboat is 9mm ply, & she's 24ft.
You'll bend 9mm ply to that shape dead easy. Reduce your lands to about 24mm.

The scantlings in your plan are for solid timber are they not? Ply is way stronger.