Plywood Carvel Question

I know this is a stupid question because I haven't seen it anywhere. I want to build a boat designed for Carvel planking (Atkins Sallie Hyde). It will live on a trailer. I want to sheath the boat with epoxy/glass on the outside, epoxy on the inside. I understand standard solid Carvel planks will eventually move too much. Why can't I use plywood planks with bead and cove edges epoxied together?

 

Where did you get the information that planks will move too much? If you epoxy and glass both sides, there won't be enough moisture change to affect the dimensions of the wood.

 

strip planking is probably the best method for what you want.

 

There's no such thing as a stupid question. What's stupid is ignoring a thoughtful answer when it's not what you want to hear.

Good Luck,

MIA

 

Yes, you can use plywood in carvel planking, but you can't carvel plank the plywood, it will have to be glued. This is because the solid planks are edge set with caulking, this along with moisture gain keep it tight. Plywood being much more dimensionally stable will not swell enough to remain water tight, so you'll have to glue it. Also a plywood version of a carvel, will weigh a little more and have about 1/3 less longitudinal stiffness, then a solid planked carvel.

Sallie Hyde has a lot of shape that isn't developed, but large portions of her are, so you could use full sheets in many places. In other areas, like the bow and the stern, you can rip the plywood into 8' long pieces, maybe 2' wide and fit the planks to the radius changes in these areas.

The logical method for this boat would be a double diagonal plywood build, with planking ripped length wise and laid down, at about 30 degrees to perpendicular, of course at opposing directions for each layer. It's easier to bend the thinner material and you'll cover the seams of the preceding layer, so no leaks. Naturally, this would be sheathed with cloth or Xynole to offer additional water proofing and abrasion resistance.

 

Thanks PAR. I think I'll try the two layer approach.

 

You'll need to revise the planking scantlings. What did Billy call for in planking widths and thickness, also the species? 3/4" cedar?

 

I don't know yet. I'm still waiting on the plans.

 

why not strip plank in cedar. should be able to almost match original weight. I do know someone who has built dozens of atkins hulls and converted them to sheet ply. they all float on there marks right. he uses 1/2 inch ply and rakes the stem out to make it easy to sheet.

 

Strip planking might be worth the extra effort. It would make for a much prettier boat.

 

Strip planking a hard chine hull isn't a wise decision.

 

Triple layered strip planking with high quality marine grade plywood produces very stiff and robust hulls but the quality of the staples or pins is critical to longevity of the hull. More than one owner builder reduced cost by using zinc plated pins that corroded under the sheath. Best is use high grade s/steel or bronze pins.

Turks are building timber carvel plank hulls to 50m loa using 60mm solid single plank or double layered diagonal 30mm plank. The single layer has edges machined to a radial tenon/socket for epoxy filled jointing. The joints are sound but I don't like the planking style, they don't taper the planks out to the tail they run parallel width full length then fit fillets into the gaps. Hulls are glass sheathed to prevent moisture causing movement but their boats also come out every winter for 4months so how well they would last if wet all year is hard to say. Timber used is mostly African mahogany, Black Sea Chestnut or high resin Black pine.

 

Triple planking anything shouldn't rely on fasteners, but adhesive. Plastic staples or brads can solve corrosion issues. Unless molding, triple planking isn't necessary, particularity on a 6.7 meter boat. Without actual scantlings, schedule speculation is silly.

 

Unfortunately staples or pins are required to secure strip planks in place while the epoxy cures, same as when hulls were built with aircraft/marine resinol glues

 

There are other methods, though brads and staples are common and generally assumed, they are typically temporary and often removed. There's nothing wrong with resorcinol as an adhesive. It's worked well for generations.