Simple and Inexpensive
"Composite" Planking
by George Buehler
(excerpted from Buehler's Backyard Boatbuilding)

Recently I designed a 64-foot boat to be built in wood. When you get up to this size range, the planking in particular is pretty heavy, and it's difficult to handle alone. It's also difficult to find 2-1/2-inch thick planks, and when you can they're expensive.

By good fortune I had been talking with Bob Pickett at Flounder Bay Boat Lumber, and remembered his description of "epoxy/strip" building. The idea here is that you put on an inner, strip-planked core, then cold-mold a layer or two of thinner, wider strips over it and coat the whole works with a layer of glass cloth and epoxy. Not only is it a very strong way to build, but the epoxy coating protects the hull from bug damage, a concern in the tropics.

Since my design used a single-chine hull, I didn't see any reason to actually "cold mold" a lot of thin layers. That method is too expensive: You have to mill the wood really thin, it's laborious to deal with all the little pieces, and you use epoxy by the drumful. Yet the basic idea is sound, and it is easier to deal with little pieces than big ones. After a bit of thought, here's what I came up with: a thick core of edge-nailed, edge-glued strips mechanically fastened to the frames, topped with two layers of plywood (see Figure 7-19).

You'll need a framework to start. Many cold-molded boats don't have frames, just relying on the glue to hold the hull skin together. That ain't for me. Some cold-molded racing sailboats might have frames every 4 feet or so, which is probably good enough since those boys subject their boats to tremendous strains, apparently oblivious to the forces at work.

Well, on a large boat I'd space frames on at least 3-foot centers; on the 64-footer I specified them on 2-foot centers. These are all main frames, with no intermediates, like the framing used with a plywood hull. In other words, the hull for this composite planking system is framed up just the same as you would a hull destined to be planked with plywood.

Since it isn't a good idea to glue heavy timbers together (because of possible expansion breaking the joint), the whole keel has to be laminated up, too, as you would if cold-molding a boat. I'd use 2-by stock regardless of hull size for these laminate components; on the 64-footer I specified 2 x 12s. I'd install a 2x4 sheer batten notched into the frames, running along the sheer, just as you would for a plywood hull. Although you probably wouldn't need to indent the planks into the keel with a rabbet, I'd do it anyway. However, I'd only do the inner planks of the laminate, allowing the outer layer of plywood just to butt square to the keel.

Start strip-planking the topsides first, but unlike traditional carvel planking, this time start at the chine, using a 2x4 for the first plank. Since the chine will be overlapped rather than mitered, you can rough-saw the angle of the chine into the plank's bottom before nailing the piece up.

The rest of the planking will be 1-1/2 by 2-1/2 inch strips, made by ripping a 2x6 in half. These will be fastened to the frames and to the plank below with galvanized finish nails and glue. (Actually there's probably no need to edge-nail the planks together, but it can't hurt anything—sort of a belt-and-suspenders approach that may add measurably to your feeling of security some night hove to off Cape Horn.) If you're gluing the planking together then you'd better nail it too, so the seams are pulled together tightly. Drill pilot holes when edge nailing so you don't split the piece.

click image to enlarge

There's no need for butt blocks or caulking seams either, although I'd stop the strips on frames so I didn't see butts "standing proud" on the inside of the hull.

I wouldn't glue these strips to the frames; I'd rely on the fastenings. I don't care what the epoxy people say. If moisture does get into the wood, something's going to move. This is especially true on thicker stuff, like this ripped 2 x 6 planking.

Keep nailing on strips in this fashion until the sides are covered. As you approach the sheer, just let the strips run along the sheer stringer, and saw them off parallel to the sheer line. You may need a few wider fillers near the bow.

Now, cross plank the bottom with 2 x 6s, just like conventional planking. You could use edge-nailed strips here, too, but it may be more hassle than it's worth. Overlap the bottom pieces at the chine, and saw them off flush with the side.

Now you're ready to install the second layer. I'd use plywood, and on the 64-footer I specified two layers of 1/2-inch.

You need some sort of goop between the plywood and the planked hull. You could use epoxy, but I don't think I would.

Have you ever tried to separate two boards stuck together with ATCO roofing patch? If not, spread some on a board, lay another on it, and try to separate them a week later. That stuff is sticky as hell, dirt cheap, and the petroleum oils in it act as a wood preservative. And even if it isn't ultimately as strong as epoxy, it doesn't really matter; unlike the epoxy boys, I "praise epoxy but pass the nails." We're not relying on glue, in other words. The outer layer is mechanically fastened. I just wouldn't feel comfortable further from shore than I could swim back to in a boat without mechanical fasteners.

We've talked about plywood grades before. I think if I was adding two layers I'd use MDO single overlay, with the overlay side against the planks (in the tar), so you'll have the plywood "bare to bare" when gluing them together. If I was using three or more layers, I'd use single-sided MDO, then one or more layers of T1-11 siding, then a final layer of single sided MDO.

I'd use 1/2-inch ply only on larger boats. Two or three layers of 3/8 inch or even 1/4-inch ply would be a lot easier to deal with on most boats under 45 feet or so, although MDO doesn't come in 1/4 inch. An alternative is Lauan plywood from the Philippines, which is fairly inexpensive and fairly decent stuff.

So, just as with plywood planking, make a pattern of 1/4 by 3 inch strips (or tack up the first sheet and scribe around it). Mark on the hull where it will go, spread tar or glue on the area, and nail on the sheet with galvanized ring nails. Continue this until you've covered the whole hull.

If the hull shape has a lot of curve in the ends and the 1/2 inch ply doesn't want to bend to it, stop the sheet back where the hull is still flat enough for it, then fill in the ends with two layers of 1/4-inch ply.

Or, you could rip the 1/2 inch stuff into 1-foot-wide strips and install it diagonally.

After the whole hull is covered, glue on a second layer. I'd bet you could use roofing tar here, too, but I think I'd use glue—maybe even epoxy. If you use it, be sure to get one that kicks off slow. This might be a good place for Areolite glue, since it has a long pot life. If you've had to piece-in the ends, then have this second layer (and/or third and fourth at the end) overlap the first by a few feet.

After the whole hull is covered, round off the chine a bit with a disc sander to make it easier to wrap cloth around. Also, make a paste from epoxy and sawdust or some other filler and fill in where the plywood hits the keel to make a bit of a radius there, too. It might be easier to saw out a triangular strip that more or less fits the angle and nail it on.

As mentioned before when we were talking about plywood building, coating the hull with epoxy and cloth won't be fun. Again, since the cloth is there just to protect the plywood and not for strength, I'd think one layer of moderate weight would be enough, with maybe a second layer overlapping the chine. Just as with plywood planking, I'd staple the cloth lightly to the hull first, then put on the resin. I'd hire guys from the local mission to sand it smooth.

This type of planking could be done on any size hull of course, but the interesting thing about it is that it makes big boats feasible and affordable for backyard projects. Since the inner planks are protected by the outer layer, there's no need to use top grade planking stock. I'd use a good grade of construction fir. The prices are low!

Without figuring waste, here's how the costs broke down; The 64 footer is a very trim design and has roughly 1,130 square feet of hull area. Good fir 2x6 is available in the lumberyard near me for 29 cents a board foot, which works out to 58 cents a square foot of planking. That's $655 for the inner layer. It would take about 70 sheets of 1/2-inch plywood to cover the hull with two layers, which would cost maybe $1,000. Figuring a few hundred bucks for nails and tar, that's a pretty reasonable price for planking a 64-foot boat, less expensive than using plywood alone. Of course there'll be some waste, and the epoxy isn't cheap, but just the same, it's within the range of the "average" working guy. Plus, it's a system that a guy can do alone, without fancy tools, and it would make a bullet-proof, long-lived hull. There's no reason why it wouldn't work on most round-bottomed hulls, too, although you might need to rip the ply into 8- to 12-inch wide strips and put them on diagonally.

This same idea works on smaller boats too, of course. Smaller boats might use a layer of 3/4- by 2-inch strips, covered with two layers of 1/4- or 3/8-inch plywood. I'd use whatever combination it takes to make the completed skin the same thickness as single-layer planking would have been.

Recently I've learned about a few options that make both composite and straight plywood construction worth a closer look. A compulsive friend down the way is building a huge dome house, and he ordered all his plywood direct from a mill in southern Oregon. By buying in quantity, he was able to get a low price, but best of all, the mill ran a house construction grade (AC or AB, I forget which) for him that used a fir core. This gave him the same quality cored ply as marine or MDO, with more voids, of course, at a far lower price.

Also, I've been noticing advertisements in industrial supply catalogs for epoxies, frequently at prices as low as 1/5 of the advertised prices in the yachting magazines. A friend called one of these places and asked why it's so cheap, and was told that the epoxy works fine, but it's "crude." They said it is very temperature-critical and can't be applied when the temperature is below 66 degrees, unlike the newer formulas which can be used down to freezing. For 110 a gallon, it might pay to rent some big heaters when coating a hull.

Anyway, this composite construction seems worth a try, and maybe I'll do it some day. The only big disadvantage I can see is that the finished hull will look rather boring. Like a 'glass hull, there'll be no caulking seams to look at, and I do like the look of a planked hull. But if you decide to try this system, let me know how it works.