: Most S&G kayaks (unless they have a flat bottom) longer than 16ft. will have
: two panels meeting at the keel. Since the preferred joining method is to
: scarf panels together, how do you reconcile this long butted joint?

Nothing really to reconcile here. There are several different ways of joining wood and this one works at this location, while something else is needed elsewhere.

I think you may be confused about the reasoning behind the use of the scarf joint. And, I think you may not understand the strength underlying the joint along the keel--which is the same as any other joint along the chine lines of a kayak.

First let's look at the joint along the seams. Historically, two pieces of wood forming a corner were reinforced with an interior piece of wood, unless they were thick enough to be able to provide sufficient surface area for a glue joint, big enough for the reinforcing of a mortise/tenon joint (or a dowel, screw or nail) or they could be made into an interlocking joint like a dovetail joint.

With panels of thin wood it is difficult to insert dowels. Also it is hard to insert mechanical fasteners such as screws or nails in such as way as to join just the panels. So, a backing strip of thicker wood (in many cases this is called a batten or a chine) is fitted to the joint, and the two panels are secured to this thicker, stronger, batten or chine. In conventional boatbuilding where chines are inserted at all seams, not only do they serve as places which will hold nailes and screws, but they also serve as seals against water coming in through the joint. this is frequently accomplished by filling any gaps between the chine and the attached paenels with a "gasket" of cloth soaked in varnish or paint or resin, or with a thick tar, or with some type of caulking.

Let's shift gears for a few minutes. I want you to think about a fiberglass fishing pole. I assume you've handled one at some time--or at least seen one. Typically the tip on a fishing pole is about 1/8th inch in diameter. That increases towards the base, where the reel is mounted. The thinner tip is very flexible, yet strong enough to support the weight of a good sized fish. The base is sometimes 1/2 inch in diameter (or more) and much stronger. The fiberglass is considerably stronger than a piece of wood of the same size, and tapered in the same way would be.

Now imagine that the seam between your two panels of plywood is not backed with a piece of wood, but with a piece of fiberglass of the same dimensions as the wood might be. If, from years of experience with building boats, we know how stronger we need the backing piece to be, then replacing it,size for size with a fiberglass piece would give us a stronger backing. WE can go the other way on this, though. Instead of replacing the wood with a similarly sized fiberglass part, why not replace it with a fiberglass part which is the same or slightly greater in strength. Such a part could be made smaller than the original wood piece. And also suppose that we could somehow replace the gasketing materials which make that joint waterproof.

With stitch and glue construction that is essentially what we do. We draw the plywood panels together with wires, then when they are in the correct position we use those panels as a mold. Then, in this mold (really jsut the corner seam area) we cast a continous strip of fiberglass-reinforced resin. This "V" shaped strip, which runs along the entire length of the joint, we call a fillet. The thickness of the "V" is further increased by covering it with layers of glass cloth and resin. If you could make this joint, then dissolve the wood around it, you'd find it to be thicker than a fishing pole, and because of the "V" shape, more rigid than a fishing pole. The resin which forms the fillet, and which binds to the glass tape or cloth which goes over the fillet, seeps into the wood and seals the entire length of the joint, too. No gasket or caulking needed.

On a boat with wood chines the plywood panels are secured to the chines with glue, small screws, nails, or staples. The glue does almost all the holding. The mechanical fastenings simply serve to hold the parts in tight contact while the glue hardens. Epoxy resin makes a very strong glue joint, and it does not need to be tightly clamped to do so--so mechanical fasteners other than the wires holding the panels together, are not needed. Soem ball park figures: A small nail or brad holding the plywood panel to a wood chine can be removed with about 15 pounds of pulling effort. A staple or larger nail might take 50 pounds of effort. A small screw would hold 100 to 200 pounds of direct pulling force. You might put screws about every 6 to 8 inches apart. But epoxy resin has a bonding force of several thousand pounds per square inch. Lets be very conservative and say it is only 500 pounds per square inch. Over a 6 inch span of the fillet in each seam, assuming the fillet is 3/8 inch wide, you have 2 square inches of contact area, or a holding power of 1000 pounds. That is about 5 times the holding power of a screw, and 40 times the holding power of a brad. If this is covered with a 2 inch-wide tape (I like to use 3 inch tapes, but that's just me) then each panel has 1 inch of tape bound to it. Over the same 6 inch length you have a holding power of 3000 pounds.
Heck, each inch would support 2 2/12 times your entire body weight.

Mind you I am being fairly accurate on the holding power of brads and scress. Take a hammer or pliers and pull one out yourself to see if it takes much more effort than I've described. At the same time I've been very conservative on the strength of the epoxy. Actual practice suggests their strength is at least a dozen times higher than the numbers I'm using here.

Oh, lest I forget, the glue bond on a wood panel glued to a wood chine is based on the suface area of the parts being joined. You could use a waterproof wood glue, or epoxy. With a chine 1/2 inch or 3/4 inch wide, you would have a slightly larger gluing area than with a fillet alone (but it would be smaller than a fillet covered with glass tape or glass fabric). Even so, it would not be as strong. It is well known that the glues are stronger than the underlying wood. With the strongest glues the joint would fail when enough stress caused the wood to come apart. The fiberglass and resin fillet is the same material as the "glue" and has no such built in weakness.

Now for the scarph joint (or scarf if you prefer that spelling). We use it on joining panels of plywood for just reasons I can see: Appearance, and smooth bending of the panels. Strength is not an issue at all, but since this is also a strong joint then we are relieved of the need to do anything further to reinforce it. That is a convenience in building, but not a necessity.

The alternatives to a scarf joint would be the join the panels with the square edge of one panel butting against the square edge of the adjacent panel. If this is just secured with glue we have a very thin glue line--siomething jsut the thickness of the panels. We would get more bonding area if we used a a "butt block" reinforcing the joint. This is typically a piece of scrap plywood of the same thickness as the panels, which is placed on the "bad" side of the plywood and glued so it evenly overlaps some area on each side of the joint line. In the area where the butt block is located, your panels are twice as thick as they are elsewhere. In this area they will be less flexible than anywhere else. If you try to bend these panels in a long smooth curve, you'll get a flatter spot in the area where the butt block is located. The thicker the material of the butt block, the less the panels will bend in that area.

If, instead of using a block of wood to reinforce a butt joint we use a layer of fiberglass cloth on one or both sides of the joint we can get slightly better flexibility, but that area is still going to be a bit stiffer, and not bend as evenly. If the butt joint is cut in the form of interlocking jigsaw-puzzle pieces we still have the flexibility issue, but the length of the joint area over which the panels are flexing is longer, so the curves are a little smoother.

So, we come to a long tapered joint to replace a simple butt joint. the idea here is taken from the scarf joints used with solid wood to create long pieces. The longer the taper, the greater the glue area and the stronger the joint. With epoxy resin you could probably make a scarf joint with a 45 degree angle on each piece of wood. With 4mm plywood that would give a joint about 6mm long, or about 1.5 times the thickness of the materials. historically, a scarf joint is more on the order of 10 to 12 times the thickness of the materials. With modern glues we can shorten this to as little as 6 times the thickness. Anything in between is probably just fine. For convenience many builders make their scarf joints 2 inches wide. With 1/4 inch plywood this is an 8:1 joint. With 4 mm plywood this is roughly 12:1

Remember that plywood is made up of layers of wood veneer. each layer is composed of several smaller pieces of veneer which are edge joined (butted together). A sandwich is formed with layers of veneer arranged with the grain at opposing angles. Butt joints in the veneer in interior layers are surrounded, and reinforced, but the layers of veneer above and below them. Butt joints in th surface veneers are carefully made so there is a fine surface, and these veneers are glued down to, and supported by, the rest of the plywood. If you were to cut a piece of plywood in a stair step manner, slicing along the thicknesses of the veneers, you could joint that piece to one with a matching cut and have a very strong joint.

But why be so extreme? A simple half lap joint will also do well for joining plywood panels. Make each lap with a face 1 to 2 inches wide and you'll have a very strong, flexible joint, with just a thin seam line showing on each face. When any of these seams are later covered with fiberglass cloth and resin, the glass layer becomes a seamless top layer to the stacks of veneer in the plywood. It reinforces the entire stack.

: . . . Seems
: most people just tape it, at least on the inside seam. I'm just a little
: concerned about bursting apart at the seams, since on a good day I'm about
: 6ft., 190lbs.

So, on a bad day are you shorter, or are you taller? Either way, I wouldn't spend much time worrying about bursting apart. Few people actually do burst apart, even after consuming very large meals.

Sorry, couldn't resist two such wonderful sttraight lines in a row. Thank you for them.

Seriously, though. I see. You are worried about the boat bursting apart under your weight. Well, reassure yourself. Once it is afloat, there is about as much watter pressure on the outside, trying to press up on the bottom of this boat, as there is pressure on the inside, from your weight, trying to press down on that same bottom panel. On dry land you would put a lot more stress on the bottom of that boat, as there would be no water on the outside.

The deck around the cockpit needs to be much stronger--and indeed it is reinforced better than the bottom of the boat.

: I'll search the archives tonight after work, since most
: topics have probably been discussed to death anyway...

Nah, we like to rehash things. Just like old soldiers fighting the last battles, we keep coming up with ways to win which weren't thought of before.

Hope this helps.

PGJ