But first a looming question needs an answer: Butt Joint or Scarf? Big Debate # 2, concerning plywood again.
The Core Sound 17 and almost all other boats available as plywood plans are quite a bit longer than standard 8' sheets. This creates an issue for us. Somehow we need to fashion a near invisible transition between the two or more lengths of ply that is also of some integrity.
Brief synopsis of the commonly used methods for solving this: Butt jointing involves, yes, butting two square edges against each other and doing all the structural glassing and so forth on the hidden back side. With scarfing you carve two matching tapered edges, which will overlap each other with glue holding them in place.
I have never hesitated at this junction. While the initial appeal of butt jointing is the dead easy layout and even simpler execution, the general esthetics will pass no test. It is damn hard fairing the joint, and equally troublesome keeping a hairline crack from messing with the paint job. Out the door.

Scarfing is a tricky, time consuming process that takes practice to do well. But it CAN be done well, unlike butting.
Here's one way:
Cut the taper. A successful plywood scarf needs to be 8 times longer than the thickness of the material, or 2" long in 1/4" ply, which is what we will use in this example. First stack the two edges to be joined parallel, with the top one 2" back from the other. Mark another parallel line 2" back on the top sheet. Grab a razor sharp block plane and remove all material between the top line and the bottom outboard edge of the lower sheet, creating a 4" long sloping ramp. The glue lines in the plywood visually helps keeping things in line, as you work the material down. Take your time and do it well. If the result is unsatisfactory first time around, saw off the tapers and start over. Most layouts have enough waste to try at least twice.
Dry fitting. Find a dead flat spot on the floor and put down a sacrificial piece of plywood where the scarf will go. Cover that area with plastic sheeting and lay down the first piece of ply with the taper up. Fasten it with a couple of screws. With the taper facing down slide the mating piece up to the first one. Using string lines make sure the long edges of the two sheets line up straight.
Take a close look at the scarf. Nudge the two sheets towards each other until a straightedge laid across the scarf shows no gaps. The joint is now perfect if we didn't need the inconvenience of gluing. But since a healthy application of thickened epoxy takes up several microns in thickness we must slide the mating surfaces apart about a 16th of an inch. Not doing this will result in an utterly unfairable scarf, bad on any boat.
After you're happy with everything, sink a few screws in the top sheet and clearly mark its location with bridging tick marks. When absolutely sure you can reposition it exactly to this place, back out the screws and flip it over, leaving the first sheet buttoned down.
Bonding. Mix some goop and brush on a clear, unadulterated coat on both surfaces. Add wood flour to the remaining epoxy until it is the consistency of a good brand of organic peanutbutter, and smear a generous layer on the fixed sheet. Flip the other over again, find the reference marks and the original screw holes and fasten it down. More plastic next and then go look for a piece of plywood 6" wide and as long as the scarf is wide. Predrill two lines of pilot holes in this piece, down each side of the scarf, and suck it down with screws long enough to go thru the whole sandwich and into the floor, if permissible. Step back and hope the best. You will know tomorrow.
There we go. The first crucial thing to master. Read other descriptions of scarfing too, and settle on a system. But whatever you find out, don't forget to back the scarf apart as described, prior to gluing.
Two comments: With scarfing, as opposed to butt jointing, you can benefit from making up the pieces for the bottom and the sides before marking and cutting out the final shape. So prior to gluing find out the widest spot on these parts, add an inch for good measure, rip the plywood and then scarf them together into some really long, unwieldy planks. Comment # 2: Don't forget you lose 2-3" of material length per scarf!

In the first article we mention some of the tools needed. Well, there's one more item of great importance in the kit. It cannot be purchased, but you will need several of them. It is that boatbuilder's special called a batten, namely a thin, long piece of wood that will bend to fair curves for laying out and marking boat like shapes. Here's what to do: Get hold of the longest piece of clear one-by pine you can find. Promptly rip this costly board into 3/4" strips and scarf (more on this later) them together to produce 2 lenghts of flexible battens, at least 18' long. Plane one down to 1/2" square and keep the other one fat for gentle runs like keel profiles. Out of the remaining stock you can make an assortment of skinnier battens for tighter curves of various lengths. Until needed, lay them all ceremoniously to rest on a fully supported rack out of reach of young boatbuilders.

Okay, back to boatbuilding. With the rough side and bottom panels all scarfed together, draw the final shape of these various parts on the plywood itself. A 20' long table will come in handy here.
The plans contain all the measurements needed. Transferring these onto the wood gives you a number of pin points, which you will then fair into a smooth curve using those long battens just described . Tack a small finish nail in every point and bend the batten around. Now sight the resulting curve from all possible angles, while looking for flat spots and bumps. Some of the nails will hold the batten from running fair. Try removing those. Others will not even touch the batten. Ignore them.
Again you will benefit from a calm attitude and lots of time with this important step. The ultimate shape of the boat relies on your judgment. What you're doing here, besides working on a boat, is developing an eye for fair curves and an ability to know when things run smooth together into something just right. Don't underestimate these skills. Your boat depend on them.

When cutting long curves in plywood my tool of choice is a skilsaw, AKA circular saw. Set the saw's blade depth a hair deeper than the thickness of the plywood, and you can follow even the tightest curve on this boat much smoother than with a wandering jigsaw. The general woodworking ethos of sawing to the outside of the mark applies here as well. ALWAYS leave the line and things will inevitably end well. Plane the cut down to the line with the block plane. Sight the curves again and fuss over them till satisfied. Then fuss some more.

Now things get interesting. The hull will rather quickly take shape during the next couple of steps, which is the charm of stitch and glue. To quickly summarize this method: The prescarfed, precut panels you just made so carefully, will now be stacked inside face against inside face and crudely laced together with wire. Then you will open up the whole package like a book, with much creaking and groaning from plywood and builder alike until, almost magically, the shapely hull of the boat sits on the floor.
The CS 17 need a little fiberglass support in addition to the ubiquitous wire ties. In the plans you will find out how to arrange all the panels on the floor prior to applying this glass tape on the bows. Oh wait..the plans are somewhat vague regarding this. In fact the precise angle of the bottoms in relation to the sides is your call, within several degrees of variation. Rather scary, since the consequences of a misalignment are impossible to foresee!
Also, some builders, myself included, have looked at those short strips of temporary glass holding the front of the assemblies together, and questioned their integrity. Tremendous force is concentrated here when tweaking the hull panels into 3D. A cheap insurance is to bridge the aft end of the glass joint with a small cleat of 9mm plywood. Make up two pieces for each side, about 2"x4" and bolt them together across the chine, one piece inside the boat, the other outside. Leave them on for a week or two after unfolding.
Stitching is next. A low stress job. To ensure even tension on all those wires, which will translate into a fair joint, twist the wire tight on top of a short piece of 3/8" dowel temporarily laid against the end grain of the plywood.
Now enlist at least one willing helper, preferably a fellow boathead. Study the plans together. Have rope, clamps, screwgun and a full day at hand. Drink a cup of calming herb tea, and commence to unfold the boat. Since this is not a low stress job the biggest challenge may be to remain friends with your assistant.
Set the stack upright from its position on the floor. The long side panels, daintily attached to the bottom by that patch of glass, quickly becomes a major liability as they arch out in space like enormous moth wings. Rig rope supports or get more manpower. Securing the bow vertically to something solid may also be an advantage. With the wings under control, keep opening up the bottom panels until the temporary middle mold can be screwed in, then fit the forward bulkhead with wire ties.
Now bend the wings around these two stations, wiring them to the bottom as you go. To keep things relatively balanced, do a little on one chine, then switch side, etc. Finish off by wiring in the transom.
There it is. A big tub of a boat. After getting the heartrate down a bit, it's time to start fussing over details, once again. Determine the working height that will suit you for the remainder of the project, and build cradle supports based on the three bulkhead/mold patterns. Stick the hull on these. Go over the chines and keel joint making sure they meet in a fair curve. Loosen and tighten wires accordingly. Whack recalcitrant spots in place with a rubber mallet. Sight the joints from all angles. You know the routine by now.
Then level the boat athwartship using the best spirit levels you can borrow. If they are too short, as they surely will be, bridge the sheer with dead straight lumber. To make any sense these beams have to be absolutely perpendicular to the fore-aft centerline, which you will achieve by horning. Ehh, what? Remember the isosceles triangle? Or, simpler said, measure back from the bow to a point on the sheer, repeat on the other side the same distance, and you have a line across the boat that is square to the CL.
Like a poorly directed movie, where all the drama is played out in the first 15 minutes, anything further done on this boat is anticlimactic. Sorry. Tedium will rule now. But it is still boatbuilding.

A major step in completing a stitch and glue job is turning all those rickety seams with their wires into something more solid. This is accomplished with a combination of modified epoxy fillets and fiberglass tape, as mentioned in the first installment.
First tackle the inside. Actually first decide what to do with the wires. One school of thought dictate removing them after building up the fillet, but before it turns rock hard. Timing is essential here. Another says to tack weld the seam with globs of epoxy on the outside, snip the wires and put in the structural fillet. Then there are the folks who leave the wires in, because they use copper instead of steel. Your choice. I use a combination of them all, but lean towards the tack weld one.
Okay, back to the fillets. The idea is to build up a gentle radius of thickened goop, filling the entire length of those inside seams. This adds strength to a critical area, in addition to accommodating an overlay of fiberglass tape, which otherwise pull away from tight corners.
Wet out the surrounding wood. Mix a dose of epoxy and colloidal silica, AKA cab-o-sil, to the consistency of nut butter and smear it liberally into the seam. Grab a premade plywood stick with one end rounded smoothly (a one inch radius will do all the filleting on this boat), and run this paddle firmly along the joint. With enough epoxy in place the result is a nicely rounded fillet. Notice how the radius changes when you angle the paddle. Use this leeway to accommodate different areas on the boat. Make little chisels out of tongue depressors, and scoop up the excess material pushed off to the sides, and deposit it further along. The neater you are during filleting, the better the glass will lay. When you get good, no sanding is needed between these two steps, as long as the glassing gets done roughly within 24 hours.

Glassing. The Core Sound 17 needs 2 staggered layers of 3" wide tape running inside and outside of all chines, keel joints and bulk head connections. The end result will be a very strong, rigid structure. First the inside. With the fillets in place, get out the tape and decide on one of two different approaches. One, lay the tape dry along the joint, neat and straight, and come back with a brush and epoxy to wet it all out. Or two, on the workbench saturate the strip, roll it up and then unroll it where it needs to go. I'm with method one, so that's what we'll work with here.
When laying down the strips notice how one of the selvage edges has a thick roll of fibers along it, while the other is flat. Since we're doing two overlapping layers, it is best if this thick edge doesn't end up under the top layers, creating a hard to fill pocket of air. So think of this when positioning the first strip. Smoothen out all wrinkles. At sharp bends it may be impossible to achieve this, so snip the tape with scissors perpendicular to the run.
The glass takes a little time to wet out, and will shift under aggressive brushing with the relatively viscous epoxy. To get started I first place fat globs of epoxy at random intervals and let this soak through. With the strip now smothered to the fillet, it is a simple, though time consuming job to wet the rest out. Still watch out for creep and shifting, which will trap air, the bane of fiberglassing. Don't starve the glass; but conversely avoid floating the tape on too much epoxy. Work over the job again and again, with a wet brush. Some areas are slow to saturate, and others pull away from the substrate as you turn your back. Sometimes, when the epoxy hardens a little you can get adhesion is those trouble spots.
So, if you got more in you, return to the starting point while things are still a bit sticky and lay down strip # 2, staggered half an inch or so. Repeat the whole wetting out scenario until satisfied, which indeed makes for a full day.

Take a break from the chemical lab and install some structural members of real wood: Inwales and keel batten. These tasks pose only a few challenges, so now is a fine time for a quick discussion of suitable wood species for boatbuilding. This is a major subject, and distilling it to fit our limited space here is hard, but I'll try.
Regional choices hardly matter in this world of petroleum fired shipping, but I'm in the Northwest, Coastal Douglas Fir grow here, and that's what I use. It's a wonderful all round wood, when lumber of reasonable quality is obtained. Decay resistant, strong, pretty and smells good. Splinters a bit along the edges, but nothing is perfect.
I find all my Doug Fir at the local lumber yard, sold as construction lumber at construction prices. Only a few boards in those immense stacks of 2X material are worth getting, so the trick is to be around when they make it to the surface. Befriending the yardworkers is a good way to start a lumber collection. Whenever I need something there, I always swing by the yard and scan the lumber piles. You learn quickly what to look for: Tight ring count on the endgrain, tan or beige color instead of pink, small sound knots or, lo and behold, clear.
However broad spectred and utilitarian Doug Fir might be, occasionally a job calls for something stronger than a mere softwood. White oak was a tremendous choice until finicky boatbuilders discovered compatibility issues with epoxy. Want to glue it? Look for something else. Honduras Mahogany? Hardly any different in strength and density from Doug Fir, so why pay a premium for another acre of tropical deforestation? No, for those odd jobs I keep a few boards of ash around. It's an affordable, very strong, easily worked species, unfortunately encumbered with limited decay resistance. Treat this downfall with epoxy encapsulation.
That's about it for my shop, but the list is off course long much longer: The cedars, southern yellow pine, spruce, cypress, etc.

Where were we? Oh yes, the inwales. Mine would most definitely not bend willingly in the bow, with the pieces cut to the specified scantlings. A neat gunwale/inwale job calls for some end tapering anyway, so back at the bench I planed the forward 5 feet gradually down to 1 1/8" x 1/2". Some resistance was still encountered, but we got them in.
The keel batten AKA keelson has a changing bevel on the underside to fit in the bottom vee of the boat. If you don't get it right a little epoxy will save the day. More important is a straight run fore-aft with the keelson centerline directly above the bottom CL. I installed a super tight stringline and dryfitted the keelson under it. Shores to the ceiling will help hold everything in place as you square the line down. When satisfied I made lots of tick marks and glued the thing in precisely where it had been, using the same shores again.

A mighty good looking boat now sits on the shop floor waiting for the next installment, where we will finish things off.