Construction:
02/27/2008: I've never built a boat before, so I think I'll start with something really simple -- like building the strongback that the rest of the boat will be built upon. If you don't know what a strongback is, it's essentially a workbench to which you attach all of the boat's frames while you build the hull. Since, if the boat is to have reasonable performance, the hull must be straight and free of any unwanted twists (else they wouldn't be unwanted, of course), it is important that the strongback be as straight as possible.
I built the strongback from 3/4" MDF, with pine 2x3's as a frame to stiffen the MDF:
I laid the 4x8 MDF on a quartet of saw horses, snapped a chalkline down the center of the MDF to separate the 4x8 sheet into two 2x8 sheets, then used a circular saw to cut along the chalk line. Then I glued the 2x3s along the outside edges of the MDF...:
... and clamped the 2x3's to the MDF until the glue dried:
At this point, I ran into the bane of woodworkers everywhere: I need more clamps. Fortunately, Home Depot was having a special :)
03/03/2008: Today, I started cutting the transom and frames from a sheet of 3/4 inch exterior grade plywood that I had left over from an earlier project:
The completed transom and forward seat frame:
The next step is to build the sides of the boat. First, I took the two 4x8 sheets of 4mm plywood, measured the centerline, and snapped a chalkline down the center. Then, I used a circular saw to cut the two 4x8 sheets of plywood into four 2x8 sheets of plywood. I wasn't particularly fussy with getting the cut perfectly straight -- stich-and-glue construction is rather tolerant of builder sloppiness, since the epoxy and cabosil putty will easily fill gaps between the pieces of wood to be joined, and the fiberglass tape used to reinforce the joints is stronger than the plywood itself. Here is a photo of the plywood after snapping the chalkline and before cutting it into 2x8 sections:
Now that you have four 2x8 sheets of plywood, it is time to join the 2x8 sheets at the end to make two 2x16 sheets. There are two ways to make the joints: you can either butt the joints and reinforce with fiberglass tape, or you can scarf the joints. Scarf joints are stronger, but butt joints are easier to make. I've heard of boat builders successfully using both techniques, but I just feel a little better about the scarf joint so that is what I plan to use. If I find I don't have the skill to make a good scarf joint, I'll regroup and make butt joints instead, since I should have more than enough plywood :)
Anyway, to make a scarf joint, you first have to shape the ends of the plywood sheet into very shallow wedges. Then you turn one of the mating sheets of plywood over so that it's wedge mates with the wedge on the second sheet of plywood. Cover both surfaces of the joint with epoxy and cabosil, join them together and clamp until the epoxy dries.
The trick, of course, is getting the wedges to match each other. The simple way to do this is to stack your plywood together and cut the wedges at the same time. Here's how you do it:
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On each sheet of plywood, mark a straight line that is between 2 and 3 inches back from the edge of the plywood. Two inches will give a slope of 8:1 for 1/4" plywood; three inches will give a slope of 12:1 for 1/4" plywood. I will be using a slope of 8:1 (two inches).
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Now, place the bottom sheet of plywood even with the edge of your work table and stack your plywood sheets one at a time so that the edge of the top sheet lines up with the line you have drawn on the sheet under it, like so:
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Clamp all of the sheets to the work table so that they won't move while cutting the scarf:
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And use a power sander or block plane to cut the wood so that it resembles a smooth ramp rather than a set of stairs:
Notice the waviness in the scarfs -- you want to avoid this as much as possible. Assuming that the laminations in the plywood are of equal thickness throughout the sheet, a smooth scarf will show as a straight line. I tried to straighten out my scarfs as much as I could, but working with 4mm wood, I found that I was starting to scallop the edges of the plywood, so I am hoping that epoxy and cabosil will fill the imperfections. I'll let you know how it works :/
Now, mix up a batch of epoxy and wet out the fiberglass cloth...
(Note the white areas in the cloth -- this is where the fiberglass is not yet completely wet out. Make sure there are no more white areas in the area that will be laminated to the plywood.)
...and spread a little epoxy on the *outside* of the plywood sheets where the fiberglass will cover the plywood (to keep the plywood from leaching all of the epoxy out of the fiberglass cloth), and smear a little more epoxy on the scarfs.
Set aside about half of your remaining epoxy, then mix some cabosil into the other half of the epoxy remaining in your cup until it makes a paste -- not quite as thick as a putty, but thick enough not to run out of the scarf joint. Spread the epoxy paste over the joint, and join the two sheets of plywood together.
Now cover the joint with a patch of fiberglass tape, and wet out the tape with the pure epoxy that you set aside earlier.
Notice the white line at the seam in the picture where the plywood sheets are joined together? This is another resin-dry area. Unfortunately, my plywood sheets were a little warped, and wouldn't lay flat on the table. This resin-dry area is where the plywood panels are pulling away from each other. To avoid this, clamp the panels together after wetting out the epoxy patch over the plywood.
Now, while you can still make adjustments, take the time to double-check your work. Make sure your plywood sheets are lined up straight and true, make sure your fiberglass patch is properly wet out (no resin dry areas, no pools of excess epoxy), etc. If something isn't quite right, make sure you fix it now. If you wait until the epoxy starts to set, you will have to cut a new scarf joint to fix the problem.
