VIII. Centerboard Case and Seat Tops
Building the Core Sound 20
April 25, 2008
NOTE: This article is divided into chapters. Click here for the Table of Contents.
With the inside of the seats sanded and sealed in epoxy, the next step is to install the centerboard case and the seat tops.
The centerboard case is made from 1/4" plywood reinforced with 3/8" plywood doublers at the pivot pin and some hefty fir stringers which run along the top edge and get tied into the forward seating.
Like the centerboard itself, building the case is fairly labor-intensive. After cutting the pieces roughly to shape, we spent a lot of time carefully shaping the bottom edges of the case sides to match the hull contour. Then we used strings, levels, and tape measures to determine the appropriate height so that the top of the case would be perfectly even with the tops of the seats. Patience and careful measuring are crucial at this stage.
When we were satisfied with our measurements, we glassed the inside faces of the case sides and then glued the pieces together, leaving the top edge a bit long so that we could re-measure later and cut the top off at the desired height on the table saw. Then we epoxied the heavy-duty stringers along the top edge. This procedure worked superbly. The finished case came out level, plumb, and perfectly even with the seat tops.
Before installing the case in the boat, we used a drill press to bore the holes for the pivot pin. First, we bored them oversized with a 7/8" forstner bit, then filled the holes with thickened epoxy and re-drilled them to 5/8" so that the hole would be surrounded by an epoxy bushing that will prevent the pin from chafing or cracking the surrounding plywood.
Next, we installed the centerboard case in the boat. To ensure the sides of the case remained perfectly parallel while bonding, we cut a full-length spacer block, coated it in packing tape (to prevent epoxy adhesion), and stuffed it inside the case. Without a spacer block, there is a danger that the sides of the case may bow in or out, resulting in the wrong amount of clearance for the centerboard.
After measuring and triple-checking the alignment of the case with a box level, we clamped the case into place and applied partial fillets to tack it to the hull. Four hours later, after the partial fillets had time to stiffen, we applied full-size fillets to bond the case to the hull. We also covered the fillets with two layers of 6 oz. fiberglass tape to smooth and strengthen the joint. Once cured, a final examination with the tape measure confirmed that the case was aligned correctly with the hull. What a relief! It's very satisfying to have such an important and labor-intensive part of the boat come out perfectly level, plumb, and even with the seat tops. All our measuring and patience really paid off!
The next day, we installed the cross-bracing that supports the case at the forward end and provides a landing for the forward seats, which span across the entire cockpit for the first 34" aft of the forward bulkhead. Now all the interior framing is complete except for the seat access hatches and the stringers that will support the deck!
For the seat tops, we began by cutting out a cardboard template which we used to transfer accurate lines to plywood panels. Here, it is important to have the plywood grain run across the seat (not lengthwise) to obtain adequate stiffness. Since our plywood panels are only 4 feet wide with the desired grain orientation, we had to scarf four pieces of plywood together to obtain the necessary 13 feet of seat length.
As with the hull panels, we scarfed the plywood together before cutting out the seat pieces. When it came time to cut, we deliberately cut the seats wider than necessary and used a block plane to make precise, minor adjustments to the fit until everything settled nicely into place. The process is somewhat tedious, but if you're patient, the results are superb. Our seat panels came out terrific thanks to the care we exercised while making our cardboard template, and while scarfing the panels together.
We decided not to glue in the seat tops until after the centerboard case was installed (see above). The order of assembly probably makes little difference, but we wanted to clamp support bars to the seat faces in order to hold the centerboard case level and plumb while the epoxy fillets cured. This turned out to be a wise decision that made installing the centerboard case quick, easy, and accurate.
We also added an extra support stringer in the fore and the aft quarter of each seat, where passengers are most likely to step when coming aboard or going forward to handle the foresail. The stock design is plenty strong to support our own body weights, but the 1/4" seat tops feel like they would be a bit too flimsy if any 200+ lbs. passengers happened to come aboard. So, just to be safe, we opted to reinforce the high-traffic areas.
Meanwhile, back in our garage, we coated the underside of the seat tops with three coats of epoxy to seal the wood and protect it from moisture. When these seal coats were cured, we installed the seats by laying a thick bead of glue along the stringers and pressing the seat tops into place. Since the seats are flat and horizontal, clamps aren't needed at this stage. Instead, we used bricks, books, water-filled jugs, and other heavy objects to exert downward pressure on the seats until the epoxy cured.
Six hours later, we trimmed down the oversized edges of the seat tops with a flush-cut router bit. Then we went back over the edges with a 3/8" roundover bit to give the seats a more finished look and to make the edges of the seats more comfortable and more chip-resistant. Although not structurally required, we ran a fillet along the seat-to-hull joint to give the seats a more "finished" look, then covered the seat tops in a layer of 6 oz. fiberglass cloth to protect the soft Okoume from nicks, dents, and scratches.
Now the grunt work really begins: We spent the next several days sanding, sanding, and more sanding. Then some filling. Then some more sanding. Then a little more. Then, at last, we were done. No, wait a minute, there's still a little more sanding to do. Oh wait, there's some more yet. (The sanding saga goes on and on, believe me.)
At long last, the sanding results finally lived up to our high expectations. The interior was looking smooth and ready to be epoxy-sealed. In order to prevent drips, runs, and sags (which would create more sanding work), we used foam brushes to apply extremely thin coats of epoxy. In fact, it was more like "dry-brushing" as we dragged the brushes along to seal the surface of the entire cockpit. If you follow our example, be sure to have lots of foam brushes on hand. They deteriorate quickly, and we used about 20 brushes per coat to seal the interior. Three coats of epoxy later, we had a smooth, shiny, gorgeous interior that should yield a beautiful painted finish.
Obviously, we still have to install and fiberglass the front panel that bridges the two seats (forward of the centerboard case), but first we want to paint the space beneath it while it's still easily accessible. Otherwise, the interior is entirely finished except for the hatches that provide acccess to the storage beneath the seats.
© 2008, Wesley Kisting