X. Finishing the Hull Bottom
Building the Core Sound 20
May 17, 2008
NOTE: This article is divided into chapters. Click here for the Table of Contents.
With most of the interior completed and the seat hatch rims well underway, we decided it was time to flip the boat. This can be one of the most enjoyable stages of the entire boat-building process. Partly because you finally get to see how well the hull came out (and how much fairing work you have yet to do), but primarily because it means you get to gather friends and family to help with the flip, then grill out afterward. (Hey, if you're going to invite your friends over to do manual labor, you better make it fun!)
Many builders flip and finish the hull bottom very early into the building process, before the interior work. We chose to do the interior work first because we wanted the hull to be thoroughly reinforced before flipping. Also, we find interior work to be much more tedious and time-consuming than bottom work because it's more difficult to work inside a confined space. We knew that if we completed the interior first, working on the hull bottom would feel delightfully easy by comparison, giving us a terrific morale boost late in the (long) building process (just when it's most needed!). It's a great feeling when you can flip your boat and know that you're more than halfway finished.
So, after the interior was nearly complete, we waited for a sunny Saturday and called up our friends to invite them to a boat-flipping grill-out in our back yard. Eight people answered the call. (Thanks to Mark, Ted, Derek, Christina, Billy, Rob, Hitchcock, and Gina!) This turned out to be plenty of help. The boat felt amazingly lightonly about 400 to 450 lbs.so the flip went very easily. In a matter of minutes, the job was finished and everyone began helping themselves to burgers, brautwurst, soda, snacks, and cold beer. Not a bad way to spend a Saturday!
With the boat upside down, we can finally see just how well our careful work at cutting and prepping the panels (back in the very early stages of building) had paid off. The hull is already impressively fair and symmetrical as-is, which is a terrific tribute to the precision of the designer's blueprints and the care with which we stitched the panels together. We couldn't be happier because this means less sanding and filling work for us! In fact, the only significant labor needed is to fill some small gaps along the seams, smooth out the edges of the scarf joints, and fill in the screw and wire holesall of which can be easily accomplished in just a day or two.
Seeing the boat upside down, it's easy to appreciate the brilliance and simplicity of the hull design. There's a sharp V-shape near the bow that allows the boat to slice cleanly, smoothly, and efficiently into the waves, but these aggressive lines flatten quickly as they sweep toward the rear, where the nearly-flat stern section ensures that the boat will plane easily. This is the secret to the Core Sound's superb speed and handling. It's a huge leap forward over the perfectly flat hull of our last sailboat, which had an annoying tendency to "slap" and "pound" noisily (and inefficiently) whenever we encountered short, choppy waves.
The next two days were occupied with sanding, fairing, and fiberglassing the hull. Although we were lucky only two days of filling, fairing, and sanding were needed (sometimes this stage can take weeks of prep work), the sanding is still exhausting and the Georgia heat and humidity make it even worse.
On the third day of bottom prep work, we fiberglassed the hull with 6 oz. cloth. We fiberglassed each section of the hull individually, overlapping the cloth at the seams to give them extra strength and abrasion resistance. We also added a few strategic reinforcement patches and some fiberglass tape to the forward portion of the hull, just for good measure. Later, we will also add a stainless steel "rub strip" running the full length of the keel to protect the wood.
After the fiberglass cured, we spent another day sanding and blending in the edges of the fiberglass patches we applied. The results came out great. Although it took a lot of sweat, we sanded so precisely that no fairing putty was needed and all of our fiberglass patch work came out seamless.
A day later, we cut out the slot for the (pre-installed) centerboard case. This is nerve-wracking work because the slot is large and any mistakes here can destroy the hull we've worked so carefully to build. First, we used a flush-trim bit in the router to cut out the hole. Since the hull is angled, we also taped a block of wood to one side of the router base to make it sit level as we routed the slot. The results were superb, and the process was virtually idiot-proof since the flush-trim bit rides on a little round bearing that follows the inside walls of the centerboard case to prevent the bit from gouging into the wood.
After cutting the slot, we used a 1/4" roundover bit to round off the edges to acceptfiberglass tape. We also used some thickened epoxy to fill in some very tiny gaps around the perimeter of the slot where the glue chipped out instead of cutting off cleanly from the router.
With the slot cut, rounded, and smoothed, we applied a layer of 6 oz. fiberglass tape over the edges of the centerboard slot. To prevent epoxy from running down inside the case as we worked, we wetted the tape beforehand in a disposable tray, then carefully squeezed out any excess epoxy resin before applying the wet tape to the hull. Again, we couldn't have asked for better results. The tape adhered perfectly to the edges of the case.
Next, we cut and attached the shallow keel to the bottom of the hull. Earlier, we planed down the centerline of the hull to create a 1" wide flat "landing" area for this purpose. Then we cut the keel from an 18' long piece of 5/4" x 3" Douglas Fir. The keel is 3" deep at the transom, but it tapers gradually as it runs forward and ultimately blends into the curve at the bow. We used screws on 16" centers to hold the keel in place until the epoxy glue cured. Since we plan to remove these screws later, we also applied epoxy fillets and fiberglass tape to both sides of the keel for reinforcement.
A day later, there was more sanding to be done (seems like it never ends) to blend the fiberglass tape along the keel and centerboard slot into the rest of the hull. We also removed the fasteners from the keel and planed it down to a more graceful profile that blends imperceptibly into the bow curve. Then we filled the screw holes in the keel with thickened epoxy and applied three thin fill coats of epoxy to the entire hull to smooth out the finish and ensure all of the wood is adequately sealed. Now our hull is really looking sharp and ready for paint.
© 2008, Wesley Kisting