Building a new composite coaming
May 09, 2010
I'm back in the boat shop. After nearly 4 years since her launch, my cedar strip baidarka is languishing on the kayak rack with a leaky front hatch. With repeated exposure to moisture, the hatch, which used to fit with an airtight seal, warped just enough at the edges to allow some water through the rubber gasket.
In addition to the leaky hatch problem, she has a tendency to leecock which can be dangerous in a strong wind. I don't think this is a problem with the original Shooting Star design, but is probably specific to this particular build. I think it demonstrates how sensitive to trim the Shooting Star design is. While touring I could fix the trim by packing more gear in the front. Moving the seat forward would fix it permanently. Unfortunately, I built the cockpit "ocean size" so there was very little room to move the seat forward. In addition, the front bulkhead served as my footrest, so moving the seat forward requires moving the front bulkhead forward as well. This meant that I had a lot of modifications to get this kayak seaworthy again: rebuilding the hatch, the cockpit, and the forward bulkhead.
I'll start by describing how I expanded the cockpit opening from a small “ocean” cockpit to a “keyhole” opening. The old coaming was incredibly tough. I used a combination of hand saws, a saber saw, a Dremel tool, and a router to cut it out. In retrospect, the best tool for the job would have been the Dremel tool with the diamond wheel, which I used to cut the new coaming lip to size.
After cutting out the old coaming I drew the shape of the new, larger cockpit opening on the deck, cut it out with a saber saw, then cleaned up the edges with a rasp and sandpaper.
Next I fashioned a mould for the new coaming lip with a 3/4 in layer of minicell foam. I've also seen insulating foam used which is easier to shape. I chose minicell only because I already had plenty of it lying around. I shaped the foam with coarse sandpaper and fixed it temporarily to the deck around the opening with hot glue. Then I covered the foam with duct tape to keep the epoxy from sticking to it, being careful to keep the inside edge of the deck exposed though so that the coaming rim would stick to the deck. I stretched the duct tape over the foam to keep it as smooth as possible. Every wrinkle in the tape is one that will need to be sanded out later, and any unfairness in the mold can only be fixed by adding more layers of glass and carbon fiber.
After protecting the deck surrounding the coaming with masking tape and plastic, I placed several layers of carbon fiber over the mold. I wet it out by laying it flat on a plastic sheet and spreading epoxy over it with a spreader. Then I shaped it around the mould and around the inside of the opening and underneath the deck. The fiber is cut on the bias (i.e, cut so that the fibers run diagonally at a 45 degree angle to the edges). This makes it easier to shape the cloth around curves. Sometimes the cloth fibers would sag and drop off the underside of the deck. As it cured it left a lot of sharp edges that needed to be ground down later. I don’t remember how many layers I put on-- probably 4 to 5.
After the epoxy cured I removed the mold. This part was difficut. I had to dig underneath the foam with a knife to separate it from the hot glue that held it to the deck. I recommend using hot glue sparingly. I dug into the side of the mold and grabbed hold of the duct tape with a pair of pliers. Once I got a good hold of the duct tape, I was able to pull the foam out in large segments. A lot of hot glue residue was left on the deck. Paint thinner seemed to work to remove it.
Now I had a coaming lip with rough, sharp edges. I ran a white crayon about 1 1/4 inch from the inside edge and cut the lip to size along the line with a Dremel tool diamond wheel. It cut like "a hot knife through butter"! To clean up the rough and irregular outside edge of the coaming rim I ran a fillet of epoxy thickened with sanding dust along the outside and covered it up with a layer of fiberglass.
After the epoxy cured all I needed to do was sand the whole thing down. Now it is ready for varnish. The underside of the deck is still a little rough despite all the grinding and sanding. I'm planning to pad the underside with minicell foam. I don’t really recommend this method of replacing a coaming because it's difficult to get a good cosmetic result around the outside of the rim and underside of the coaming lip. I did it as a shortcut.