One of the few drawbacks to fiberglass construction is its lack of stiffness when used in flat panels, which is why boat designers use multiple curves to eliminate flat areas and sandwich core materials (like foam or balsa) to stiffen fiberglass panels. But in small areas like a cockpit floor hatch, a flat and unreinforced fiberglass panel may flex like a trampoline. The solution to this so-called 'oilcanning' is to add a fiberglass reinforcing beam under the deck, and eliminating this annoyance is a weekend project that is both easy and inexpensive.

You can make an extremely strong but lightweight deck beam from fiberglass molded into a half-round shape. Use a cardboard mailing tube as a mold, cutting it in half to get the half-round shape. Pick a tube with dimensions suitable to your project both in length and diameter. Waxed paper will prevent the fiberglass from sticking to the mold, so cover a flat surface with waxed paper and then wrap the halved mailing tube separately, taping the paper on the inside for a seamless mold. Center the half-tube on the flat surface.

Cut a piece of 2-ounce fiberglass mat (shown in photo) to cover the tube and extend for several inches onto the flat surface on each side to create 'tabs' for attaching the finished beam. Wearing gloves for protection, use a throw-away brush to saturate the mat thoroughly with properly catalyzed laminating resin until no white spots of dry material remain. Add alternating layers of mat (random fibers) with 24-ounce woven roving (heavy weave) for strength. Don't worry about neatness: You'll be trimming your beam to size after it cures.

After several layers of mat and woven roving have been applied, add a final outside layer of 10-ounce fiberglass cloth to give the beam a finished appearance when it is installed. Again, be sure to thoroughly saturate the material, but be careful not to dislodge previous layers of mat or create air voids, particularly in the corner between the tube and the flat surface. Use your brush to force the material into the seam. Laminating resin will remain slightly tacky, but it will still peel off the waxed paper mold. Be wary of sharp edges.

In the photo, two sides have been trimmed with a fine-toothed jigsaw while the other two sides show the original rough edges from the mold. When trimming, leave as much tab on each side as possible, as this will provide maximum area for bonding the beam under your deck panel. Sand away paint on the under-deck surface and lightly roughen the area before using pads of mat saturated with bonding resin to adhere the beam to the deck. Clamp the beam for a tight bond. After the resin has cured, overlay the entire beam with a layer of fiberglass cloth with finish resin for added strength. Paint to match. You've ended the 'flex deck' problem.

March 2004