"Wood-Epoxy Boats Covey Island Style"
by Eric Sorenson
WoodenBoat Magazine #120, September/October, 1994.
Used by permission of the author and the blessing of WoodenBoat Magazine.
John Steele knew he was on to something when he had to move his seven-year-old business to larger quarters in 1986. He had developed and refined an innovative approach to building one-off, custom boats using wood saturated and sheathed in epoxy. He hasn't looked back since, with his yard having now turned out some 67 wood-epoxy boats ranging in size from a 35 foot lobsteryacht to the 70 foot schooner TREE OF LIFE.
Covey Island Boatworks, 75 miles down the coast from Halifax, Nova Scotia, uses a blend of traditional woodworking craftsmanship and space-age materials to build one-off custom boats in strip, plywood, or double-diagonal planking. Whichever method is used - and strip-planking gets the nod most often - the wood is impregnated with epoxy resin during the construction process, preserving and bonding the hull components. The outside of the hull is sheathed with several layers of epoxy-saturated fiberglass or E-glass bidirectional cloth. This sheathing not only seals the wood and provides good abrasion resistance, but it also adds tremendous rigidity and strength to the hull. Impact resistance, particularly important in faster boats, is very good.
The yard's construction methods produce mirror-smooth boats which look as good as any popped out of a fiberglass production mold. The end result is a very strong, reasonably priced (particularly for U.S. customers at today's exchange rate) custom hull that should last indefinitely.
Wood-epoxy composite boatbuilding is a tried and true method of one-off custom construction. It produces a very strong, long-lasting hull of light to moderate weight that requires little maintenance. Wood has one of the highest ratios of stiffness and tensile strength to weight of any material, and the one problem that has always plagued wooden boats - rot - has been solved by epoxy resin saturation.
Covey Island uses only epoxy resin, which is far stronger and has better adhesion than the vinylester and polyester resins used by the majority of fiberglass boatbuilders. Only a non-woven biaxial cloth is used in the sheathing layup; fiberglass mat, used in most polyester laminates to bond alternate layers of high-strength woven or stitched roving, is resin rich and adds thickness and weight but comparatively little strength. A pure cloth laminate makes for a very strong and efficient sheathing, especially when combined with epoxy resin. This exclusive use of cloth and epoxy resin provides abrasion resistance and results in greater tensile strength and impact resistance than a standard polyester laminate of the same weight and thickness.
With the wood core (strip planks on laminated frames), these boats are insulated naturally and don't sweat like the interior of an uninsulated solid fiberglass hull. The wood core offers higher impact resistance compared to some conventional composite layups.
Epoxy resin, especially when coated with a barrier coat paint, does a better job of resisting osmosis and water vapor transmission than vinylester and polyester resins, reducing or eliminating the absorption of moisture in the wood core. Epoxy is also used on the interior and in the bilges, as well as over and between layers of the plywood deck. Most builders who sheathe their boats with less expensive polyester or vinylester resin on the outside still seal the interior with epoxy, since the interior of the boat is particularly susceptible to vapor absorption.
Covey Island has since had a couple of their earlier boats back for overhaul. John Steele reports that there have been no signs of moisture absorption (his stock typically has an 8-12% moisture content during construction), nor have the waterlines of the boats changed noticeably over the years. The three layers of hull sheathing outside of the boat, good natural ventilation below decks, and three coats of epoxy resin as a sealer do a good job helping to keep the core dry with stable moisture content.
Covey Island has recently finished a 41' Spencer Lincoln-designed sport fisherman. Its wood-epoxy strip-planked hull was built with the same techniques and materials used for most of Covey Island's boats. The complete boat displaces around 23,000 lbs with a full load of fuel, passengers and gear onboard and about $275,000. (US) complete, in 1993. Delivered as a bare hull, epoxy sheathed with a mirror-smooth finish and epoxy sealed inside with bulkheads installed, the same boat would cost $58,000. (US). Here's how such a boat goes together at Covey Island.
Building the Jig
The male building jig or framework used by Covey Island is straight forward to construct, and is built of lumber found in almost any lumberyard. The one-off jig and the boat itself are built upside down, with gravity making life easier for the builder. Fastening frames to the jig and planks to the frames, and then laying up three layers of fiberglass cloth over the faired hull, is completed with far greater ease, speed, and accuracy than if it were done with the hull rightside up.
The jig starts with a pair of strongbacks, in this case two 2 x 12's laminated and glued together in a slight curve (inplan view, or looking from above) to conform generally to the boat's deck line. A water level is used to get the strongbacks level and into position. One is set slightly lower than the other to allow for any shimming and leveling that may be needed later on.
Covey Island lofts the boat's lines and sections traditionally, normally lifting stations at 3' centers for molds. These station molds are cut to shape in plywood, with reductions made for plank, frame, and batten thicknesses.
So far, Covey Island doesn't feel that computer-lofted lines offer any great savings in lofting or setup time.
With all the plywood molds attached to the strongbacks, the hull form is well defined. The boat's sheer clamp is laminated and cut in place. The sheer clamp gives the boat's deck line its continuously curved shape, so it has to be right; any waves or dips are easily noticeable, especially with the types of highgloss epoxy and polyurethane finishes used on many of these boats. And this hull doesn't have anything remotely resembling a flat spot, with generous flare forward gradually changing to pronounced tumble home aft, along with continuously curving sections. Actually, all these curves make life easier for the builder intent on a mirror-smooth, highgloss hull. Producing the same visual results with a straight, flat section is far more difficult since the eye instantly detects irregularities in flat areas.
The next step is to fasten ribbands of spruce strapping running fore-and-aft to the plywood molds. These battens are doubled up to ensure a fair curve between the molds and at butts in the strapping. These are shimmed as necessary to keep the lines fair, and then screwed in place.
The Transom
At this stage of construction the boat's transom is laid up and attached to thejig. A typical transom is made of three layers Of 1/2" Douglas-fir plywood epoxied together. The transom sides and bottom will be trimmed off later on, after the frames are installed. The transom serves as the last frame, and the planking is allowed to run by it for final trimming.
The Frames
The next step is to lay up the first members (after the sheer clamp and transom) that will actually be part of the boat. These Western white spruce frames are laminated of either five layers of 3/16" or four layers of 1/4", ending up with overall frame dimensions very close to 1 x 2 1/2". The thinner strips are used on the sections of the hull that have a radius too small for the thicker ones to follow, primarily aft and amidships.
The four or five layers of wood that form the frames are saturated with epoxy and wrapped in a plastic bag for ease of handling and to keep the frame from sticking to the battens. The frames are then screwed to the battens and the glue is allowed to cure. Screws with big washers are used to hold the laminated frames down; it would take too many clamps to do the job, and screws are faster. C-clamps are only used to help out where necessary with the more stubborn frames.
Frames are on 16" centers, except on the bottom in the area of the engines, where they are on 8" centers to handle the extra local stresses. Each frame is numbered and marked where it meets the sheer clamp so that it can be put back in the same spot later on. After the glue has set up, the frames are removed for dressing; the epoxy that has oozed out is planed off and cleaned up. The screw holes are filled, and the inboard corners of the frames are rounded off. The frames are then re-installed, any necessary shimming is done, and the hull is ready to receive its 1 1/8" planking.
The Planking
Planks of 1 1/8 x 1 1/8" strips are ripped from clear spruce 2 x 12s. The first strip is laid out temporarily along the bilge, about halfway between the sheer and the garboard. An equal distance is measured to the sheer from this strip and marked at each frame, and the first strip is fastened midway up the boat's side between the bilge and the sheer. This process allows most of the strips to be fastened in place working from sheer to keel (with the hull upside-down). making it quicker for the builder and also making for good epoxy saturation and bonding at the strip edges. The strips at the sheer and garboard are allowed to run by and are later trimmed.
Three men work each side of the boat as it's being planked, with one man underneath cleaning off the excess epoxy. The strips are edge-nailed to each other with galvanized box nails and screwed to the frames with stainless-steel screws. The function of the nails is essentially to clamp the wood together until the epoxy glue dries. Once the glue sets up, the bond is stronger than the wood, putting the nails out of a job. As the planking progresses, thickened epoxy resin is used to fillet the inside corner of each frame to the planks inside the hull. Since the inboard corners of the frames had already been dressed with a router before they were permanently installed, there isn't a sharp corner or crevice to be found anywhere in the hull.
The frames are allowed to "float" as the planking goes on, starting at the sheer. They are not fastened to the ribbands nor are they fastened permanently to the garboard or keel until the planking has progressed past the turn of the bilge. This makes a hull that is about as fair as you can get, with the strips evening out any misalignment of the frames. It also cuts down on the amount of hull planing, sanding, and fairing needed before the epoxy-glass sheathing goes on.
The Epoxy Sheathing
After the planking is completed, the wood is faired with hand planes and circular sanders, and a coat of unthickened epoxy is rolled on. Seams are filled with a mixture of aeroseal and epoxy resin, and fastening holes are bunged. Then three layers of epoxy-saturated 11-oz nonwoven biaxial fiberglass cloth are applied, giving a 1/8" thick sheathing. After the three layers are sanded, a clear coat of epoxy resin is rolled on, and then a microballoon-thickened coat is applied for final fairing. On this boat, Valspar two-part epoxy tank-liner was used to seal the surface for a final sanding. Several coats of International two-part epoxy paint follow.
Finishing the Job
The building jig is disassembled and removed from under the hull as it's being planked and epoxy sheathed. Once the hull exterior is finished (but before primed and painted), the boat is turned right-side up, 3/4" plywood bulkheads are installed so the hull keeps its shape, and the inside of the hull is sealed or encapsulated with three coats of epoxy.
Two 2 x 12 's sandwiched together form the engine beds. They are epoxied to the forward and after engineroom bulkheads and encapsulated with plywood and epoxy gussets at each end. The three structural bulkheads are watertight to the sole, an important consideration if you're interested in being able to keep the boat afloat in case one of the compartments floods. Shaft glands are used where the propeller shaft penetrates the aft engineroom bulkhead.
Epoxy is used to fill and seal the lower bilge pockets so the bilgewater will drain to the bilge pumps. This also strengthens the frame-to-keel joint at the garboard.
The exposed clear-coated epoxied interior is sanded with 80 and 120 grit sandpaper. The surface is cleaned and several coats of International varnish builder are applied before the final two coats of varnish. This provides an attractive, mirror smooth clear finish and protects the epoxy from ultraviolet light.
On the outside of the hull, epoxy-saturated laminates of spruce are temporarily fastened in place to form the spray rails forward and the rubrails aft. Once the epoxy has set, the rails are removed and dressed down to their finished dimensions. The rails are then permanently reattached and covered with epoxy-saturated cloth.
The decks are constructed of two layers of 1/2" plywood, bonded with thickened epoxy and covered with one layer of 11-oz cloth. The foredeck, supported by laminated mahogany deckbeams and carlin, gets an extra layer of cloth. The sides of the pilothouse, made of two layers of 3/8" plywood, are covered with a layer of 10-oz cloth. The house sides are finished off like the hull, to a smooth finish. The hull is now complete and ready for system installations.