BoatUS.com Articles and Tips from DIY Magazine

FAMILY BOATBUILDING: Building the 100-Hour Dinghy

Here’s a simple method to build a boat that requires few tools and little experience. It’s ideal for first-time builders and one that can involve the entire family.

— Story and photos by Jan Mundy

To raise money for The Hospital for Sick Children, DIY decided it to build a boat at a boat show. Since I was the only one with any boat building experience, I got the job but finding a boat that we could start and complete in nine days was difficult. We contacted Noahs (Tel: 416/232-0522 Web: www.noahsmarine.com) a supplier of boat building materials, and ended up purchasing plans for a Glen-L Eight Ball–SG dinghy. Measuring 8'6" (2.6m) in length, a beam of 4'4" (1.3m) and weighing about 83lb (38.5kg), this vee-bottom pram can be rowed, sailed or powered with a 3 hp maximum outboard. To simplify building, we opted to build the rowing model, excluding the daggerboard and case, mast step, rudder, mast etc.

Stitch and glue is the ideal construction method for amateurs to build a boat. It combines plywood, fiberglass and epoxy resin to create a lightweight but exceptionally strong boat. It requires no mold or building frame. Instead, a boat is “stitched” together with wire or plastic wire ties. The completed boat has all the virtues of a fiberglass boat. Except for routine painting and brightwork varnishing, it’s completely maintenance free.

Glen-L (Tel: 562/630-6258, Web: www.glen-l.com) offers all types of sail and powerboats available as plans, patterns or kits. I elected to purchase the plans only, as I had an inventory of resin, glass and the like from other projects. Plans for the Eight Ball-SC (US$41) included full-size patterns for all components, a step-by-step building booklet, materials list, source list and even a list of the tools necessary for construction. Glen-L offers a wooden parts kit (US$453) plus an epoxy kit, which includes the wire, resin, fiberglass tape, fillers and all other assembly materials.

Few tools are needed. Besides the basic woodworking tools and supplies, you’ll need epoxy resin and fillers, 16-gauge copper wire (or purchase plastic wire ties or mallable iron wire), disposable containers and stir sticks for mixing the resin, flexible squeegees, putty knives, foam rollers, cheap glue brushes in several widths and a box of “rubber” gloves (purchase the ones sold for medical use at a pharmacy).

Here starts my day-by-day account of our dinghy’s construction. After reading this, we hope it inspires you to build one of your own.

Day 1

Prior to the show opening, I transferred the full-size paper patterns onto the wood, outlining the cut lines using a fabric wheel and carbon paper. Three sheets of 1/4" (6mm) Luann plywood (waterproof spruce or mahogany ply is a good option) made up the bottom panels, side panels, transom, bow, seat bases and seat tops. Knees, gunwale, outboard support and bow cap were fashioned from 1" (2.5cm) solid mahogany. A circular saw worked best to cut straight lines, a jigsaw for curved lines. Alignment marks on the patterns were transferred on the side and bottom panels. Plywood edges were then sanded smooth to removed splinters. Sides and bottom edges were also drilled with 3/32" (2mm) holes for wire “clamps,” spaced about 4" (10cm) apart and 9mm (3/8") from the panel edges. Gunwale reinforcing strips and seat cleats were glued on to save assembly time at the show.

Day 2

DIY TIP:

Fillet Filling Tubes

To quickly and easily dispense thickened epoxy, such as in filleting applications, consider using 810 West System fillable caulking tubes. These tubes fit into a standard caulking gun and can be refilled until the epoxy cures. With a set of two costing only US$2.20, it’s a much better method than messing with plastic bags.

We setup a building site in our DIY exhibit. The panels that form the hull lay on a long table (no need for a mold or building frame), though sawhorses would be a better choice as they allow access underneath. While I assembled the panels, a helper cut the copper “stitch” wire into 8"- (20cm-) long pieces. Starting with the two bottom planking halves laid flat, I began threading wires from the inside. This involved inserting a wire, pulling the seams together, and then twisting the wire to loosely tighten. This job is best done with two people. Stitching the bow was next, then sides and transom. Where panels bent in to meet the bow and transom, it was necessary to drill more holes, closely spaced together, to secure the panels.

Stitching proved to be a major challenge and fortunately, an experienced stitch-and-glue kayak builder dropped by to help me wrestle with the panels. When all ties were in, we stood back and closely surveyed our dinghy, looking for skewed or twisted panels. Once all was aligned and hull lines were fair, we twisted each tie tight, being careful that the panels didn’t shift as we tightened. It’s impossible to match all joints, though gaps of up to 1/4" (6mm) are acceptable. All seams were now covered with wide strips of masking tape to stop resin fillets from dripping through the seam gaps. With help from some onlookers, we turned our hull over to begin the interior finishing.

Day 3

Fillets made of thickened epoxy seal the gap between the panels and create a smooth form over which is laid fiberglass tape. Both operations had to be done in one day.

Working a small section at a time, seams were first coated with unthickened epoxy, rolled and brushed on, then filled. To mixed epoxy resin, I added enough lightweight fairing powder and colloidal silica at a 1:1 ratio to form a putty slightly thicker than peanut butter so it wouldn’t run or sag. This mixture was then poured into a small plastic freezer bag with one end cut and, using it like a pastry bag, applied the compound to the seam in a thick, continuous bead. Fillets butt up to the wire ties. Otherwise, you get a bump in the fillet where it contacts the tie that requires filling or sanding once cured. Bonded wire ties are also nearly impossible to remove. Passing a stir stick over the fillet gave a nice smooth and round seam. Before moving on to the next stage, which is coating the seams with epoxy and applying fillets, I cleaned up any spills. It’s much simpler to do this when resin is in a liquid state, rather than sanding cured, hard epoxy.

Before fillets had fully cured, I began taping. This was done by wetting out the 3"- (7.6cm-) wide fiberglass tape and laying it over the seams. This saved a day of waiting for the fillets to cure and then I prepped and sanded prior to taping. Some builders prefer to apply unthickened epoxy resin directly to the seams and then lay down the tape dry. Instead, I saturated pieces of tape in a tray of resin and laid them over the seam, using a squeegee to remove wrinkles and air bubbles. To obtain a straight edge, it’s recommended to strike a pencil line along each seam and set the tape to the line. Another good practice is to mask the areas outside of the pencil lines to eliminate some of the mess caused by excess resin and drips. Since tape ends fray, in order to produce a clean edge, I cut the pieces slightly longer and trimmed them to size with a sharp utility knife when the epoxy was tacky but not fully cured.

Day 4

DIY TIP:

Let it Cure

Polyurethane paints, such as Interlux Brightside, won't cure properly when applied over "young" epoxy. Better to let the resin cure for about seven days before painting.

The fiberglass tapes had fully cured overnight, so I scrubbed the seams with water and a ScotchBrite pad to remove the amine blush, a waxy by-product of epoxy resin that, if not removed, can prevent adhesion to additional resin applications. Panels and fillets were sanded with 80-grit paper to remove any high spots and drips. This took a full half-day, but even then I wasn’t pleased with the results but time was pressing, so it was on to the next step. The bow and transom knees and backing blocks for the bow eye and outboard motor were epoxy glued on.

After wiping the entire interior with solvent to remove any contaminants, I rolled and brushed on three coats of unthickened epoxy resin. Rather than waiting for each coat to cure, subsequent coats were applied while the epoxy was in the “green” stage, which means slightly tacky but not so it can be disturbed when touched with a brush. This usually occurs in one to three hours, depending on the ambient temperature.

DAY 5

At this stage, the hull was very rigid. With help from a few spectators, we turned our dinghy over. After removing the masking tape, I pulled out the stitch wires using pliers. For wires that were epoxied, the ends were cut off with side cutters and punched below the surface. Sometimes, applying heat with a soldering iron will soften the epoxy and release the “stitches.” But I was without an iron and having used copper wire, I wasn’t too concerned. To fill all wire holes, any gouges or low spots in the plywood and the gaps between the sides and bottom, I mixed batches of epoxy resin and filler powder (microballons). This non-structural putty was applied with a flexible putty knife and/or squeegee and spread on thickly so it lay proud of the plywood. Joining edges were molded with putty to form a round bead.

DAY 6

DIY TIP:

Mixing Epoxy

• Epoxy must be mixed at the precise ratio or it may not cure. Purchase metered pumps that dispense epoxy to the exact ratio.

• After mixing the resin with the hardener, stir for at least one minute before adding filler, and then stir for another minute.

• Scrub cured epoxy with water and a ScotchBrite pad to remove the amine blush before another coating or gluing application.

• Clean surfaces with solvent (I prefer lacquer thinner) before applying epoxy resin or glue. Aggressively wipe the surface and change rags frequently to prevent spreading the dirt around. After cleaning, don’t wipe the surface with bare hands or you risk recontamination and jeopardizing the epoxy bond.

• Time is of the essence when sheathing, so I prepare containers of resin only in advance. Add the hardener and stir when needed. Continually check the surface, looking for dull spots that signal a “dry” laminate. Recoat such areas with epoxy resin. Look for drips and remove before the laminate begins to cure.

• Cured epoxy is hard stuff and very difficult to sand. When you do a procedure, such as filleting, laminating or taping, be sure you tip off any drips, runs or sags with a brush or roller before exiting the work area. Time spent in reducing the amount of sanding is time well spent.

Using a wood rasp and Sure Form, the filled seam joints were radiused and then sanded with 80-grit sandpaper in preparation for sheathing. The entire hull was sanded with 120-grit paper until it was almost perfectly fair. Any high or low spots or dings were refilled, as needed. These tend to show like dots on a mirror after fiberglassing and painting. Flattening the transom and keel slightly with a block plane prepared the hull for the skeg.

Next step was to vacuum the hull to remove all dirt and sanding dust, followed by a solvent wipe to remove any contaminants. I then rolled on a coat of unthickened resin to seal the plywood hull.

DAY 7

After curing overnight, the hull was ready for a scrub with water and ScotchBrite pad to remove the amine blush. Once dry, it was given a solvent wipe in readiness for sheathing with fiberglass and application of sealer coats. Building instructions call for taping the outside joints, but I opted to sheath the entire hull exterior in 6oz fiberglass cloth for added abrasion resistance.

Rather than pre-wet the hull with resin and then apply the cloth, I applied cloth on a dry hull. Sheets of plastic covered the floor to catch drips — this is a messy job. Cloth was draped over the hull with the weave parallel to the keel and wrinkles smoothed out by gloved hands (worn to prevent contamination from skin oils). Luckily, the cloth was wider than the boat so joining pieces wasn’t required. I like to dab a brush load of unthickened resin on the cloth, which sticks the cloth to the hull so it doesn’t move, then spread epoxy with a resin-soaked roller and use a squeegee to smooth out the cloth to remove wrinkles, air bubbles and excess resin. Where the sides meet the curve of the bow, it was necessary to slit the glass to form darts. When just slightly tacky, about three hours or so, excess cloth was cut off flush with the gunwale with a sharp knife. While the laminate was still green, I rolled on a coat of unthickened epoxy to fill the weave, followed by another coat four hours later. Applying subsequent coats “green on green” — when epoxy becomes tacky but is not yet set — eliminates having to wait for a full cure and then prep washing the surface prior to recoating. If the cloth starts to pull away from the surface or it shifts, the laminate is still too green.

DAY 8

After an overnight cure, the hull was thoroughly washed and scrubbed to remove the amine blush. I epoxy glued the skeg in place and let it harden. The entire hull was then sanded with 120-grit paper, primed and painted. I used Interlux Brightside, a single-part polyurethane, to ensure a quality, durable finish.

DAY 9

The next stage was to finish the interior. Again, the hull was turned right side up. Seat tops were glued to the seat sides and cleats on the side panels. Undersides of these tops had previously received three sealer coats of unthickened resin. I used a block plane to radius edges, and applied fillets to edges where seats joined the hull sides. Seats are fully watertight and act as buoyancy compartments.

DAY 10

After a final sanding of the interior with 120-grit paper, it was primed and painted. We used Interlux Brightside in white, mixed with a flattening agent to reduce glare.

DAY 11

The last day of the show and the Eight Ball was almost complete. Oarlocks and a stainless-steel bow eye were installed. Mahogany rubrails and the bow cap were epoxy glued and screw fastened. There wasn’t time to varnish these pieces so we gave the winner a can of varnish and some brushes.