Geoff HendersonThis Shellback dinghy, built purely for rowing, has a clean, bright-finished interior unmarred by temporary fastenings.
For a winter project last year, I decided to build the Joel White-designed Shellback dinghy. I was drawn to the boat’s elegant lines and lapstrake plywood construction. The plans were excellent and Eric Dow’s book How to Build the Shellback Dinghy was very helpful.
Conventional lapstrake planking is fastened together with rivets. Glued lapstrake planking is just what it sounds like: strakes of plywood are glued together over temporary molds or permanent frames and glued together with epoxy. There are no permanent mechanical fastenings but, typically, temporary screws driven through the laps hold the planks together while the glue cures; the screws are then removed and their holes filled. Because plywood is dimensionally stable—that is, it doesn’t swell and shrink when wetted and dried—the glued plank lap seams remain durable and watertight.
The plans are for a sailing dinghy, but I chose to build mine purely for rowing, so the interior is simple and clean, with no daggerboard trunk, maststep, or rudder. The hull is built of meranti marine plywood, and the stem, ’midship frame, gunwales, knees, breast-hook, and keel, are of red oak. I used cedar for the thwarts and glued it all together with epoxy. In keeping with the theme of cleanliness and simplicity, I wanted the boat’s interior to be bright-finished, without any filled holes where temporary screws had been.
Geoff HendersonEach clamp is simply a slotted rectangle cut from 1/2″ plywood.
To avoid the blemishes of screw holes, I first tried using slot-and-wedge clamps to hold the planks together while the glue cured. These clamps are simply a long and narrow U-shaped piece of 1⁄2″ plywood whose arms reach around each side of a plank, and whose tips can thus be located on either side of the lap.
The long slots of the clamps are 7⁄8″ wide. A wedge driven between the plank face and the clamp arm “tightens” the clamp, in theory squeezing the epoxied planks together. I say, “in theory” because on my initial attempt at this, the clamps tended to slip off on the inside due to the angle between the planks.
Geoff Henderson2. A section of polyurethane fuel tubing wired to the end of each clamp keeps the clamp from slipping out of position. 3. The section of tubing also concentrates the clamp’s pressure.
Subsequent experimentation showed that a short length of polyurethane (PU) fuel tubing wired to the end of the clamp worked beautifully. It concentrated the clamping pressure, and flexible PU has a high coefficient of friction against wood, thus it does not slip. PVC, polyethylene and nylon are harder and slipperier, so they would not hold.
Geoff HendersonThe dinghy’s planks were glued together with epoxy, and clamped using purpose-made, closely spaced clamps. Pressure is applied to the clamps with wedges.
The PU tubing is 1⁄4″ in diameter and is nestled in a half-round groove filed into the plywood tips of the clamps. The wedges have a slope of about 1 in 15. In the photos, note the difference in penetration depth of the wedges into the clamps (all of which are identical), as the angle between the planks increases from bow to midships.
Geoff HendersonWith the glue cured, the clamps were removed and the dinghy rolled over for fitting out and finishing. The varnished interior will require no filling.
To finish the interior, I first sealed it with two coats of low-viscosity epoxy resin and followed this with varnish. The exterior is a standard, brushed-on enamel finish, with one primer coat and two finish coats.
She’s as pretty as a picture on the water and a joy to row.
Geoff Henderson built his first boat, a 6′ plywood pram, when he was 14. He began his career as a fitter in the local shipyard in his native Scotland, moved up to the design office, and in the 1980s emigrated to Canada to work in manufacturing. He is now retired, and this Shellback is his first lapstrake build.