Bottom Board, Transoms, and the Garboards

The first two days build the foundation: the bottom board, both transoms, and the garboards, the first plank on each side. Everything after depends on getting these fair.

Set up the bottom board

A pram’s backbone is its flat bottom board. On many museum prams it is made of several boards edge to edge: one Fredrikstad eke has three bottom boards side by side under four strakes a side ¹. In a Holmsbu pram the bottom board lies under the two lowest strakes ².

Edge-join the bottom boards if needed (cleats across the inside hold them until the frames go in). Mark the centerline, then lay out the outline, tapering toward both transoms. Watts’s design starts the same way: the keel plank is bent to rocker first ³. Set it on two sawhorses and weight or prop the ends up so it takes a gentle fore-and-aft curve. That rocker lets the boat turn and lifts the bow transom clear of the water.

Make the transoms

Each transom is a few boards joined edge to edge and held with cleats across the inside. The bow transom (the nose) is small; the stern transom is wider and taller. Both lean outward, or rake. Cut them oversize: traditional builders fit the transoms after planking by eye, while Watts supplies transom shapes and stands them up first as guides ³. For a first boat, standing them up early, braced to the bottom board and to the shop overhead, makes the planking easier to judge.

Spile the garboard

The garboard is the first plank on each side. To find its shape, tack a straight, thin spiling batten along where the plank will go, and at intervals mark on it the distance to the edge of the bottom board. Transfer those offsets to a plank blank, connect them with a flexible batten, and cut. In the traditional Norwegian method the planking is shaped by eye ³: spile the edge that must fit, and judge the other edge by sighting. Cut it a little full; you can always plane off.

Bevel the lap

Where a plank laps onto the one below (or onto the bottom board), the lower piece’s edge is beveled so the upper plank lies flat against it. The bevel changes along the boat as the hull twists, so check it every foot or so by laying a batten across from the lower plank to where the next will sit, and plane until it fits. Harry Bryan recommends a full-length batten to guide the bevel between stations, and a lap width of about twice the plank thickness ⁴.

Gains at the transoms

Near each transom, the laps can’t stay lapped: the planks must finish flush to fit tight against the transom’s edge. So for the last stretch of each lap you cut a gain, a long tapered rabbet in the edges of both planks, so the lap fades to nothing at the transom. Make the gain at least 12 inches long on 3/8-inch or thinner planking, 18 inches or more on heavier stock, using a rabbet plane or chisel ⁴.

Hang and rivet the garboards

Clamp the garboard in place with your shop-made clamps, the way North House’s pram students do ⁵. Check that it lies fair (sight along it) and that the lap is even. Then drill and rivet along the lap, roughly every few inches, working from the middle toward the ends; the riveting procedure is in the next lesson. Do the second garboard to match, checking by measurement that the boat stays symmetrical.

Try it

Before cutting good stock, spile and fit a garboard from 1/4-inch plywood or cardboard. The template will show you where your spiling is off, at the cost of one sheet.