The backbone of a plank boat is a single straight keel 8-10 cm square for a 4-5 m hull, stems at bow and stern carved from naturally curved wood so the grain follows the curve, or a flat transom at the stern, and a groove called the rabbet cut along the keel and both stems to hold the plank ends. In seasoned hardwood at 600-800 kg/m³ the keel alone weighs between 17.3 kg (8 cm section, 4.5 m, 600 kg/m³) and 36 kg (10 cm section, 800 kg/m³), so it is a two-person lift and the first timber to get right.
Part of Canoe & Rowing Boat Construction
The backbone is the structural spine of a plank boat: a keel running the length of the bottom, stems rising at bow and stern, and the grooves in which the planking ends sit.
Everything Hangs From The Spine, So It Is Built First And Checked Twice
The carrier calls the keel the strongest piece in the boat, a single straight timber of oak, ash or the hardest available wood, free of defects. The stems are the curved pieces where the keel ends turn upward; they are traditionally carved from a naturally curved tree root or branch, a grown knee. The reason is grain direction: wood is far stronger along its fibres than across them, and a curve sawn from a straight board has short grain at the bend, while a grown knee has fibres that follow the curve end to end. A transom, a flat board across the stern, replaces the stern stem for simplicity and gives a mounting point for a rudder.
Because the planks end in the keel and stems, a groove (the rabbet) is cut along them so each plank end seats in a recess rather than butting against an edge. The rabbet’s angle changes along the length as the plank approaches the keel, which is the fiddly part.
What You Need
- A keel timber 4-5 m long, straight and sound, with no large knots near its middle
- Two stems: natural knees, or one stem and a transom board
- Hardwood for the keel and stems, the hardest local wood (densities in Logistics & Load Planning)
- Scarf and fastening materials if the keel must be joined (see Mast Construction for scarf practice)
- Level blocks, string, a plumb line and a long straightedge
Method: Shape And Set Up The Backbone
Step 1: Check the timber. Sight along it; reject any sweep that will not come out with a plane.
Step 2: Dress it square to 8-10 cm and mark the rabbet line on each side.
Step 3: Cut the rabbet with a chisel and gouge to a depth that lets the plank ends sit flush.
Step 4: Join the stems to the keel with a scarf or a lap joint, fastened through, and check that the joints seat tight.
Step 5: Set it up on level blocks, with a string line from stem to stem. Measure the diagonals and plumb the stems; correct before anything else is fixed.
Worked Example: How Heavy Is The Keel?
Keel volume = length × section = 4.5 × 0.09 × 0.09 = 0.0365 m³ for a 9 cm section. At the corpus’s seasoned hardwood range of 600-800 kg/m³ the mass is 0.0365 × 600 = 21.9 kg to 0.0365 × 800 = 29.2 kg. For the extremes of the carrier’s range: 8 cm gives 4.5 × 0.08² = 0.0288 m³, or 17.3 kg at 600; 10 cm gives 4.5 × 0.10² = 0.045 m³, or 36 kg at 800. Green timber (800-1000 kg/m³) would add a quarter to a third to that (800 ÷ 600 = 1.33; 1,000 ÷ 800 = 1.25), which is another reason to season the timber first.
Common Mistakes
- A keel with a sweep or a twist — it carries into the planking; check by eye and string before cutting.
- Sawing the stem from a straight plank — the grain runs out at the curve and the stem splits at the first impact.
- Cutting the rabbet too deep — it weakens the keel where it is already narrowed.
- Fastening the stem to the keel with a butt joint — scarf or lap it.
Safety
Heavy timber, edge tools
A 36 kg keel falling from trestles can break a foot. Lift with two people, block the timber so it cannot roll, and keep both hands behind the edge of a chisel or drawknife.
What’s Next
- Learn next: Planking Methods — covering the backbone
- Combine with: Ribs & Frames — the internal skeleton the backbone anchors