Ribs are curved members running across a plank hull from gunwale to gunwale that give it its cross-section and resist crushing; space them every 20-30 cm along the hull, which gives 16 to 24 for a 4.5 m boat, and cut them about 3 cm by 4 cm in oak, ash or another wood that bends well. A 3 cm rib needs about 71 minutes in steam at 93-100 °C, must be bent in under a minute, and needs a metal strap if the bend radius is under about 45 cm (15 times the thickness).
Part of Canoe & Rowing Boat Construction
Ribs, also called frames or timbers, form the internal skeleton of a plank boat and give the hull its shape, in clinker added after the shell and in carvel set up before the planks.
A Rib Carries The Hull’s Shape, So Its Grain Must Follow The Curve
The carrier gives the rule: in a clinker boat, ribs are steam-bent and inserted after planking, fitted to the hull’s natural shape; in a carvel boat the frames are built first, shaped to plans or a mould. Either way the rib resists crushing forces and holds the planks in section. A rib made by steam-bending a straight piece has grain that runs from end to end around the curve, which is why bent ribs outlast sawn curves.
Steam softens the wood so fibres can slide past each other. The corpus procedure (Steam Bending) is: steam in an enclosed, not sealed, box at 93-100 °C for 1 hour per inch of thickness, bend within 30-60 seconds, overbend by 5-10% to allow for spring-back, and use a thin metal strap along the outside of the curve if the radius is under about 15 times the thickness.
What You Need
- Rib stock: straight-grained white oak or ash, 3 × 4 cm in section (the carrier’s size for a small boat), cut oversize and green or air-dried
- A steam box and a boiler (see Steam Bending)
- A thin metal strap and end blocks for tight bends
- A way to hold the bent rib against the planks until it dries: clamps, wedges or temporary ties
- Fasteners for fixing the ribs to the planks (see Riveting and Lacing)
Method: Steam, Bend, Fit
Step 1: Check the radius. Bend across the 3 cm dimension. The corpus gives a minimum radius of 10-15 times the thickness: 10 × 3 = 30 cm to 15 × 3 = 45 cm. If the inside of the hull curves more tightly than that, use the strap or a thinner rib.
Step 2: Steam. 3 cm ÷ 2.54 = 1.18 inch, and at 1 hour per inch that is 1.18 × 60 = 71 minutes. Do not start the clock until the box is at temperature.
Step 3: Bend at once. Take the rib from the box and bend it into the hull in no more than 30-60 seconds. Practise the sequence dry first; two people make it easier.
Step 4: Hold it. Clamp the rib in place until it cools and dries, overbending by 5-10% so that it relaxes to the correct curve.
Step 5: Fasten to the planks.
Worked Example: Rib Count And Wood
Hull length 4.5 m at 20-30 cm spacing gives 4.5 ÷ 0.30 = 15 intervals, so 16 ribs, up to 4.5 ÷ 0.20 = 22.5, so 23 or 24 ribs. If each rib is about 1.9 m long (an assumed gunwale-to-gunwale length; measure yours), one rib’s volume is 0.03 × 0.04 × 1.9 = 0.00228 m³, and in seasoned hardwood at 600-800 kg/m³ weighs 1.37-1.82 kg. For 16-24 ribs the total is 16 × 1.37 = 22 kg to 24 × 1.82 = 44 kg, which is a useful figure for planning the wood: it is comparable to or more than the 17-36 kg keel in Keel, Stems & Backbone. Allow extra for ribs that crack during bending; the corpus warns that bends with no strap below the radius limit tear the outer fibres.
Common Mistakes
- Bending the wrong dimension — bend across the thin side.
- Steaming too briefly or too long — too short and the wood cracks; kiln-dried wood bends poorly.
- Taking too long to place the rib — it cools and stiffens within a minute.
- Placing ribs at uneven intervals — the planks flatten between wide gaps.
Safety
Steam and hot, springy wood
A steam box delivers scalding steam from its opening, and a hot rib that slips from a clamp whips back. Keep the box not sealed (a sealed box can burst), wear leather gloves, open the lid away from your face, and keep onlookers well clear of the path of the bent rib.
What’s Next
- Learn next: Caulking & Waterproofing — closing the shell
- Combine with: Keel, Stems & Backbone — the spine the ribs attach to