The best caulking fibre for a wooden boat is oakum, tarred hemp or jute, made from old rope by unlaying it and tarring the fibres; twisted cotton wicking works but wears out sooner; the seam is then sealed over with pine pitch, putty or a modern marine sealant. On a hull with 63 m of seam and a 20 mm plank, a pitch seal 7 mm deep and 3 mm wide over the fibre needs only about 1.3 L, so the fibre and the labour, not the pitch, are the real quantities.
Part of Canoe & Rowing Boat Construction
Caulking materials are the fibres driven into the seams of a plank boat and the sealants laid over them to keep the water out.
A Caulked Seam Is Two Layers With Two Jobs
The fibre goes in first. It must absorb the sealant, swell slightly when wet to improve the fit, and resist rot in continuous contact with water. On top goes a sealant that fills the last third of the seam and keeps water out of the fibre. The carrier’s list of fibres is oakum (tarred hemp or jute, the traditional choice) and cotton wicking (twisted cotton cord, less durable than oakum). Boat Sealing adds sphagnum moss, cattail fluff and shredded inner bark as alternatives.
Oakum’s strength is the tar: tar is what resists rot. If you have old rope, unlay it into its strands, tease them into loose fibre and tar them (see Pine Tar and Tar Properties). Cotton wicking, from candle-making supplies or unravelled cotton fabric, is quicker to make but rots faster unless tarred.
What You Need
- Fibre: old rope for oakum, or cotton wicking
- Tar or pitch, for the fibre and the seal (see Pitch Making)
- Caulking iron and mallet
- Putty or a salvaged marine sealant for patches and repairs
- A heat source for pitch and a safe pot (see Caulking & Waterproofing)
Method: Choose And Prepare
Step 1: Prefer oakum if old rope exists. Unlay it, pull the strands into loose fibre, and soak in warm tar.
Step 2: Use cotton wicking if rope is unavailable; tar it.
Step 3: Choose the seal. Pine pitch is brittle but waterproof; pitch with tallow is more flexible; a modern marine sealant works if it is rated for continuous immersion. Do not use silicone on a hull you may later paint or pitch: it leaves a film that nothing sticks to.
Step 4: Estimate the quantity by a test metre. Caulk one metre of seam, measure the loose fibre it took, and multiply by the length of seam.
Step 5: Keep the sealant for the last third of the seam depth, with the fibre filling two thirds (the proportion in Boat Sealing).
Worked Example: How Much Pitch?
The 4.5 m hull from Carvel (Smooth Hull) has 14 seams of 4.5 m, so 14 × 4.5 = 63 m. Assume planks 20 mm thick and a seam 3 mm wide (assumed values). If the fibre fills two thirds of the depth, the pitch occupies the outer third, about 7 mm. Cross-section area = 7 mm × 3 mm = 21 mm², or 2.1 × 10⁻⁵ m². Volume = 63 m × 2.1 × 10⁻⁵ m² = 1.32 × 10⁻³ m³ = 1.3 L. That is small: a single pot of molten pitch covers the whole hull, which is why the labour of driving 63 m of fibre, not the pitch, is the limit.
Common Mistakes
- Untarred cotton — it rots, as the carrier says.
- Sealant over wet or oily wood — it does not bond.
- Silicone near pitch work — nothing will stick over it later.
- Using one fibre for all seams — wide seams need two layers.
Safety
Hot pitch and tar
Never let water touch hot pitch; it spits and burns. Keep it away from flames, use a double-boiler or a bed of coals, wear gloves and eye protection, and keep tar-soaked fibre away from fire. Keep tar and its fumes away from food and drinking water.
What’s Next
- Learn next: Surface Treatment — the outside of the planking
- Combine with: Clinker vs Carvel Construction — which hulls need all this