A plank hull is made watertight by driving fibre into the seams with a caulking iron and mallet and sealing over it with pitch or putty, then soaking the hull so the planks swell, and judging the result by measuring the leak in litres per hour. For a 4.5 m boat with a waterplane of about 3.3 m² and 25 cm of freeboard, roughly 825 L can enter before the gunwale reaches the water, so a 10 L/h seep gives about 82.5 hours but a 200 L/h leak gives about 4 hours and a 20 L/min leak about 41 minutes.
Part of Canoe & Rowing Boat Construction
Caulking is the driving of fibre into the seams between hull planks, followed by a pitch or putty seal, so that a wooden boat keeps the water out.
A Leak Is A Rate, Not A Yes-Or-No
Carvel boats, and old clinker boats whose laps have opened, need caulking. New carvel planks start dry and shrunken, so the seams weep at first; as the wood takes up water it swells, and a good caulked seam tightens. A leak that remains after a day or two of soaking is a defect. Measure it: that turns a worry into a number, and the number tells you what to do.
Material and technique details are in Caulking Materials and Boat Sealing. This page is about the process and the test.
What You Need
- Caulking fibre, a caulking iron and mallet, and a seam reamer for narrow seams (see Caulking Materials)
- Pitch, putty or sealant and a safe way to heat it
- A calm mooring where the hull can sit afloat or half-afloat for a day
- A bucket with litre marks and a clock to measure the leak
- A bailer you have timed
Method: Seal, Soak, Measure
Step 1: Open the seam if needed with a reamer, so the fibre has room.
Step 2: Drive the fibre in with firm, even taps; too hard and the planks are pushed apart (the carrier’s warning).
Step 3: Fill the rest with putty, pine pitch or marine sealant, flush with the plank.
Step 4: Soak the hull for a day with some water in the bilge or afloat at a mooring, so the planks swell.
Step 5: Measure the leak. Bail the hull dry, mark the water level, wait one hour, and bail into a marked bucket. The volume is the leak rate in L/h.
Step 6: Repeat until the rate is acceptable, then re-test after the first rough use.
Worked Example: How Long Before The Gunwale Goes Under?
Assume a waterplane area of 3.3 m² (4.5 m long by about 1.0 m wide at the waterline, with a fine hull shape; an assumed value) and 25 cm of freeboard. Each centimetre of extra draught takes 3.3 × 0.01 = 0.033 m³, or 33 L. Over 25 cm, the reserve is 25 × 33 = 825 L. Dividing by the leak rate:
- 10 L/h: 825 ÷ 10 = 82.5 hours
- 200 L/h: 825 ÷ 200 = 4.1 hours
- 20 L/min = 1,200 L/h: 825 ÷ 1,200 = 0.69 hours, 41 minutes
These are upper bounds. A hull with water sloshing in it behaves worse than the sum suggests, because the free surface reduces stability (see Loading for Stability), and the bailing rate needed is the leak rate. Time your own bailer; if the leak exceeds what one person can bail, the boat should not leave the shore.
Common Mistakes
- Launching a dry, new hull and loading it — it will leak until it swells; soak it first.
- Caulking over dirt or old pitch — clean and dry the seam.
- Overdriving the fibre — see the carrier’s warning.
- Measuring only after sailing hard — a hull that is tight in calm water can open when worked by waves.
Safety
Hot pitch
Pitch is heated to about 150-180 °C in Boat Sealing, and is extremely flammable. Heat it in a pot inside a larger pot of water or on coals, not in flames, keep water nearby, work away from the boat and the building, and never leave it unattended. Fit no flame or hot pot into a boat afloat.
What’s Next
- Learn next: Caulking Materials — oakum, cotton, pitch and sealants
- Combine with: Surface Treatment — protecting the outside once the seams hold