What Is Caulking? The Basis of Watertightness in Wooden Boats

Caulking is making the seams between planking on a wooden boat watertight. This craft, which has kept boats afloat for centuries, is the wooden boat's most critical safeguard when done right.

Why is caulking needed?

Wood swells with moisture and shrinks as it dries. This movement opens and closes the seams. The caulking material must be flexible enough to accommodate it — holding water out without cracking.

Traditional and modern methods

The traditional method drives oakum (cotton/flax) into the seams and covers it with a stopping compound. Modern applications use flexible polyurethane/polysulphide sealants. The right method is chosen for the boat's construction and age.

When should it be renewed?

A boat that leaks, has lifting seams or dried, cracked stopping is due for caulking. Postponing small leaks can rot the wood beneath.

We do caulking and planking under our wooden boat refit service. For more on wood care, see our varnish care article.