Leaks can shrink themselves. In some traditionally built wooden boats, water entering the wood makes the planks expand, narrowing the seams through which that same water initially passed. The apparent flaw helps. A hull that has dried ashore can therefore leak when launched, then become more watertight as its planks absorb moisture and press against their neighbors or the caulking between them.
The trick is structural. Wood is hygroscopic, meaning that its cell walls absorb moisture, where water molecules associate with cellulose and other components and push the structure outward. Direction matters more than bulk. Because this swelling is anisotropic, planks expand much more across the grain than along it, so their increasing width can reduce gaps between adjoining edges. Absorption also has limits. Once the cell walls approach the fiber saturation point, additional water mainly occupies cavities rather than producing much further swelling, which explains why soaking does not make a hull expand without end.
Swelling is no cure. In many carvel-built hulls, where planks meet edge to edge, the narrowing seams compress fibrous caulking that helps restrict water passage without making the wood itself impermeable. Fit still earns its keep. How effectively a boat tightens depends on its timber, construction, condition and previous drying, while rot, split planks or loose fastenings remain defects requiring attention rather than faith in soaking. The bargain remains conditional. Beneath the waterline, a sound seam can draw closed, leaving the material that admitted the sea to hold it back.
