Wikipedia says "Traditional caulking (also spelled calking) on wooden vessels uses fibers of cotton, and oakum (hemp fiber soaked in pine tar). These fibers are driven into the wedge shaped seam between planks with a caulking mallet and a broad chisel-like tool called a caulking iron. The caulking is then covered over with a putty in the case of hull seams, or in deck seams with melted pine pitch in a process referred to as paying."