Caulking, in wood shipbuilding, is the operation of driving oakum or untwisted rope into the seams of the outside planks, or of the deck planking, to render them watertight. The quantity thus driven in depends on the thickness of the planking; it varies from 1 to 13 double threads of oakum, with 1 or 2 single threads of spun yarn. The caulker first raims or reens the seam—that is, drives a caulking-iron into it, to widen the seam as much as possible, and close any rents or fissures in the wood; he then drives in a little spun yarn or white oakum with a wood mallet and a caulking-chisel, and afterwards a much larger quantity of black or coarse oakum. The fibres are driven in until they form a densely hard mass, which not only keeps out water, but strengthens the planking. The seam is finally coated or payed with hot pitch or resin.
In iron or steel shipbuilding and boilermaking the term covers the operation of driving the edge of one thickness of plating firmly against the other thickness upon which it is superimposed, or to which it is adjacent, thus rendering the joints watertight. The tool employed is a specially formed chisel, struck by a metal hand-hammer; but endeavours have been made to supplant this by steam-driven machines, so far with but indifferent success.