oakum

Oakum is a loose, fibrous sealant and packing material historically used in wooden shipbuilding and other mechanical applications. It consists of fibers—traditionally hemp, jute, or flax—that are unraveled from old tarred rope, combed to separate the fibers, and then treated with tar or pitch to make a waterproof, sticky filler. The material is then packed into seams or glands and driven in with a caulking mallet to create watertight joints.

In maritime practice, oakum was the primary filler for the seams between wooden hull planks, helping to seal gaps and prevent leaks. It was also used to pack stuffing boxes and glands on steam engines, pumps, and other machinery to reduce leakage. The waterproofing arises from the tar or pitch coating and the swelling of the natural fibers when exposed to moisture.

Oakum was most common from the 18th through the early 20th centuries, particularly on wooden sailing ships. With the advent of synthetic sealants, elastomeric gaskets, and modern shipbuilding methods, its use declined. Today it survives mainly in traditional ship restoration, certain forms of wooden-boat construction, and craft contexts.

Safety and handling considerations include the sticky, messy nature of tar-treated fibers and potential tar odors or fumes, so proper ventilation and protective gloves are advisable when working with oakum.