The hull and deck of a wooden ship is made up of individual planks fitted (ideally) close to each other. Without something closing up the gaps between the planking, it is more like a strainer than a skin. In the modern conception, caulking is usually thought of as a substance that the squeezed from a tube into a gap, where it adheres to the edges and cures into a rubbery substance. While this in theory could be done to seal the gaps between a ship’s planks, it would not accomplish the other function of caulking; that of stiffening the hull up.
The planking is securely fastened to the frames with large spikes, bolts, and lag screws. Regardless of the size and number of these, the forces applied to the hull by the sea will still cause the structure to bend and twist. Something is needed to make the planking act more like a solid panel than a collection of individual sticks.
The gap between the planks is specially shaped to be open at the surface, tapering down to have adjacent planks tight against each other where they rest on the frames or deckbeams. Into this tapering gap, traditional caulking is driven in with thin wide tools called caulking irons. Into the narrowest portion of the seam a fluffy rove of cotton is driven, its fine fibers packing tightly into the tight space. Atop this oakum is driven in. For centuries oakum was made of worn-out hemp rope picked apart (frequently in prisons) and soaked in pine tar; modern oakum is tarred jute fiber. Its coarser fibers pack into a denser wad in the wider section of the caulking seam.
The process of driving this caulking material into the seam acts like a wedge, generating tremendous pressure. This pressure not only creates a seal to keep the water out, but also locks each plank against its neighbor. This limits the ability of the planks to slide past each other, thereby stiffening the entire hull. After the caulking is driven tight, it is protected from the weather by a layer of putty or melted asphalt pitch, depending on whether it is in the hull or deck.
Caulking of course also must prevent water from leaking into the ship, either through the hull or deck. This requires meticulous attention to every nook and cranny that water might find a way in (Think Like A Raindrop!™). Fresh water leaking through the deck and topside planking promotes rot, which necessitates major repairs.
The process of caulking involves specialized tools and techniques, but it is one of the skills that the PSPG exists to preserve and teach. It requires a commitment to learning and practice to get the knack for it, but it is vital to the survival of our favorite Barkentine.
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