Caulking is what keeps the seams of a wooden hull watertight, and on a traditional gulet it is a skill that has to be done by hand and redone over the boat's lifetime. Here is how the process works and why it still matters on modern wooden boats.
What caulking actually does
Wooden hulls are built from individual planks fastened to a frame, and no matter how precisely those planks are cut and fitted, there will always be a seam between them that can let water through. Caulking fills those seams with a fibrous material driven in tightly, then sealed, so the planks can still move slightly with the wood's natural swelling and the hull's flex without opening a path for water.
This is different from simply sealing a crack with a sealant. Caulking is a structural part of how a traditional wooden hull is built to work: the seams are meant to take up material and swell tight once the boat is in the water, and a skilled caulker prepares them with that behaviour in mind rather than trying to force a permanently rigid seal.
The traditional process, step by step
The process begins with opening and cleaning each seam, removing old, deteriorated caulking material and any debris so the new material can be driven in against clean wood. A caulker then works fibrous material, traditionally cotton for finer seams and oakum for wider ones, into the seam using a caulking iron and mallet, packing it to a consistent depth and tightness along the entire length of the plank.
Once the fibre is properly set, the outer part of the seam is finished with a compound, historically a pitch or putty-based mixture and today often a flexible marine-grade sealant, which protects the caulking material and gives a smooth, paintable surface. The work is slow and repetitive by nature, since a full hull can involve hundreds of metres of seams, and consistency along every one of them is what actually keeps the boat dry.
Materials: cotton, oakum and modern compounds
Cotton has long been the standard material for finer seams on quality wooden boats because it swells evenly and holds a tight, consistent pack without excessive bulk. Oakum, traditionally made from tarred hemp or jute fibres, is used for wider seams, particularly lower on the hull or in older, heavier construction, where a bulkier material is needed to fill the gap properly.
Modern flexible sealants and compounds have largely replaced traditional pitch-based finishing materials for the outer seal, offering better adhesion and longer service life while still allowing the underlying fibre packing to do its job. Many gulet builders and restorers use a combination of traditional fibre caulking with modern sealants, keeping the structural logic of traditional caulking while benefiting from materials that hold up better over time in a Mediterranean climate.
Signs a hull needs recaulking
A hull that needs recaulking usually gives warning signs before it becomes a serious problem: visible gaps or cracking in the seam compound, damp patches or active dripping inside the hull that track back to specific seams, and a general increase in bilge pumping that cannot be explained by anything else. Older caulking material that has dried out, shrunk or lost its elasticity is one of the most common underlying causes, particularly after a boat has spent time out of the water and the wood has dried more than usual.
Regular haul-outs and hull inspections catch most of these issues early, when recaulking a section is a planned, manageable job, rather than after a slow leak has already caused damage to structural timber or interior finishes that costs far more to put right.
Why this craft still matters for gulets today
For a traditional gulet, caulking is not a one-time fix but an ongoing part of hull maintenance across the boat's working life, tied closely to how the wood moves with the seasons and with time spent in and out of the water. A well-maintained caulking programme protects not just watertightness but the structural timber itself, since a leaking seam that goes unaddressed can lead to rot and much more expensive repairs.
Yeke Marine Services Ltd. carries the Yeke Group's carpentry expertise, built up around teak decks, interiors and traditional wooden boat work, into caulking and hull maintenance for gulets and other wooden vessels, treating it as the specialised craft it is rather than a routine touch-up.