Caulking succeeds or fails on the condition of the plank edges, not on the material driven between them. A seam with sound, square edges will hold caulking for years. A seam whose edges have been rounded by successive re-caulking will not hold it properly no matter how skilfully it is driven — and the honest remedy at that point is edge repair or plank renewal.
What caulking actually does
In a carvel hull the planks sit edge to edge, and the seam between them is deliberately shaped so it is slightly open on the outside and tight on the inside. Fibre — cotton, oakum or similar — is driven into that opening under controlled compression. Three things follow. The seam becomes watertight. The planking is wedged apart very slightly, tightening the whole hull against its frames. And the hull gains stiffness, because the caulked seams resist the shear between adjacent planks.
That last point is why caulking is structural. A hull whose seams are failing is not merely leaking; it is losing stiffness, and it will work more, which opens the seams further.
Seam preparation is most of the job
- Rake out the old materialCompletely, without gouging the edges. This is slow, careful work and it sets the ceiling on everything that follows.
- Inspect the edgesSquare, sound edges can be caulked. Rounded, crushed, split or soft edges cannot, and pretending otherwise wastes the whole job.
- Repair what needs repairingEdge repairs, graving pieces or plank renewal where required, as covered under hull repair and caulking.
- Clean and dryCaulking into a damp seam traps moisture against end grain — exactly where decay starts.
- Drive the fibreTo a consistent depth, with consistent compression, leaving room for the stopping above it.
- Stop and finishSeam compound over the caulking, faired, then coated as the hull’s system requires.
Depth and compression are the craft
Too little and the seam leaks. Too much and the caulking forces the planks apart, straining fastenings and sometimes splitting plank edges from the inside. This judgement is why caulking is a specialist trade, and why it is not sensible to have it done by whoever is available.
Doing it on a hull that has dried out
A hull sitting on the hard has shrunk. Its seams are wider than they will be once it is afloat again. Caulking those seams as if the width were permanent means that when the planking swells, the caulking has nowhere to go, and it pushes the planks apart. The result is a hull that leaks worse after the work than before, sometimes with sprung fastenings as well.
Experienced hands allow for this. The rule the desk applies is simple: caulking is judged against what the seam will be when wet, not what it is when dry, and where a hull has been up a long time in strong sun the caulking work is scheduled late in the programme rather than early. The related behaviour after relaunch is described in the article on relaunch and take-up.
Materials
Cotton, oakum and combinations of the two remain standard, chosen by seam size and by where on the hull they sit — finer material in tighter topside seams, heavier material in wider seams below the waterline. Above the caulking sits a stopping compound whose job is to keep water out of the fibre and to fair the seam for coating. What matters more than brand choice is compatibility: the stopping must be suited to whatever coating goes over it, and to a hull that will move.
What determines service life
Edge condition at the time of work
The single strongest predictor. Good edges, long life. Worn edges, short life, regardless of skill.
How much the hull works
A vessel driven hard in a seaway flexes more, and seams open sooner than in a boat used gently.
Drying cycles
Repeated hauling and drying is hard on seams. A hull kept afloat and used regularly is kinder to its own caulking than one that alternates.
Fastening condition
Slack fastenings let planks move relative to their frames, and moving planks destroy seams. Caulking over a fastening problem does not last.
The diagnostic question
When a hull leaks along its seams, the question is never “how do we caulk this?” — it is “why did the seams open?” Sometimes the answer is age and fair wear. Often it is fastening wastage, hull distortion from poor support on the hard, or a drying cycle the vessel should not have been put through. Answering that question first is what separates a repair that lasts from one that returns at the next haul-out, and it is the reason inspection precedes quotation under survey and condition reports.