Caulking Decks and Hulls


Caulking is defined by driving oakum, cotton or general rope fibres in the seams of a ship's wooden deck or side / hull to make it watertight. It also plays a structural role in tightening up the hull / deck of a ship and reduces the longitudinal movement of neighbouring planks.

It is very easy to demonstrate the principles behind caulking a vessel, however, it is a skill where practice is  important but also individual vessels present their own issues. Usually you will have to warm to the task, understand which iron you  have to use, and how much oakum. There is a lot in the skill that simply comes from experience and new builds present different  issues to old boats.

The final pitching of the seam was not demonstrated because it was not occurring at the time of the  demonstration.


The process is demonstrated in the accompanying video clips with a step-by-step guide. The conversation with the boat builder, in this case, Chris Rees is unscripted and covers the technique from his experience.

The technique is very simple but the subtleties come with experience:

  1. Clean the seam to be caulked.

  2. Twist a length of caulking material (oakum or cotton). The length of material to prepare will increase with experience and  speed.

  3. Lay the material into the seam using the making iron and caulking mallet (some people use a normal wooden mallet but never  a hammer). It is tapping not hitting the irons. Wide seams may need ‘choking’ with a double gather of caulking material.  If seams are too tight they may have to be worked with the dumb iron first, this will open up the seam so caulking can occur.

  4. Seam is gone over with the hardening iron to firm up the caulking material in the seam and caulking is complete.

  5. Seam usually paid with pitch or tar (not demonstrated).

If a seam is stopped midway, leave the oakum attached if possible. If  you have to break off the oakum or run out and need to start a new bail / ball, twist the ends together so they become one.

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Tools Required

  • Caulking Mallet
  • Various caulking irons – making, hardening, dumb, cranked versions
  • Mallet
  • Box to keep the oakum or cotton clean and off the floor
  • Vacuum cleaner
  • Suitable scaffolding for the hull side if required
  • Personal Protective Equipment (PPE)

Materials Required

  • Oakum or cotton
  • Pitch if paying the seams