Caulking (Calking)


Making the seams between the shell or deck planks of a ship watertight. Caulking is a technique whereby the gaps between the shell strakes or deck planks are sealed using fibres, cord and pitch.

We shall first look at the implements, machines and tools required.
We shall then explain caulking step-by-step.

Hook / Raking Iron: Caulking is undertaken for new-build and shall have to be repeated several times during the life of a ship. When re-caulking, the previous filler material shall have to be removed first. This can be done using a specially developed hook (raking iron) or similar blade-shaped implement

Caulking Cotton & Oakum: The initial filler material is applied after the seams have been neatly scraped out and are empty. The photographs show caulking cotton (marline) and "oakum".

Mallet: The caulk is applied using a specially developed mallet. The end of the mallet is fitted with horn pegs to strengthen it.

Caulking Irons: Caulking irons have between one and three grooves. There are also hardening irons. Hardening irons have a single, double or triple groove. Using the hardening iron the oakum and then the cord are driven evenly into the seam. A caulking iron is never sharp. A sharp iron could cut through the fibres or the cord.

Pitch: After the oakum and the cord(s) have been inserted the seam is covered (paying) with pitch


Removing the old 'oakum'; removal can be undertaken using a specially developed hook (raking iron) or, for very fine seams, a flat and sharp object such as a knife (image 1). For new-build the planks are provided with a caulking seam. See "Butt sawing".

Preparing the 'oakum'; the oakum first has to be rolled into a workable material. The fibrous oakum is rolled gently by an expert to create a loose-tension cord. (image 2, 3, 4 & 5)

Inserting the 'oakum' into the seam; the example in the photographs (image 6, 7, 8, & 9) shows the seam being sealed with oakum and a 3-ply tarred marline. This marline is coated in tar. The expert drives in 20 cm of oakum at a time, immediately followed by the marline, and thus completes the seam in steps.

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Tools Required

  • Hook/Raking Iron
  • Caulking mallet
  • Caulking iron

Materials Required

  • Caulking cotton (marline)
  • Oakum
  • Pitch