Inside Traditional Ship Caulking Tools and Techniques

Last month we described the function and importance of caulking for the health of the ship. Now let’s discuss the materials and tools involved in traditional ship caulking. The whole process is an involved and nuanced craft that can only be learned by doing and practicing it, but there are many ways for new folks to get involved with no prior experience.

When planks are attached to the ship, the edges (ideally) meet tightly along the faces against the frames or deckbeams, with a bevel cut along one or both edges to create an opening of ¼” to 5/16”at the surface of the planking. The very point of this space is filled with caulking cotton; its finer fibers packing down tightly. Atop this, oakum is driven, with the tar and coarser nature forming a thicker wad in the wider gap.

Caulking cotton comes from the manufacturer essentially ready to use, as a strand of fluffy untwisted cotton fiber. Oakum, on the other hand, requires preparation before it can be used. It is compressed into bales for shipping, and the strand needs to be loosened and fluffed up in a process referred to as “rolling oakum”. This is somewhat similar to felting wool, but is a skill that can really only be learned first-hand.

If a seam has previously been caulked, the old material needs to be removed and the seam cleaned out before it can be re-caulked. This is primarily done with a tool known as a reefing hook, which has a flattened hook-shaped end sized to slip into the seam. It is tapped underneath the oakum or cotton with a plastic faced mallet, and using the long handle lifts the material out of the seam. It is important that all debris is picked and vacuumed out of the seam before re-caulking begins.

A good deal of skill and judgment goes into caulking a seam, adjusting how much cotton and oakum is driven in based on the shape of the seam, and to a lesser extent the variability of the rolled oakum. The fibers are tucked into the seam in a series of small loops, then driven into a tight wad between the plank faces. This is done using fishtail-shaped tools known as caulking irons. A set of irons has a range of edge thicknesses to match the width of the tapered caulking bevel at various depths. Caulking irons are driven with a particular type of mallet, with a head  shaped very much like a scaled-down croquet mallet.

On large ships with heavy hardwood planking like Gazela, an additional step is normally done to drive the caulking tighter, known as hawsing. For this process a large caulking iron attached to a long steel handle is guided down the seam, while a second person repeatedly whacks the iron with an oversize mallet known as a beetle.

Once all of this is completed, the oakum is protected from the weather by pouring melted asphalt pitch into the remaining gap between deck planks, or in the case of hull planks by squashing in a soft oil -based putty. 

Traditional caulking is one of the esoteric skills that PSPG exists to preserve and teach. There are aspects of the process that are accessible to folks of nearly every level of ability. If you are not up for swinging a hawsing beetle, you might find rolling oakum just for you!

Email volunteer@gazela.org to pitch in.