Oakum
Museum number 1985-228-3
This caulking material consists of tarred hemp fibres, taken from old rope, and would have been driven into the hull seams to keep them watertight.
When the oakum was wedged solid, the seams were covered with melted pitch.
Links:
[1] https://www.mcmanus.co.uk/category/collectiontype/history-working-life
[2] https://www.mcmanus.co.uk/category/objectcategory/marine-ship-building
[3] https://www.mcmanus.co.uk/category/objectname/oakum
[4] https://www.mcmanus.co.uk/sites/default/files/adlib/no_photo.jpg