The technique used is called Caulking

 

The main material used is oakum.

 

Caulking is the action of putting oakum on board joints of the decks and then covered with tar, so it avoids water filtration. Caulking needs a different process depending on the type and thickness. When it is "soft" wood, a chisel is introduced among boards until they cause a gap; for hard wood it is necessary to make a bevel to the sides of boards of 2 cm, on the outside.

 

In the video you will see how is the caulking process on a boat.