a skiff that they had managed to build with canvas and ropes that they had brought with them, and wood that they had cut in the forest where they had hidden ; they had made this boat waterproof by means of a thick layer of tallow. These prisoners, from Arras to the wood in which they had hidden near the coast, had walked only at night, using the moon and the stars to guide them. The administration of the Navy tested the canoe made by these four prisoners; 6 men embarked, steered it with oars and held the sea without a drop of water entering.

What they are describing there is a Currach.