Arthur's ship is mentioned several times in the course of the tale of "Kilhwch and Olwen."  Its name was Prydwen, and under that appellation it is alluded to by Taliesin in his Preidden Annwn, the Spoils of Hell.  In that mystical poem, which appears to be full of allusions to traditions now no longer intelligible, various expeditions, consisting of as many warriors as would have thrice filled Prydwen, are represented as setting forth on different enterprises, from each of which only seven returned.
    The ancient chroniclers speak of these treasures of Arthur's with due reverence.  Sometimes, however, they bestow the name of Prydwen on his shield instead of his ship. (Mabinogion, Guest, 1910)

    Davies thinks that Prydwen means "the ark," from Prûd = κοσμος, and Wen, a female: the Lady of the world.  I am more disposed to think the bard means Britain, and that the Druids adorned their own island with a name which has since been universally adopted.  In the poem Mic Dinbych, Ysgrifen Brydain is translated, the writings of Britain; but a note suggests, "The writings of Prydain, who was the same as Hu-God."  The two words are certainly interchangeable; for in the preceding stanza, Britain is written Prydain, which is surely the same as Prydwen.—Davies' Celt. Myth. p. 510. (The Doctrine of the Deluge, Harcourt, 1838)