 Specifically, the poem "Lausavisor" by Vígfúss Víga-Glúmsson describes a Knörr being used as a battleship.The fast design of Viking ships was essential to their hit-and-run raids.For instance, in the sacking of Frisia in the early 9th century, Charlemagne mobilized his troops as soon as he heard of the raid, but found no Vikings by the time he arrived.