Cogs were the first vessels at the end of the first Millennium AD. They were later superseded by the similar Hulcs (pic.).
An important characteristic of these vessels are the palisades (my term) at the bow and stern that were important for protection against attacks by Viking long-boats, to which they were virtually invulnerable. The defenders could defend from a greater height.
Hulkesmouth (History of Shoreham)
by Andy Horton
Hulkesmouth Publishing Company
Hulc is the Old English word, from which hulk is derived. It means a towed ship (i.e. without
sails or oars). A cog, on the other hand, was a small single masted trading vessel. It was usually
rigged with a single square sail - Viking long ship style. It was, in fact a derivation of the long
ship, though usually much broader in the beam and with a raised forecastle and quarterdeck. True to
its origins it was most popular in Scandinavia and in the Hanseatic States.
From: Michael Snell <firstname.lastname@example.org>