I'm interested in naval warfare in the early and high middle ages, particularly on the Atlantic. Looking on wikipedia and other places it seems like Mediterranean navies continued to use galleys and tactics similar to those of Antiquity, with certain developments. But what about North and West Europe? What kind of ships did they use and how did they use them in battle? Also wikipedia mentions that cogs were able to defeat Scandinavian longships and that this signaled the type of ship to be used from then on. What allowed the cogs to defeat the longships, and why was this so important?
Disclaimer: This is off some research I did a few years back, so, some facts may be misremembered!
But what about North and West Europe?
They used galleys as well. While they aren't suited to sailing out into the open, rough waters of Northern Europe, they were perfectly fine at sailing along the coasts, just like they did in the Mediterranean.
In any case, beyond galleys, one of the most common medieval ship types was the cog. All sorts of other ship types show up in the records, but it can often difficult to classify them, as chroniclers often had no experience with sailing, so, the descriptions can be rather vague. It is pretty certain to say that the Cog, and ships like it, were one of the more important types used in Northern/Western Europe.
One of the main tactics in naval warfare of the period was to board your foe, so, height was a major advantage. If your ship was lower than your enemy, you would have a more difficult time boarding the opposing ship, and your lower position made you a particularly vulnerable target. One example, from the Orkneyinga Saga, shows that Vikings in the Mediterranean, on attacking a tall Arab vessel, found that “it stood so high that they couldn't reach the gunwale [The top of the side of the ship] with their weapons." (1) This is probably the answer to your question about cogs having a big advantage over traditional viking Longships. Amusingly enough, the Vikings in this record solved the problem by cutting a hole in the opposing ship's hull and boarding it that way.
For war, these ships usually had "castles" mounted on the stern and bow (Hence why later ships had sections at the front and rear called the forecastle and the aftcastle). They were essentially wooden tower-type fortifications for archers and other ranged-weapon wielding folks to rain down projectiles onto nearby enemies. They were generally not permanent parts of the ships, as they made them unwieldy, but were assembled before battle and removed after battle. Medieval commanders often chained their ships together, both to keep a coherent formation and to allow soldiers on one ship to assist others. Essentially, many of these large naval battles became land battles, except on the water. Of course, there was the added factor that fleeing from one, once a ship was locked together with a foe, was nearly impossible. Look at The Battle of Sluys for instance.
For further reading:
Lewis, Archibald and Runyan, Timothy. European Naval and Maritime History, 300-1500. Bloomginton: Indiana University Press, 1985.
Meier, Dirk. Seafarers, Merchants and Pirates in the Middle Ages. Translated by Angus McGeoch. Rochester, New York: The Boydell Press, 2006.
Rose, Susan, Medieval Naval Warfare, 1000-1500. Routledge, 2001.
Trim, D.J.B and Mark Charles Fissel. Amphibious Warfare 1000-1700: Commerce, State Formation and European Expansion. Lieden: The Netherlands. Brill Academic Publishers, 2006.