At the Battle of Sluys in 1340, the French fleet operated:
in accordance with the usual medieval tactics of a fleet fighting on the defensive. Quiéret and Béhuchet formed their forces into three or four lines chained together, with a few of the largest stationed in front as outposts.
The limitations of this tactic seem fairly obvious – severely restricted manoeuvrability – but, according this article on Medieval Maritime Warfare,
There are multiple contemporary accounts of ships being linked together with chains or cables in order to ensure formation integrity
Tying ships together was also used by, for example, the Vikings, to create a fighting platform (i.e. naval battles were often actually land battles fought at sea).
I’m thinking that one reason this tactic of tying ships together died out was the increasing use of cannons on ships, the first use in Europe believed to have been at the Battle of Arnemuiden in 1338.
When was the last battle in which ships were tied together?
The answer could be for anywhere in the world but it should be a battle involving at least 20 ships tied together. Note that this question refers only to ships of the same fleet being tied together – the use of grappling irons for boarding does not count.
As a supplementary question, were ships always tied together side by side or were they sometimes (for defensive purposes) tied bow to stern?