- "When we take them by surprise, our rams splint and crack against their hulls, carved from rare hardwoods of the Summer Islands and fitted together without a single nail. Worse, every swan ship carries a company of archers armed with goldenheart bows, which can hurl the shaft hard enough to pierce steel plate."
- ―Euron Greyjoy
A swan ship is a large ocean-going vessel designed and built in the Summer Islands. They are called "swan ships" because their prows are carved in the shape of a swan's head and neck.
Every swan ship always keeps a company of archers aboard for defense against pirates, armed with powerful Goldenheart wood bows, capable of piercing steel plate. For this reason, Euron Greyjoy describes the ships as "tough prey".
In the books
Summer Islander swan ships are so-named for their large sails (which look somewhat like wings) and their prows which are carved in the shape of swan heads.
Summer Islanders consider women equal to men, and many of their ships have female captains.
Swan ships are smaller than full war galleys, but they are easily the best in their size class. They are built out of sturdy rare hardwoods from the Summer Isles, making them very tough while still very fast and maneuverable. Due to prior attacks from slaver raids thousands of years ago, the Summer Islanders always keep a company of well-equipped archers on each of their ships for defense. With good winds they can easily just outrun threats on the open sea, or outmaneuver and fight even larger ships. In becalmed waters, however, they are helpless against a full sized war galley.
Fundamentally, it is stated that swan ships are not meant to be true "warships", because the Summer Islanders are not warlike: they are sturdy, fast, and well-defended vessels for trade and exploration. They aren't even as large as war galleys, though they can fend one off with good wind in their sails. Therefore it is a bit of an unfair comparison to say that aren't quite as capable as full war galleys: they're not war galleys, they're smaller trading ships of such high quality that they can actually "play up" against war ships of larger size classes.
The Summer Islanders are great mariners with a better knowledge of cartography than many other peoples; combined with the sturdy construction of their swan ships, this means they are unafraid to sail them over the open ocean. Most other trading vessels stay relatively close to the safety of the coasts, but the Summer Islander swan ships will travel in a straight line to their destination for a shorter voyage, with no fear that they will become lost beyond the sight of land or fear of large storms on the open sea.
There are three main isles in the Summer Islands, which from north to south are Walano, Omburu, and Jhala. Walano is the most populous but Jhala is the largest. Three out of four swan ships, however, are built on Koj - a smaller island between Walano and Omburu. Koj has been the heart of shipbuilding and mapmaking in the islands for thousands of years, making it disproportionately important relative to its size - but nonetheless it still isn't counted as one of the three "main" islands in the chain.
In the novels, Samwell Tarly and Gilly make the long trip to Oldtown by first taking a local ship to Braavos, and from their booking passage on a Summer Islander ship named the Cinnamon Wind - which itself previously encountered Daenerys Targaryen at the docks of Qarth. Thus a significant part of their narrative is spent on a swan ship among a Summer Islander crew, giving a glimpse into their culture. As they approach Oldtown itself they are attacked by ironborn ships, now harassing the coasts of the Reach at the command of their new king, Euron Greyjoy. The ironborn are skilled raiders but the swan ship manages to fight off and evade them, and successfully reach Oldtown.