In the The Thousand Isles the confusingly named Sloop of War refers to a two masted square rigged ship with a single gun deck that typically carried up to eighteen guns.
In build this ship is actually closer to a Brigantine then a Sloop, being simple in size and armament. The two square rigged masts though give the ship significantly different sailing characteristics, making it faster then the Brigantine but less maneuverable and notably worse at sailing into the wind.
These characteristics make it an attractive option as a naval or pirate ship but less so as a merchantman.