From YPPedia

A flagsit is a slang term used by players for an uncontested blockade. It refers to the fact that the contender must still field vessels and influence buoys in order to ensure a win. Buoys are sometimes called "flags" by players, and should not be confused with the grouping of crews also called a flag.

A typical flagsit uses the same resources as a small-scale non-sinking blockade. Jobbers must still be paid because a minimum crew is required for a vessel to influence buoys. Small ships such as sloops and dhows can be used, and there is no risk of sinking in a true flagsit.