Patrol boat

A patrol boat is a relatively small naval vessel generally designed for coastal defence duties. There have been many designs for patrol boats. They may be operated by a nation's navy, coast guard, police force or customs and may be intended for marine (blue water) and/or estuarine or river ("brown water") environments. They are commonly found engaged in various border protection roles, including anti-smuggling, anti-piracy, fisheries patrols, and immigration law enforcement. They are also often called upon to participate in rescue operations. Vessels of this type include the original yacht (from Dutch/Low German jacht meaning hunting or hunt), a light, fast-sailing vessel used by the Dutch navy to pursue pirates and other transgressors around and into shallow waters.

They may be broadly classified as inshore patrol vessels (IPVs) and offshore patrol vessels (OPVs). They are warships typically smaller in size than a corvette and can include fast attack craft, torpedo boats and missile boats, although some are as large as a frigate. The offshore patrol vessels are usually the smallest ship in a navy's fleet that are large and seaworthy enough to patrol off-shore in the open ocean. In larger militaries, such as in the United States military, offshore patrol vessels usually serve in the coast guard, but many smaller nations navies operate these type of ships.

...
Wikipedia