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 lawenforcement. 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.
During both World Wars in order to rapidly build up numbers, all sides created auxiliary patrol boats by arming motorboats and seagoing fishing trawlers with machine guns and obsolescent naval weapons. Some modern patrol vessels are still based on fishing and leisure boats. Seagoing patrol boats are typically around 30 m (100 ft) in length and usually carry a single medium caliber artillery gun as main armament, and a variety of lighter secondary armament such as machine guns or a close-in weapon system. Depending on role, vessels in this class may also have more sophisticated sensors and fire control systems that would enable them to carry torpedoes, anti-ship and surface-to-air missiles.
Their small size and relatively low cost make them one of the most common type of warship in the world. Almost all navies operate at least a few offshore patrol vessels, especially those with only "green water" capabilities. They are useful in smaller seas such as the North Sea as well as in open oceans. Similar vessels for exclusively military duties include torpedo boats and missile boats. The United States Navy operated the Pegasus class of armed hydrofoils for years, in a patrol boat role. The River Patrol Boat (PBR, sometimes called "Riverine" and "Pibber") is a U.S. design of small patrol boat type designed to patrol waters of large rivers.
Harbour security boat (PBI) - 4 newly built 80 ton class harbour security / patrol boats, and more are planned in order to take over the port security / patrol duties currently performed by the obsolete Shantou, Beihai, Huangpu, and Yulin classes gunboats, which are increasingly being converted to inshore surveying boats and range support boats.