A patrol boat (also referred to as a patrol craft, patrol ship, or patrol vessel) is a relatively small naval vessel generally designed for coastal defence, border security, or law enforcement. There are many designs for patrol boats, and they generally range in size. They may be operated by a nation's navy, coast guard, police, or customs, and may be intended for marine ("blue water"), estuarine ("green water"), or river ("brown water") environments.
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Per their name, patrol boats are primarily used to patrol a country's exclusive economic zone (EEZ), but they may also be used in other roles, such as anti-smuggling, anti-piracy, fishery patrols, immigration law enforcement, or search and rescue. Depending on the size, organization, and capabilities of a nation's armed forces, the importance of patrol boats may range from minor support vessels that are part of a coast guard, to flagships that make up a majority of a navy's fleet. Their small size and relatively low cost make them one of the most common naval vessels in the world.