One use of the term "gunboat" is to refer to the PBR (Patrol Boat River) boats deployed by the U.S. Navy since the 1960s. The boats themselves are military adaptations of commercial boats that were designed and manufactures by United Boatbuilders of Bellingham, WA. These boats would usually be manned by four men stationed at the cockpit, bow, stern, and mid-section.
Gunboat warfare was new to the U.S. Navy when riverine warfare was introduced during the Vietnam War. Naval advisors quickly grasped the potential of the Vietnamese Navy's riverine fleet and decided that time was of the essence in manufacturing these new boats so that U.S. could maintain naval domination in Vietnam. So the Navy went shopping for a slick little commercial boat from United Boatbuilders and got to work with equipping it with armaments.
The first U.S.-made PBR "Mark I" made its debut in 1966 to form the foundation of the Navy forces in the Mekong Delta, with a length of 31 feet and a width of 11 feet. It could reach a top speed of 28 knots with its twin 215 horsepower GM diesel truck engines and Jacuzzi water jet propulsion pump. The Draft was 18" fully loaded. This boat would cost $75,000 (of course, at the value of the dollar in 1966)
In 1967, The Mark II came out which was longer, more powerful, and more armoured, with aluminum gunwales on the hull, to prevent the breaches and slices that rocks would often create on the hull that plagued the Mark I design.
After the Vietnam War, the Navy released the Mark III version of their PBRs, running with 450 horsepower engines, reaching a top 50 knots. The boat is ultra-manueverable as hairpin turns and dead stops from top speed are possible in a single boat length. All the power and manuverability at the cost of stealth, as this boat was the loudest of the three.
In addition to these three specifications, the term PBR is also used to designate a variety of different boats and designs.
My father was deployed on a PBR for some time when he was serving in the U.S. Navy during the war. At night missions, the boat crew would cut the engines and drift with the current. To detect the boats at night, VC guerrilla fighters would hide in the trees and shine a tiny red LED lamp on one side of the river, so that an observer on the other side could just see the lamp through the complete darkness. When a PBR would pass, the red light would be obscured, signaling that there is an object between him and the lamp - a boat. At such a signal, he would push down the detonator for a claymore mine designed to spray shrapnel into the hull of the boat and sink it. To avoid sinking, the PBR crew would drive the boat at full throttle, so that the boat nearly hydroplanes on the water and the water would not enter the hull.
This is just one of many imaginative strategies that this versatile craft would allow for its driver.