River monitor River monitors were heavily armored , and normally mounted the largest guns of all riverine warships . The name originated from the US Navy ' s , which made her first appearance in the American Civil War , and being distinguished by the use of revolving gun turret s . On 18 December 1965 , the US Navy , for the second time in one hundred years , authorized the reactivation of a " Brown Water Navy " for riparian operations in South Vietnam . In July 1966 , Secretary of Defense Robert S . McNamara authorized the formation of a Mobile Riverine Force ( MRF ) ; a force , that would bring back the heavily armored single turret " River Monitor . " River monitors were used on inland waterway s such as rivers , estuaries , deltas and lakes . Usually they had a shallow draft which was necessary for them to be able to operate in enclosed waters ; but their displacement , size and draft varied depending on where they were used . Most river monitors were lightly armored although this varied , with some carrying more armor . Exceptional examples , however , most notably the Royal Navy ' s Lord Clive Class monitors , which could operate in coastal or certain riparian / esturine situations , bore extra - thick armor plating and heavy shore - bombardment guns , up to a massive 18 - inches ( 457 mm ) in size . Typically , however , river monitors displayed a mixture of gun sizes from 3 - inch ( 75 mm ) to 6 - inch ( 152 mm ) , plus machine gun s . This type of vessel overlaps with the river gunboat . Asia . On Asian rivers , large river monitors were first used on the Amur by the USSR and Japan . They were up to 1 , 000 tons displacement , armed with 130 mm guns . During the Vietnam War , the United States Navy , in conjunction with other riverine craft , commissioned 24 " Monitors " , ten of which mounted a single 40mm cannon in a Mk 52 turret , eight which mounted an M49 105mm Howitzer within a T172 turret , and six " Monitors " which mounted two M10 - 8 Flamethrowers from two M8 turrets located on either side of the vessel ' s 40mm cannon . Referred to as " River Battleships " by their men , they provided the firepower of the Brown - water navy . Europe . On the Danube , river monitors were employed during World War I by Austria - Hungary , then by Hungary , Romania and Yugoslavia ( approximately 500 tons displacement ) . Smaller monitors ( 70 - 100 ton displacement ) were used by Poland in 1939 on the Pripyat River and by the Soviet Union in 1941 on the Pipyat and Dnepr rivers . Hungary used smaller monitors PM - 1 class and lightly armored fast patrol crafts Őrnaszád class during World War II ( 1941 : Balkans , 1944 : Hungary ) as scout , fire support , fast minelaying PAM - 21 class and anti - aircraft ships . United States . River monitors were used during the American Civil War , playing an important role in the Mississippi River Campaigns . They also played a role in the Battle of Mobile Bay . The American Civil War river monitors were very large , weighing up to 1 , 300 tons . During the Vietnam War , the US Navy ' s " Brown Water Navy " , operated its " Monitors " as part of their " River Assault Flotilla One " , which " initially " consisted of four " River Assault Divisions " ( RAD ) ; with " RAD " 91 containing 3 Monitors , " RAD " 92 having 2 Monitors , " RAD " 111 having 3 Monitors , and " RAD " 112 operating 2 Monitors . The Vietnam " Monitors " were divided into two programs ; program 4 would consist of the 40mm gun Monitors , while the later program 5 would entail the eight " Monitor ( H ) " Howitzer versions , and the six " Monitor ( F ) " Flamethrower models . All of the Monitors were converted from World War II 56 ' long all steel " Landing Craft Mechanized " ( LCMs ) Mk 6 ' s . When completed , they were 60 ' long , 17 ' wide , with a draft of 3 ' , had two screws driven by two Gray Marine model 64NH9 diesel engines , could do 8 . 5 knots and were manned by usually 11 or more crewmen . They usually carried about ten tons of armor . References . Carrico , John M . " Vietnam Ironclads . " 2007 ; Brown Water Enterprises . ISBN 978 - 0 - 9794231 - 0 - 9 . Friedman , Norman . " U . S . Small Combatants : An Illustrated Design History . " 1987 ; United States Naval Institute . ISBN 0 - 87021 - 713 - 5 . Kramer , Charles ( Illustrator ) . " Popular Mechanics " . Magazine , 1967 ; Charles Kramer of Zik Assoc . , Ltd . cutaway view of Vietnam " River Battleship . "