Because of the extensive marshlands of South Vietnam, and the frequent difficulty of moving propeller-driven craft through water thick with grass, the United States and South Vietnam had to develop a river craft giving its own troops the mobility enjoyed by Vietcong using sampans. What they turned to was the air-cushion vehicle, a modified Bell Aerosystem craft which moved on a base of air about four feet thick. Its speed was up to 75 knots. United States Navy and Army Special Forces personnel used air-cushion vehicles to patrol extensive areas of the Mekong Delta. Generally, the air-cushion vehicles were plagued with problems, including heavy fuel consumption, noise, and frequent breakdowns.