Chrysler Marsh Screw Amphibian

The Chrysler Marsh Screw Amphibian (MSA) was an experimental amphibian that moved on two large contra-rotating, screw-bladed pontoons instead of wheels or tracks. It was built for the US Navy under a 1962 contract funded by the Advanced Research Projects Agency (ARPA). The requirement was for a vehicle to carry six combat-equipped men and a driver through fresh and salt water, beach sand, rice paddies, swamps, mud and bogs, and occasionally across paved roads. The prototype was completed in January 1963.

Design

Each pontoon is an aluminium drum with a spiral blade, running the full length of the vehicle. Turning the two screws in opposite directions drives the vehicle forwards or backwards. On a hard surface, disengaging one screw and spinning the other makes the vehicle crawl sideways. The body, rotors, engine block and transmission housings were all aluminium. A water-cooled slant-six engine of 116 hp drove the rotors through a three-speed transmission, with an electric clutch-brake steering system and a chain-driven final drive. The vehicle was about 4.2 m long and 2.5 m wide, with 20 in (51 cm) of ground clearance. It weighed about 1,300 kg as tested with driver and fuel, and its rated payload was 1,000 lb (about 450 kg). Design speeds were 20 mph in snow and 8 mph in water.

Testing

After Chrysler's own 100-hour trial, the MSA went through trafficability tests at the Army Engineer Waterways Experiment Station (WES) and a three-week Marine Corps evaluation in spring 1964. The US Army then ran a military-potential test in Virginia and Louisiana in late 1964. In this test it was compared with an M113 carrier, and a Thiokol Swamp Spryte was tested alongside.

The Army found it at its best in very soft, liquefied mud, reaching 12.5 mph (20 km/h) loaded and 14.4 mph empty on mud flats. It also handled floating grass mats and weed-choked waterways that defeated the far heavier M113. In open water it made about 8 mph (13 km/h). On firm, dry soil the screws had no lubrication and the vehicle could barely move, and steering became erratic on uneven ground. WES recorded an average top speed of only 1.6 mph on soil test lanes. Maintenance demands were judged excessive. Troops riding next to the engine and exhaust suffered from the heat, which would cut the practical load to four men in hot climates. The vehicle could be carried as a sling load under a CH-34 helicopter. The Army concluded that it had military potential only in open or heavily vegetated water and very soft mud.

Chrysler later applied the screw principle to the much larger Riverine Utility Craft of 1969.