Development
In the immediate post-World War II era the US Army introduced the M41 Walker Bulldog into service to fill the role of a light tank. The lifespan of the M41 was fairly short; at 25 tons it was considered too heavy to be a true light tank, and had a rather short cruising range. Plans started to build an even lighter replacement mounting the same gun, resulting in the T-71 and T-92 test designs. Two prototypes of the 19 ton T-92 were later ordered. However, as the prototypes were entering testing, information about the new Soviet PT-76 tank became available. The PT-76 was an amphibious light tank, and soon there were demands that any U.S. light tank should be able to swim as well. The T-92 was already in the prototype stage and could not be easily refitted for this role, so the design of an entirely new system started as the XM551.
The vehicle designed to mount the gun had a steel turret and aluminum hull. Unfortunately, the armor was thin enough to be penetrated even by heavy machine gun rounds, and when hit by a rocket propelled grenade the vehicle would "brew up" due to the main gun propellant being stored in cardboard tubes. Like the M113, it was also highly vulnerable to mines.
Swimming capability was provided by a flotation screen, similar to that used by the World War II, amphibious DD Tanks. The front armor was overlain by a wooden "surfboard", actually three folded layers, hinged together. This could be opened up into a sloping vertical surface in front of the driver providing a bow of a boat hull, about level with the top of the turret. Fabric formed the rest of the water barrier, folding up from compartments lining the upper corner where the side met the top of the hull, and held up at the back with poles. The front of the "hull" was provided with a plastic window, but in practice it was found that water splashing onto it made this useless, and the driver instead had to take steering directions from the vehicle commander. The M2 Bradley adopted a similar solution, but dropped it with upgraded armor.
In the Vietnam War, firing the gun often adversely affected the delicate electronics, which were at the early stages of the transition to solid state devices, so the missile and its guidance system was omitted from vehicles deployed to Vietnam. The expensive missile ended up almost never being fired in anger, despite a production run of 88,000 units.
Read more about this topic: M551 Sheridan
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