The T86 76mm Gun Motor Carriage (Amphibious) was developed in an attempt to produce an amphibious version of the M18 Hellcat tank destroyer.
A number of different methods of floating an M18 had been examined during 1943 and at the end of the year the results of this Ritchie Project were examined. Most of the methods used by the Ritchie Project had involved adding flotation devices to a standard M18, but this approach was now abandoned. Instead it was decided to design an entirely new amphibious hull that would almost turn the M18 into a boat. Two prototypes were ordered, each with a different method of propulsion in the water. The T86 would use its tracks for propulsion while the T86E1 would have two 26in propellers. The best of these methods would then be used on the T87 105mm Howitzer Motor Carriage (Amphibious). All three were to be built by Marmon-Herrington, and the contract was issued in January 1944.
The T86 would use the normal M18 turret and would keep the lower part of the hull, as far as the base of the sponsons (just above the top of the tracks). A new light-weight floating superstructure was developed, with a straight edged bow. The new hull was 30 inches taller than the standard M18 hull, so the turret was raised by the same distance.
The T86 was tested early in 1944. Its land performance was almost identical to that of the standard M18. It could reach 4.6mph in water and could cope with some surf. It was built with the standard 14in track of the M18, although the original plan had been to use the 21in track from a M24 Light Tank (Chaffee).
The T86E1 entered testing late in April 1944. It was faster than the T86, with a top speed in water of 6.2mph, but the track propulsion system was still judged to be superior and was used on the T87. The T86E1 was later modified to only use one screw.
The biggest problem with the T86 was the reduction in visibility caused by the big forward deck on the new hull. The T86 had the front corners cut off the deck, vision blocks mounted in the corners and extra periscopes added in an attempt to fix this problem.
Work on both the T86 and T87 projects was cancelled at the end of the Second World War.