AC1 Sentinel

The Sentinel tank series was a remarkable one. The use of an all-cast hull was way ahead of design practice anywhere else and the easy use of heavy guns like the 25-pdr and 17-pdr was way ahead of contemporary thought. But the Sentinel series had little impact at the time for the examples made were used for training only.

The first design, known as AC1 (Australian Cruiser 1) was to have a 2-pdr gun and two 7.7mm (0.303-in) machine guns. It was decided to use as many components of the American M3 tank as possible to make it easier to obtain and produce replacement parts. The powerplant was to comprise three Cadillac car engines joined together and extensive use was to be made of cast amour.

A second model, known as the AC2, was passed over in favour of the AC1, which had armour ranging from 25mm to 65mm in thickness.

The first AC1s were ready by january 1942 and were soon named Sentinel. The whole project from theory to the first tank rolling out of the factory had taken only 22 months, which was remarkable in that all the facilities required to built the tanks had to be developed also.

Only a few AC1s were ever built as by 1942 it was realized that the 2-pdr gun would be too small to have any effect on enemy armour. Also the "hurried" design had some bugs that needed to be ironed out. Most of these bugs were minor, for the sentinel turned out to be a remarkably sound design capable of considerable stretch and modification.

The Sentinel AC3 was equipped with a 25-pdr field gun barrel to over come the problems of the 2-pdr. The 25-pdr was chosen because it was already in production locally, but again it was realized that against enemy tanks there would still be problems and the Sentinel AC4 was built with a 17-pdr anti tank gun. This was during mid-1943 and by then there was no chance of a Japanese invasion of Australia. American M3s and M4s were pouring off the assembly lines in such numbers that all allied countries could be equipped, including Australia, thus the Sentinel project came to a halt in July 1943 allowing the diversion of the industrial potential to other important tasks.