It was eventually recognised by the Ministry of War Transport and its members were conscripted and received pay.
Thirteen of the 20 of the ships were initially built for the Ministry of War Transport (MoWT), which assigned them to be operated by various merchant shipping lines.
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Many of the ships were given names with the prefix "Mac-", in a reference to their designation as MACs; the "Empire" prefix was used on ships built by the Ministry of War Transport to distinguish them from pre-war privately owned ships.
In 1940, she was passed to the Ministry of Shipping, passing to the Ministry of War Transport in 1941 and being renamed SS Empire Chamois.
Tobruk was a 7,090 GRT cargo ship which was built in 1941 as Empire Builder by William Gray & Sons Ltd for the Ministry of War Transport (MoWT).