Clan Forbes had 20 corrugated furnaces with a combined grate area of 402 square feet (37 m2) heating five single-ended forced draught boilers with a combined heating surface of 17,780 square feet (1,652 m2) that supplied superheated steam[2] at 220 lbf/in2 to a pair of three-cylinder triple-expansion steam engines.

J G Kincaid and Company of Greenock built the four engines, whose combined power was rated at 1,370 NHP.

[1] On 16 August 1940 Clan Forbes was damaged by bombs in a Luftwaffe air raid whilst berthed at Port of Tilbury.

[2] In November 1940 she was one of the three merchant ships that took part in Operation Collar, a convoy to supply Malta and Alexandria.

She spent some of her time disguised as the submarine depot ship HMS Maidstone, having been fitted with a dummy funnel.