For surface running, the boats were nearly all powered by two 800-brake-horsepower (597 kW) Vickers two-stroke eight-cylinder diesel engines, each driving one propeller shaft.

[3] G14 was initially powered by FIAT diesels, but these proved unsuccessful, and were replaced by the standard Vickers engines.

It was originally intended to fit more efficient four-stroke MAN and Sulzer diesels to some of the class, but the outbreak of hostilities rendered such plans impossible.

On the surface, the G class had a range of 2,400 nautical miles (4,400 km; 2,800 mi) at full speed.

[5] A total of 14 boats were built at four yards: G1 to G5 by Chatham Dockyard, G6 & G7 by Armstrong Whitworth, G8 to G13 by Vickers, and G14 by Scott's on the Clyde.