Operation Substance was a British naval operation in July 1941 during the Second World War to escort convoy GM 1, the first of the series from Gibraltar to Malta. The convoy defended by Force H was attacked by Italian submarines, aircraft, and MAS torpedo boats.
The ships of convoy GM 1 sailed from the British Isles on 13 July 1941 as part of convoy WS (Winston Specials) 9C, and arrived at Gibraltar on 20 July. Ships of the Mediterranean Fleet operating from Alexandria began making heavy radio traffic in the hope of diverting attention to possible preparations for a major operation in the eastern Mediterranean. Eight Allied submarines were deployed off Italian naval bases; but the Italian fleet assumed the convoy was merely HMS Ark Royal flying-off replacement aircraft to Malta, and chose to remain in port. HMS Leinster ran aground while leaving Gibraltar on 21 July and had to return to port. The Perla-class submarine Diaspro found the convoy on 22 July and launched torpedoes which narrowly missed HMS Renown and HMAS Nestor
RFA BROWN RANGER was engaged in refuelling the convoy escorts sailing from Gibraltar on 21 July 1941
The convoy came under low level attack by nine Savoia-Marchetti SM.79 torpedo bombers coordinated with five CANT Z.1007 high level bombers. Four Fulmars met the torpedo planes head-on and shot one down before another SM.79 launched a torpedo which hit HMS Manchester before also being shot down. HMS Ark Royal launched seven more Fulmars which were unable to engage the high level bombers before they released bombs which failed to hit the convoyed ships. Three Fulmars were shot down. A later attack by two SM.79s sank HMS Fearless killing 35 of her crew. Another bombing attack near-missed HMS Firedrake causing damage requiring the destroyer to be towed back to Gibraltar. Bristol Beaufighters from Malta assisted Ark Royal’s Fulmars defending the convoy from these attacks.
HMS Cossack detected MAS boats 532 and 533 approaching the convoy after dark, but was unable to prevent them from torpedoing ss Sydney Star. HMAS Nestor towed the damaged 11,000-ton cargo ship to Malta
Seven empty ships sailed from Malta as convoy MG 1 on 23 July to be convoyed back to Gibraltar by Force H. One was damaged by an aircraft torpedo on the voyage west. HMS Ark Royal lost a total of six Fulmars defending convoy MG 1 and the Malta bound ships from Gibraltar and at least 12 Axis aircraft, in total, were destroyed by FAA fighters and the AA guns of the Royal Navy.
The six cargo ships of convoy GM 1 arrived in Malta on 24 July where they were observed by a CANT Z.506 reconnaissance seaplane escorted by 42 Macchi C.200 fighters. Malta launched 22 Hawker Hurricane fighters which shot down three of the escort without loss. An audacious attack on Grand Harbour by other MAS boats and manned torpedoes on the night of 25–26 July was thwarted by Ultra intelligence