Complex sentence: Propulsion was provided by four Wolseley engines, mounted in pairs on each of two external gondolas, one fore and one aft, that drove swivelling propellers mounted on massive extension shafts.
Split into simpler sentences: Propulsion was provided by four Wolseley engines, mounted in pairs on each of two external gondolas, one fore and one aft. These drove swivelling propellers mounted on massive extension shafts.