Boulton Paul's Turrets were Electro-hydraulic in Operation; Electric Motors located in the Turret drove Hydraulic Pumps that powered Hydraulic Motors & Rams. This was more effective than Electric Motors alone and did not require Power developed by the Aircraft's Engines as did the Hydraulic System utilised by the Nash & Thomson Design.
Rear Gun Turrets Halifax KN-W fellow Rear Gunner – smiling Flt Officer ‘Jock’ Jacks – RAF Elvington Station Gunnery Instructor The Halifax with the Switch to Boulton Paul T…
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I believe that the second half of the second sentence is wrong: the electrical power still has to come from somewhere, and that's from the engines, directly or indirectly.