In the book Gray Wolf Grey Sea the LI Brinker had both the diesels and electric motors on to give more additional speed. According to the book he disconected the electrics after 30 minutes contrary to Mohr's orders. A destroyer eventually forced them to submerge but Brinkers disconect left him enough juice in the batteries to stay down. Afterwards Mohr was mad but Brinker told him that engines can only do so much and someday you will not have me and you must listen to your LI. They were always great friends and Brinker was a great LI. He even rebuilt bearing out of cigaretepaper tinfoil! Thus you must be able to run both at the same time if you are willing to take a chance and give up precious battery charge.