The four diesel engines on a fleet boat did not turn the props. Each diesel only drove an electric generator. The electric power produced by the generators could be fed to electric motors which actually turned the prop shafts. It could also be used to charge the batteries. The more diesels powering the motors, the faster the boat could go. And the more diesels being used to charge the batteries, the sooner the batteries would be charged. So the slower the boat went while charging, the quicker the charge was completed.
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