Complex sentence: The crews were to abandon their submarines shortly before the collision with the viaduct, leaving the submarines to steer themselves automatically but during the passage from Dover, "C1" parted with its tow and arrived too late to take part in the operation.
Split into simpler sentences: The crews were to abandon their submarines shortly before the collision with the viaduct, leaving the submarines to steer themselves automatically. During the passage from Dover, "C1" parted with its tow and arrived too late to take part in the operation.