Complex sentence: Between 23:42 and 23:44, "Helena" and "Boise" reported their contacts to Scott on "San Francisco" who mistakenly believed that the two cruisers were actually tracking the three U.S. destroyers that were thrown out of formation during the column turn.
Split into simpler sentences: Between 23:42 and 23:44, "Helena" and "Boise" reported their contacts to Scott on "San Francisco". Scott mistakenly believed that "Helena" and "Boise" were actually tracking his three destroyers who had been thrown out of formation during the column turn.