Complex sentence: No secondary explosion was heard after the explosion of the first charge but after the sound of the second depth charge a third, "very violent", explosion was heard which threw up a large water column between the stern of "Christabel".
Split into simpler sentences: No secondary explosion was heard after the explosion of the first charge but after the sound of the second depth charge. A third, "very violent", explosion was heard which threw up a large water column between the stern of "Christabel".