Complex sentence: On 12 July, on a return voyage, the escort carrier "Santee", providing air cover for the convoy, was relieved by "Core"; but before "Santee" departed the area, four U-boats were discovered in the convoy's vicinity.
Split into simpler sentences: On 12 July, on a return voyage,, providing air cover for the convoy, was relieved by. But before "Santee" departed the area, four U-boats were discovered in the convoy's vicinity.