Complex sentence: Defense against torpedo boats was provided by twenty-two 51-caliber five-inch guns mounted in individual casemates in the sides the hull; these proved vulnerable to sea spray and could not be worked in heavy seas.
Split into simpler sentences: Defense against torpedo boats was provided by twenty-two 51-caliber five-inch guns mounted in individual casemates in the sides of the ship's hull. Positioned as they were they proved vulnerable to sea spray and could not be worked in heavy seas.