Complex sentence: Capps placed the rear superfiring turret, Number 3, closest amidships because it represented the greatest weight borne by the ship's structure due to its tall barbette and would be supported by the greatest amount of underwater volume available.
Split into simpler sentences: Capps placed the rear superfiring turret, Number 3, closest amidships. Since it represented the greatest weight borne by the ship's structure due to its tall barbette, this placement would allow it support by the greatest amount of underwater volume available.