Complex sentence: The court martial took place some six years after the grounding and the board blamed the master's mate and the pilot, but he had died while a prisoner in France, and the pilot "showed signs of madness".
Split into simpler sentences: The court martial took place some six years after the grounding and the board blamed the master's mate and the pilot. However, by that time the master's mate had died while a prisoner in France and the pilot "showed signs of madness".