Complex sentence: In 1977 the main engine suffered serious damage while assisting a ship named Hilda Marjanne in the St. Clair river so the engine was changed to a Caterpillar D399 V16 turbo-charged diesel engine that produces 1100 hp at 1200 RPM.
Split into simpler sentences: In 1977 the main engine suffered serious damage while assisting a ship named "Hilda Marjanne" in the St. Clair River. This led to the engine was being swapped out for a Caterpillar D399 V16 turbo-charged diesel engine that produces at 1200 RPM.