Complex sentence: The British Blanshard Pontoon stayed in British use until the late 1870s when it was replaced by the "Blood Pontoon" which returned to the open boat system which enabled them to be used as boats when not needed as pontoons.
Split into simpler sentences: The British Blanshard Pontoon stayed in British use until the late 1870s, when it was replaced by the "Blood Pontoon". The Blood Pontoon returned to the open boat system, which enabled use as boats when not needed as pontoons.