This is a very important book, aiding the understanding of how U-Boats evolved into submarines, most WWII U-Boats really being submersible torpedo boats. The term ‘submarine’ has been misused since the first primitive submersible vessels. The snorkel offered the opportunity for a submarine to remain underwater from the start of the voyage to the return to home port. Purists will say that snorkel equipped U-Boats were still not real submarines but, in tactical terms, the snorkel made the submarine. – Most Highly Recommended