Complex sentence: The snorkel device was still a new device, at that time only used by U-Boats; it was probably unknown to Launders and his crew, which is why it was thought to be a periscope.
Split into simpler sentences: The snorkel device was still a new device, at that time only used by U-Boats. It was probably unknown to Launders and his crew, which is why it was thought to be a periscope.