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.
The snorkel device was still a new device. This was at that time. This time was only used by U-Boats. It was probably unknown to Launders. It was probably unknown to his crew. His crew is. It was thought to be a periscope.