The three fishermen's vessel didn't have an Emergency Position Indicating Radio Beacon, which transmits a distress signal anywhere in the world through a satellite system, the navy said.
Meanwhile, the coastguard was not alerted to the Purbeck Isle's plight for seven hours as it was not fitted with an emergency position-indicating radio beacon or similar distress signalling device.
Sherman thought he was sending an SOS signal to the world by activating an "epirb" -- short for emergency position indicating radiobeacon -- which is a satellite-connecting beacon that resembles a floatable walkie-talkie.