Neil and David Hunter were travelling from their home in Mayland, Essex, across to Pembrokeshire, in Wales, when the engine on their £55,000 cruiser failed. The 56ft vessel, Persistent Whisper, which Mr Hunter, 47, built, drifted until it hit a rock and began taking in water at around 3.30am yesterday.

With the VHF radio broken, Mr Hunter dialled emergency services and managed to relay to the operator that his boat was sinking and that he and his son, 17, might drown before the signal broke up as they drifted out of range.

The operator, from Orange, the telecommunications company, surmised from the strength of the signal that the stricken pair were off the Essex coast and alerted Essex Police and coastguards.

Technicians were able to calculate their position to within five square miles by using the strength of the signal between a coastal mast at Holland-on-Sea and another at a pumping station in nearby Frinton-on-Sea. Coastguards then scrambled an RAF Sea King, two lifeboats and also put an alert out to shipping in the predicted area — about twelve miles off Walton on the Naze — to keep an eye out for the pair.

And it was a fishing vessel, True to the Core, which eventually spotted the father and son at 5am.

The pair, who were airlifted to safety by the Sea King, were flown to Colchester Hospital, in Essex, but were later discharged.

Jenny Thorpe, a spokeswoman for Orange, said: “We knew that the emergency call was received by two of our mast sites so we were able to triangulate the approximate position it came from.

“We can find out exactly how long it takes a call to reach a mast so that will tell us exactly how far away it is. When there is more than one mast which picks up a call you can then use the two bits of information to work out a position. We were able to give information to the emergency services within half an hour of the request coming in.”

A coastguard spokeswoman said: “Mr Hunter was not able to give much information other than he was sinking and drowning in a boat. We tried to ring him back but kept getting his answer machine. A couple of hours later a fishing vessel found the remains of the boat and a dinghy nearby with two people inside. A helicopter was sent to pick them up and take them to hospital.”

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