Sumburgh Head Foghorn Sounds Again
Published: 20 February, 2015
Brian starts the engines to raise the required 25psi of air pressure The Sumburgh Head Foghorn, which has been out of use for nearly 30 years, was officially sounded today for the first time since 1987.
A group of spectators and media assembled at Sumburgh Head this morning to hear Brian Johnson explain the process of sounding the foghorn, and how he has been involved in the full restoration of the engines and foghorn, along with the team involved in the recent restoration and refurbishment of Sumburgh Head into a world class visitor attraction.
The Foghorn was built in 1905 and was first operational in 1906. The Northern Lighthouse Board holds the original plans for the structure, signed by D. A Stevenson. The system is powered by compressed air, generated by Alley & MacLellan compressors which are driven by the Diesel Kelvin engines in the Engine Room. (these replaced the original Crosley engines in 1952)
The “character” of the Sumburgh Head Foghorn is one seven second blast every 90 seconds. This was to distinguish it from other fog signals in the area. The character is controlled by an air driven clockwork mechanism which operates valves in the correct sequence, and at the correct time, to spin a siren rotor housed within the siren chest to its correct sounding speed.
The siren spins around to create the noise, amplified by the trumpet. At the correct time a cam operates a valve which causes air to lift a diaphragm valve allowing the full flow of stored air to pass through the rotating siren thus creating the typical fog horn noise or “blast” for the seven second period. Closing the diaphragm valve stops the sound after.
The engines are set at a speed which allows them to generate the correct amount of air during the silent period to be ready for the next blast. This air is stored in the three receivers in the engine room until needed. The air pressure required is 25psl before the blast, dropping to 15psi during the blast.
Originally there were eight receivers, but three were removed during automation of the station. The two external receivers at the base of the foghorn tower are unusable owing to corrosion and have been blanked out for safety. The system has been amended to operate on the remaining three receivers located in the Engine Room which are in good sound order and have been proof tested.
Originally there were more Siren Diaphragm fog signals than any other type in the Northern Lighthouse Board estate, but the one at Sumburgh Head is now
the only working example of its type left in Scotland - if not in the world. Other foghorns, such as the one at North Ronaldsay, can operate but are of a different type and system.
The sound from the foghorn echoed out to sea, sending a real thrill of excitement through those present, as the thunderous noise could be felt as well as heard!