Foghorns - Sounding Out
At many lighthouses this was a fog signal in the form of a fog horn or diaphone. For the sound of the fog-signal use is made of compressed air. In the engine room of a lighthouse there stands a number of powerful diesel engines that compress the air. The air is then stored in large tanks on the site of the lighthouse.
If the fog signal should be given during fog, the compressed air is led to the fog horn via a pipe system. These fog horns are usually close to the coast and naturally oriented towards the sea. A clockwork opened a valve and allowed the air, at pressure, to start a small motor, which in turn caused the fog horn to operate. This clockwork arrangement also opened and shut the valves which regulated the duration and frequency of the blasts.
Like lights, fog signals also possessed different characteristics for identification purposes, such as 4 blasts every 90 seconds. The diaphone worked on a similar principle except that the air from the operating valve caused a slotted piston to reciprocate in the cylinder instead of the motion being rotary as in the case of the fog horn. From the 1970’s onwards the large air driven fog signals were progressively replaced by small electric powered sound emitters. The fog horn at Skerryvore lighthouse was a small electric sounder, and sounded every 60 seconds.
In January 2005, the three General Lighthouse Authorities (GLAs) of the UK and Ireland issued a consultation document following a joint review of Aids to Navigation of the coasts of the United Kingdom and Ireland. The review addressed the current and future requirements of national and international vesselping. Each Aid to Navigation – light, buoy or beacon - was studied in isolation, as well as in relation to the other Aids to Navigation in its vicinity. As part of this process the Northern Lighthouse Board also reviewed the need for the provision of fog horns.
The conclusion was that audible fog signals had a significantly reduced role in the modern maritime environment, as a result of the widespread use of electronic position finding aids and radar, and the adoption of navigating bridges on many vessels. Accordingly, this review, the Board have taken the decision that all of Northern Lighthouse Board's fog signals are surplus to requirement, as such the few remaining systems have been decommissioned and the very last Scottish fog horn was switched off on 4 October 2005 at Skerryvore lighthouse.
This marks the end of a long history of operating fog signals, which in their day drove the introduction of advanced technology in the form of the internal combustion engine into lighthouses, and included very complex lengths of air pipe at sites such as Hyskeir and the Isle of May lighthouses.
For more technical information about fog horns, follow the link to Fog horns.
Hear the magnificent sound of the fog horn at the Sumburgh Head lighthouse on Shetland. Start the video below.
|Sumburgh Foghorn - video||- JJ Jamieson|