During the Second World War the Navy developed a new technology to find enemy submarines underwater. This was called ‘ASDIC’, and was based on the understanding that sound signals could be transmitted into the water and any echo would be heard if the sound wave hit a solid enough target. The echo could be detected by a hydrophone and displayed on a suitable display screen, showing distance from sound source to sound echo and also the direction that the sound source was sent away.
After the war this technology was adapted; first by sending the sound signal straight down to be echoed off the bottom. This was the origin of the early electronic Depth Finders. But the same concept could be adapted to echo off smaller softer targets such as fish, simply by changing the frequency of the signal.
The first Fishfinders were made by Kelvin-Hughes in the late 1950s and were quickly taken on by the fishing industry. The Cornish fleet was very quick to adopt the early devices and the Delmar 131 was soon fitted in many of the local boats.
A beam of sound is transmitted from under the hull at a frequency suitable for getting a good echo off a shoal of fish. The echo is displayed on a roll of paper showing the depth of the shoal and to some extent the density of the shoal and therefore the chances of making a good catch in the net.
Fishing boats are not noted for there tidiness and cleanliness and these electronic black boxes needed to be particularly robust. Today’s technology is considerably more advanced, but some Cornish boats are still using their early equipment, which still serves them well.
These two Fishfinders are Delmar 131 Sounders. Produced by Kelvin-Hughes, and manufactured in Japan in the early 1960s. It is still possible to get the paper rolls to keep them working!