A High-speed echo-sounder recorder having seabed lock

An electro-mechanical recorder giving an intensity-modulated record on dry paper is described which indicates the presence of fish within 1? fathoms of the seabed (i.e. under the headline). Experiments were made with several possible types of triggered recorder to determine the best design, also with various signal delays required for seabed lock. Details are given of the moving-iron recorder, finally adopted, which displays a range of 4 fathoms with high accuracy across paper 6 in. wide. A magnetic drum storage system (described in an associated paper con cerning the complementary c.r.t. display), is employed. These displays form part of a comprehensive fish-detection equipment of improved acoustic performance, for use on distant-water trawlers.