[TG] [TG11] Depth value cycling up and down

[Tom - Raymarine - Moderator]

Hello benzy,

On yachts, this can sometimes happen when the sounder gets an echo from the fringes of the sound 'beam' off the keel or a daggerboard. For safety reasons the system treats the shallowest strong return as the bottom, and if due to a combination of depth, bottom material and structure, water conditions, heel, etc. etc. the keel echo is a similar signal level to the bottom echo, the keel echo will tend to get treated as the bottom. Once a strong signal is regained from the bottom, you'll get the correct depth again. The beam pattern from the instrument depth transducers is very wide and un-focussed, so it's relatively common for boat structure to be within the fringes of that beam.

Another possibility is that there's some noise/interference at the 200kHz frequency that the sounder operates which is being picked up and being treated as a bottom echo. This is hard to diagnose in instrument systems, but is a common cause of poor sounder performance in fishfinder systems. The source of interference could be an electrical system or acoustic noise at that frequency (on powerboats we often see hull vibration at sounder operating frequencies).

Where's the tranducer mounted, what sort of boat is it?

Regards,
Tom