[TG] [TG11] ST 40 Bidata depth failure

[Tom - Raymarine - Moderator]

Hello Olmate,

On an instrument depth system such as your ST40, the most common reason why a sounder would show a very shallow incorrect depth reading is if it's getting an echo from the fringe of the beam off the keel. The beam on these transducers is very wide and echoes from keel structure are quite common, especially either where transducers are mounted alongside the keel rather than forward, or where the keel is bulbed or similar.

What's the keel shape on your boat, is it fixed or lifting? If lifting, does the ST40 show the same thing if you pick up a mooring and then lift the keel?

As for why this has got worse over time, it could be that the display's transmit frequency or output power has drifted over time so that the transducer's no longer so sensitive or similar. Are you sure it's only 5 years old though? We retired ST40 in 2012 so we will have built it at least as long ago as this.

Regards,
Tom