[TG] [TG11] ST 40 Bidata depth failure

[Olmate]

My 5 yr old ST40 display started becoming unreliable a few years ago and would read 0.5 or 0.6, often scrolling back to that from whatever true depth and then would start working again after an indeterminate time.
The failures have become more frequent until now it seems permanent.
Cleaning the Airmar through hull transducer on my trailer sailer seemed to help once but not again so I don't think paint or anything on it is the cause. It's had several varieties of anti foul and no antifoul with no direct correlation.
Does that sound indicative of anything to point at the transducer or display?
Surely mine isn't the only one to have ever done this.

[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

[Olmate]

Thanks Tom,
It must be older than I thought. Was replaced by previous owner.
It's a flat bottom lifting keel with a round Airmar through hull transducer a couple of metres forward of the keel.
Could power supply problems cause the symptoms?
"'the display's transmit frequency or output power has drifted over time so that the transducer's no longer so sensitive or similar''.- can you tell me more about that?
Graeme

[Tom - Raymarine - Moderator]

Hello Graeme,

A power supply problem would likely show no depth rather than an incorrect depth, I would say. Regarding the possibility of degradation of the sounder's performance over time, I'm essentially speculating about possible causes for reduced performance as a product has aged, in a simple system such as ST40. I've never had the need to look into exactly what might degrade over time in one of these instruments though. Signal propagation obeys an inverse-square law though, so if power/sensitivity declines one might expect longer-range echoes to be lost before closer ones and a bias towards close-in echoes to develop. Again, I'm just speculating.

Fundamentally, if it's not a keel echo then it's perhaps a fault in display or transducer and you should consider either getting them checked by your local dealer or replace one or both parts. There are no simple tests that can be done without specialist test equipment to confirm operation of either component, so testing-by-replacement is the standard approach with these simple instruments.

Regards,
Tom