Can a Tridata see a fish?

[richardabeattie]

When sailing in deep water my new Raymarine Tridata seems to be telling the truth most of the time but just occasionally and for no apparent reason it tells me I'm shallow water and sounds the alarm. A few seconds later all's well again. Could it be that a biggish fish has gone past the transducer?

And as a secondary dumb question: the manual says "Use the Trip or Reset buttons to switch the shallow alarm lock on or off". What does locking the alarm do?

 

[cobolt]

Could be a big fish, often also happens when you have sub-surface currents with big temperature differentials.

When sailing in deep water my new Raymarine Tridata seems to be telling the truth most of the time but just occasionally and for no apparent reason it tells me I'm shallow water and sounds the alarm. A few seconds later all's well again. Could it be that a biggish fish has gone past the transducer?

And as a secondary dumb question: the manual says "Use the Trip or Reset buttons to switch the shallow alarm lock on or off". What does locking the alarm do?

 

[onesea]

Could be a big fish, often also happens when you have sub-surface currents with big temperature differentials.

Following the local ferry does it for me...

 

[Ian_Edwards]

The false reading is most likely to a second or third echo, the sound has bounced from the seabed to the surface, back to the seabed and then to the transducer. The multiple echo then "confuses" the sounder.

When I'm sailing on the West Coast of Scotland, I often get a shallow water alarm, when I know the water is at least 100m deep!

 

[antaris]

I am getting the same effect at random locations (so I presumed there must be huge white sharks swimming underneath us!)

 

[MM5AHO]

I get this too, and have always assumed that its fish. Sometimes just a single "blip", sometimes must be several of them - a whole school perhaps. Happens for different levels, but most often about 2-5m deep (under the keel), and normally for only a minute or less. On rare occasions for 5 minutes.
Most common in mackerel season.

 

[richardabeattie]

Thanks to all. And the second question - why would I want to LOCK the shallow alarm? What gets locked?

 

[macd]

I am getting the same effect at random locations (so I presumed there must be huge white sharks swimming underneath us!)

I speak with little authority, but always understood that fish showed up on echo sounders mainly because of their (air-filled) swim bladders, which sharks do not possess. As to the general question, any air bubbles in the water...in swim bladders, turbulent wakes, or whatever, would be expected to show up on depth finders (lead lines excepted...), as might boundary layers.

 

[antaris]

I speak with little authority, but always understood that fish showed up on echo sounders mainly because of their (air-filled) swim bladders, which sharks do not possess. As to the general question, any air bubbles in the water...in swim bladders, turbulent wakes, or whatever, would be expected to show up on depth finders (lead lines excepted...), as might boundary layers.

Fair enough - I was only kidding. No way would I sail around shark-infested waters! I am lucky enough to reside in the Med... (which, of course, does host sharks - albeit not many of the people-eating type. Slightly off-topic here). Interesting details about the biology, btw.

 

[macd]

Fair enough - I was only kidding.

Got that, thanks.

I am lucky enough to reside in the Med.

Lucky? I'd call it rather damp.

 

[richardabeattie]

Great stuff but what about my second question - what does locking the depth alarm mean? Why lock it? What does it do or not do when locked?