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BrickTopHarry

What is the main source of unexpected depth sounder readings?

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I've noted from time to time when passing over well charted waters with depths of thousands of feet of sea water getting the occasional readings of 48 feet, 52 feet etc.  Instrument is an old Raymarine Bidata that otherwise reads entirely consistent with charts so I'm assuming there is some phenomenon being pinged.

Is it usually marine life?  School of fish, a whale or some dolphins?  Are there thermal phenomena that can cause this?  Or debris floating somewhat under the surface?

I couldn't find much with a superficial Google search.  Can anyone point me to a good article about this?

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There is much under the sea that the government doesn’t want you to know about. Just sail on and don’t worry about it.

Also, all those things you list can give false readings. I turn my sounder off when out in the deep sea so to stop annoying me with readings thousands of meters off.

And presumably the wildlife appreciates the silence too.

The manufacturers could help with better displays. An simple indication of reliability based on echo power and deviation could help.

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  Density and/or temp differences will give some rebound to your signal. Submarines use these a lot, which is why the USN sponsored a lot of oceanography.

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Nessie.

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47 minutes ago, Rawhide said:

Sunfish

But only each December

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4 hours ago, BrickTopHarry said:

I've noted from time to time when passing over well charted waters with depths of thousands of feet of sea water getting the occasional readings of 48 feet, 52 feet etc. 

Recreational sounders haven't the power to make it to the bottom.

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The keel.

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Navigator's situational awareness gone AWOL

Cfr Vestas in the Volvo Ocean Race

:P

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7 hours ago, Schnappi said:

seismic anomaly

Whales humping?

9F5AB1F1-BBB9-449C-B059-C700C3A52547.jpeg

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Boat heeled over so far transducer is out of the water.

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Ask naviguesser or crew on Vestas, they may have some ideas.

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Cthulu, the Ancient Ones and other eldritch horrors. 

You shouldn't even be asking about this, really. 

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For starters,  you are talking about a raymarine bidata..... i have the speed version of that instrument.  It has said that my top speed on an overpowered 20 foot catamaran has been just below the speed of sound. 

fast.jpg

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It could be the submarines the government is using to track you.

 

You don't know any Russians that meddle in elections, do you?

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I only get erroneous readings when I'm in shallow water and the bottom silt or sand is getting stirred up. 

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15 hours ago, See Level said:

Google  "Thermocline"

This.

Around here there are usually two levels of thermocline bounce back.  4' and 16'  The 4' bounce back is obviously erroneous when motoring along, but the first time I saw the 16' bounceback it caused me to slam the engine in reverse and do a 180° right then.   

I was new to the PNW and hadn't memorized the bottom features yet.  Now I know most of them due to the braille method of navigation I seem to practice regularly.

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Mimsy....

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Something really fucking big.

1 hour ago, 10thTonner said:

The turd your crew just flushed.

 

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Out here we get whales, as well as thermoclines and dense schools of fish.

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On 4/2/2019 at 5:07 AM, El Boracho said:

Also, all those things you list can give false readings. I turn my sounder off when out in the deep sea so to stop annoying me with readings thousands of meters off.

And presumably the wildlife appreciates the silence too.

I used to until a submariner pointed out to me that a sailing boat with the sounder off was invisible to them. Between the Brits, the Russians and the French there are quite a few of those around in our waters...

https://en.wikipedia.org/wiki/Bugaled_Breizh

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