False Depth Meter Reading

Jerry Albright · Post by **Jerry Albright** » Jun 29th, '08, 22:53

My Datamarine Depth Meter has started giving false readings, It will work fine for hours, and then go from reading say a 10' depth to reading maybe 2.5' to 3' bounce around there for a bit and then go back to a true reading.It has happened under sail and power.
Has anyone else had the same problem ? any ideas on what to check ?

Thanks in Advance
Jerry Albright
CD30C, B Plan

tartansailor · Post by **tartansailor** » Jun 30th, '08, 05:49

Same problem here last year.
Clean the transducer.

Dick

Warren Kaplan · Post by **Warren Kaplan** » Jun 30th, '08, 06:16

I get that like clockwork in some spots on my usual sailing route. I believe someone explained to me that different currents and temperature differentiations (stratified layers if you will) can sometimes do this. Maybe even heavy vegetation on the bottom.

I know it has to be something like that because as soon as I pass through these areas, the depth sounder just gets back to normal every time.

John Ring · Post by **John Ring** » Jun 30th, '08, 06:35

Warren Kaplan wrote:I get that like clockwork in some spots on my usual sailing route. I believe someone explained to me that different currents and temperature differentiations (stratified layers if you will) can sometimes do this. Maybe even heavy vegetation on the bottom.

I know it has to be something like that because as soon as I pass through these areas, the depth sounder just gets back to normal every time.

I just installed a brand new Raymarine ST60+ Tridata unit, and I find it does the same thing. It routinely shows me sailing in 2.5 feet of water just outside of Marblehead Hbr. My old Signet unit correctly showed it at 40-50'. The hull and transducer are very clean. The ST60+ appears accurate everywhere else, and unlike the old Signet, holds bottom very well in the 150-200' range.

John

Neil Gordon · Post by **Neil Gordon** » Jun 30th, '08, 11:31

Warren Kaplan wrote:I believe someone explained to me that different currents and temperature differentiations (stratified layers if you will) can sometimes do this.

I remember the same thing. I'd have to check with an official USN Sonarman, but I believe that it's the temperature layer that does it. When I was a signalman, we'd routinely get very long messages from anti-submarine destroyers, with way too many numbers reporting temp at varying depths. I also remember from one computer game or another that subs can effectively hide just under a thermal layer because sonar does funny things there.

chase · Post by **chase** » Jun 30th, '08, 11:40

I think I got some false readings in the CApe Fear River the other day. The water is deep in the channel, but I was getting shallow soundings. The Cape Fear has the normal river flow underneath the flood tide, moving over one another, and I thought that might be what was happening. Otherwise there is something seriously wrong with my piloting.

I lose soundings all together when a big tug goes by or a large sportfisher passes and goes back to plane from aeration or stirring up the mud bottom. There are other places where I mysteriously lose soundings consistently, like Adams Creek Canal in between Beaufort and Oriental.

Chase

SPIBob · Post by **SPIBob** » Jun 30th, '08, 11:42

I was a sonar technician in the navy and we routineloy measured the water temp at different depths (the instrument was called a bathythermograph). Water temps don't usually vary gradually with depth but rather remain fairly steady within layers called thermoclines and change abruptly at the edges of these layers. Sound tends to reflect at these temp boundaries and could be responsible for some false depth readings. (Submarines routinely submerge to just below one of these thermoclines to "hide" from the surface ship's sonar transmissions.)

Another possible source of momentary false depth readings is marine life. A school of fish would do the trick. We get frequent false readings of 2-3 ft. down here when a dolphin swims under the transducer and hangs out there for a few seconds. Apparently the frequency of the sound is within their audible range and they are curious to check it out.

mahalocd36 · Post by **mahalocd36** » Jun 30th, '08, 13:01

SPIBob wrote: Another possible source of momentary false depth readings is marine life. A school of fish would do the trick.

Freaked us out a bit - in the middle of the Gulf of Maine, miles from land, our depth sounder suddenly went to I forget - something around 10 feet. It stayed there a bit, varying a foot or 2 up and down, for what seemed like a long time as we double checked charts and chartplotters and the lack of land in view in any direction. Finally it sprung back to the few hundred feet it is. We figured it was a school of fish or a really big one hanging out there.

Melissa

Dixon Hemphill · Post by **Dixon Hemphill** » Jun 30th, '08, 14:41

I too get false readings quite often. Glad to learn that others do too.

Best solution if there is one is to consult the chart depths and proceed accordingly.

wsonntag · Post by **wsonntag** » Jun 30th, '08, 16:59

All suggestions made are good ones but the problem I had with a DMI unit was water intrusion in the transducer. Only fix, replacement, never a false reading since other than the occassional large unidentified creature silently slipping beneath the keel!

Bill Sonntag

Jerry Albright · Post by **Jerry Albright** » Jun 30th, '08, 23:30

Thanks for the quick response, I can always depend on this site and the collective wisdom to help me through the issues a boat owner encounters.

Thanks Again
Jerry Albright
CD30 , B Plan

The Cape Dory Board

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Thanks for the Quick Response