Cruisers Forum
 


Join CruisersForum Today

Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 23-09-2019, 08:02   #1
Registered User

Join Date: Mar 2011
Location: Saint John, N.B. Canada
Boat: Catalina 400 2001 40 feet
Posts: 26
Depth instrument giving incorrect reading

At times when my depth reads for 350 feet (example in deep water ) , I know I am in 350 feet of water, my reading on my instruments might go to 4 feet or 7 feet, then it will go back to 350 feet what it should be. I thought my instruments might not be working correctly but it happened to another boat that was with me over the weekend. Does anyone have an explanation for this.
__________________

SeaDancer is offline   Reply With Quote
Old 23-09-2019, 08:06   #2
Are you serious?
 
sailorboy1's Avatar

Cruisers Forum Supporter

Join Date: Jul 2007
Location: On the boat somewhere
Boat: Hunter 410
Posts: 15,535
Re: Depth instrument giving incorrect reading

something floated under, like a plastic bag or seaweed
__________________

__________________
jobless, houseless, clueless, living on a boat and cruising around somewhere
sailorboy1 is offline   Reply With Quote
Old 23-09-2019, 10:37   #3
Registered User

Join Date: Nov 2012
Location: Long Beach, CA
Boat: Tayana Vancouver 42
Posts: 2,263
Re: Depth instrument giving incorrect reading

Sometimes fish, sometimes bubbles and sometimes multiple echoes from the bottom can cause those responses.
Tayana42 is offline   Reply With Quote
Old 23-09-2019, 10:54   #4
Registered User

Join Date: Nov 2013
Location: Nice, France
Boat: Hunter Marine 38
Posts: 632
Re: Depth instrument giving incorrect reading

This is inherent to echo sounders. They send out a pulse and wait for the echo coming back. If the water is very deep the echo comes back just after sending out the second pulse.

It then indicate depth based on the time difference between the second pulse send and the first pulse coming back. After all a pulse is a pulse for that machine.

Of course you can get false echos also from water layers of different temperature or salinity, etc.
sailormed is offline   Reply With Quote
Old 24-09-2019, 10:24   #5
Registered User

Join Date: May 2009
Posts: 29
Send a message via MSN to geoff3nebel Send a message via Skype™ to geoff3nebel
Re: Depth instrument giving incorrect reading

What make of depth sounder?
geoff3nebel is offline   Reply With Quote
Old 24-09-2019, 10:26   #6
Registered User

Join Date: Jan 2014
Location: Ft. Lauderdale
Boat: Watkins 29
Posts: 207
Re: Depth instrument giving incorrect reading

Bubbles! Air! You passed a wave that swept air under your hull or bubbles from some idiopathic source. Very common in sailboats where our transducers are mounted toward the bow! Nothing to worry about!
dennismenace111 is offline   Reply With Quote
Old 24-09-2019, 10:49   #7
Registered User

Join Date: Jan 2015
Location: Oz, PNG, Western Pacific
Boat: Adams 42
Posts: 72
Re: Depth instrument giving incorrect reading

I have noted that when sailing in DEEP water my depth sounder indicates a no echo situation, it still shows the last recorded depth. Usually that reading varies up to about 10 meters and (hopefully) always from a false echo.
Flotsam, fish, turbidity, temperature lenses, going astern seem to give you a false reading.
Frugal is offline   Reply With Quote
Old 24-09-2019, 11:48   #8
Registered User

Join Date: Dec 2012
Boat: Jeanneau SO45.2
Posts: 185
Re: Depth instrument giving incorrect reading

Quote:
Originally Posted by sailormed View Post
This is inherent to echo sounders. They send out a pulse and wait for the echo coming back. If the water is very deep the echo comes back just after sending out the second pulse.

It then indicate depth based on the time difference between the second pulse send and the first pulse coming back. After all a pulse is a pulse for that machine.
.

This. I used to see this regularly passing a 400-foot contour in Lake Champlain... after showing 399, the depth would steadily count down to 10' (when it was actually 410) because each echo came back just after the following pulse was sent out. Raymarine ST60. Always the same behaviour in the same place. It was weird/scary the first time, after that we'd just say that Nessie swam up to scratch his (her?) back on our keel.


A slightly more intelligent depthsounder would reduce the pulse rate (increase the interval between) as dept increased, but after 400', it wasn't much of an issue.



Radar units do something similar, changing pulse rate and duration with changes in range scale; the difference is that the range is changed manually, whereas most depthsounders do not have a "range" setting.
Redline452 is offline   Reply With Quote
Old 24-09-2019, 12:20   #9
Marine Service Provider

Join Date: Jan 2012
Location: Lake Ontario
Boat: Douglas 32 Mk II
Posts: 2,833
Re: Depth instrument giving incorrect reading

Quote:
Originally Posted by sailormed View Post
This is inherent to echo sounders. They send out a pulse and wait for the echo coming back. If the water is very deep the echo comes back just after sending out the second pulse.

It then indicate depth based on the time difference between the second pulse send and the first pulse coming back. After all a pulse is a pulse for that machine.

Of course you can get false echos also from water layers of different temperature or salinity, etc.
I disagree.

If the sounder has a max software limited range of 600 ft, and at 350 it occasionally drops to 4 feet and then returns to 350 ft, the cause is most likely loss of echo. This causes the AGC (automatic gain control) to increase.

This enables the receiver to see the transducer ring down at the end of the blanking period (duration the receiver is rendered inoperative after the transmit pulse).

Which is likely the duration of an echo from 4 ft in your case.

So in normal operation when there is a strong echo from a hard close bottom (say 50 ft), the AGC is low and the transducer ringdown at the end of blanking is less significant than the true echo which is correctly processed and distance reported.

As depth increases or bottom produces a weaker echo, the AGC increases until bingo, ringdown detected and minimal distance reported.

If the false echo was due to any other cause (debris, multiple echoes, echo from previous transmit cycle) the related distance would be random, and not always 4 ft.
ramblinrod is offline   Reply With Quote
Old 24-09-2019, 12:44   #10
Registered User
 
Nicholson58's Avatar

Join Date: Jan 2011
Location: Caribbean live aboard
Boat: Camper & Nicholson58 Ketch - ROXY Traverse City, Michigan No.668283
Posts: 4,476
Images: 84
Re: Depth instrument giving incorrect reading

False echo is most likely as noted by other posts. Is the transducer window clean?
Nicholson58 is offline   Reply With Quote
Old 24-09-2019, 16:48   #11
Registered User

Join Date: Jan 2017
Location: Fort Pierce FL
Posts: 138
Re: Depth instrument giving incorrect reading

I have a gadget that NEVER gives an inaccurate reading. It's called a "lead line".
conchaway is offline   Reply With Quote
Old 25-09-2019, 02:16   #12
Registered User

Join Date: Aug 2011
Location: West Sussex, United Kingdom
Boat: Tradewind 33, 33 foot, Parker 27 , 26 foot
Posts: 142
Re: Depth instrument giving incorrect reading

I have seen this a few times in depths that are well within the normal range of my depth sounder (20~30metres or so) it is usually caused by a thermocline, a marked temperature difference in the water at depth. During WWII , and I expect even in this day and age, submarines used to take advantage of them to sit undetected by sonar below the sharp temperature change which the sonar bounces off.
Martkimwat is offline   Reply With Quote
Old 25-09-2019, 07:48   #13
Marine Service Provider

Join Date: Jan 2012
Location: Lake Ontario
Boat: Douglas 32 Mk II
Posts: 2,833
Re: Depth instrument giving incorrect reading

Quote:
Originally Posted by Martkimwat View Post
I have seen this a few times in depths that are well within the normal range of my depth sounder (20~30metres or so) it is usually caused by a thermocline, a marked temperature difference in the water at depth. During WWII , and I expect even in this day and age, submarines used to take advantage of them to sit undetected by sonar below the sharp temperature change which the sonar bounces off.
Thermoclines occur at various depths.
ramblinrod is offline   Reply With Quote
Old 25-09-2019, 07:58   #14
Registered User

Join Date: Aug 2010
Posts: 1,202
Re: Depth instrument giving incorrect reading

Not limited to sailboats and not limited to deep water. Every boat I've owned has done this. 25......24......24....25.......4......24....25.

I'll zoom in on the chartplotter and look at contour lines, there is nothing at 4-5 anywhere near there. This occurs in heavily marked and travelled. If there were an obstruction someone would have hit it by now.

I assume it is a clump of seaweed or a large fish or a false echo. Usually whenI get a weird reading it also flashes as if even the gauge is confused by the reading.
Shrew is offline   Reply With Quote
Old 25-09-2019, 08:19   #15
Registered User
 
Mr B's Avatar

Join Date: Aug 2011
Location: Melbourne Australia
Boat: Paper Tiger 14 foot, Gemini 105MC 34 foot Catamaran Hull no 825
Posts: 2,166
Re: Depth instrument giving incorrect reading

Quote:
Originally Posted by conchaway View Post
I have a gadget that NEVER gives an inaccurate reading. It's called a "lead line".
I would like to see that working when the bottom is 12,000 feet, Hahaha
__________________

Mr B is online now   Reply With Quote
Reply

Tags
depth, men, rum

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Depth sounder reading really shallow depths when itís much deeper benzy Marine Electronics 33 25-09-2018 05:36
ST40 & C80 both showing fixed incorrect depth? danstanford Marine Electronics 3 03-09-2017 11:06
Protecting the Alternator Regulator from Incorrect Shutdown MarkSF Electrical: Batteries, Generators & Solar 30 01-03-2016 15:00
Incorrect Part Number in Manual MV-Romnya Construction, Maintenance & Refit 5 06-11-2012 11:42
Members List - Incorrect Data Wotname Forum Tech Support & Site Help 17 20-03-2011 05:15

Advertise Here


Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 21:32.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
×

ShowCase vBulletin Plugins by Drive Thru Online, Inc.