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Depth reading wrong

laika

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6 Apr 2011
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7,319
Location
London / Gosport
My depth instrument (autohelm st50) has taken to mostly reading 1.5m, although it occasionally appears to show the right depth, especially in deeper water. The waterline offset is set to 0.7m and predictably with this set to 0 depth is .8m. We only came out of the yard last week so “something sticking out of the keel” is unlikely. The boat had been hardly used in nearly 2 years prior to that so the problem may have predated the yard trip. I’ve checked the connections and wiggling wires has no effect. Next step is anchoring somewhere to swim under the boat and maybe taking some wet and dry (used very wet) to the sensor but anyone seen this before or ideas about resolution? Having the boat lifted again to replace the sensor is going to be awkward
 

johnalison

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14 Feb 2007
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30,534
Location
Essex
Our sounder failed the first time we sailed in the Frisian islands many years ago. We learned to live with it until the end of the season by making sure that we passed the right side of every buoy.
 

tillergirl

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Joined
5 Nov 2002
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7,488
Location
West Mersea
Transducers can fail. I have had two over the years; one a Raymarine (airmar) one which just stopped working, the second another make that was ok until there was less than 2.5m when it just added a metre or so for no reason.
 

Refueler

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13 Sep 2008
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10,872
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Far away from hooray henrys
The AH ST50 of OP's should have Seatalk connectors ... but a clean up wouldn't go amiss I reckon ....

Quite often its just corroded plug / socket ... especially so many use the Co-Ax type connections - even inside a boat - they get salty air corrosion ...

Idea to unpug ... clean up and connect again ...
 

laika

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6 Apr 2011
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7,319
Location
London / Gosport
Idea to unpug ... clean up and connect again ...
Unplugged last night as part of connection checking. Contacts pretty much pristine.

I’ve realised how much I rely on the depth sensor during ordinary sailing. I normally have a “I don’t need a plotter!” bravado but I found myself uncomfortably relying on one tacking down the western Solent yesterday allowing far more safety room than normal. The plotter is on its way out too but that’s another story

I’m finding it odd that the depth failure is 0.8m rather than 0
 

simonfraser

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Joined
13 Mar 2004
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6,781
my 4 yr old sounder played up for a few hours last weekend and then sorted its brain, ok again now
investigate if it is a constant issue
 

KevinT1

Member
Joined
23 Nov 2011
Messages
540
Location
Home Noss Mayo, Boat Yealm
My ST50 failed over the winter , only found out on relaunch.

Like Laika’s It showed 1.5 meters and no changing of settings, checking contacts or substituting with a different display unit made any difference.

Bought a Raymarine / Airmar transducer that glues to the inside of the hull. You can check it’s all working correctly by hanging over the side of the boat before installing. ( transducers don’t like working in air )
 

oldmanofthehills

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13 Aug 2010
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3,495
Location
Bristol / Cornwall
We fairly recently had an echo sounder regularly fail and get stuck at 1.2m if there was any water, whatever the depth - it was an airmar sensor. As it fed into an oversized, and not updatable chart plotter the whole caboodle was renewed, though not before I ran aground on a falling tide due to chart and indeed buoyage deficiency and found myself leaning over alarmingly while waiting for the water to return.

I have replaced sensors in the past and have replaced the NASA head unit due to erratic display even when swapping between alternate pre installed sensors. A small boat is a harsh environment for electronics, if its not exposed to moisture and even salt water, it is banged up and down punching into a headsea.
 

Gwylan

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31 May 2007
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3,533
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Moved ashore
They will work in air. Just they report the distance out by 4. Sound travels in air approx 1/4 the speed in water.
Other problems, dirty bottom?
 

LadyInBed

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Joined
2 Sep 2001
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14,279
Location
Me - Zumerzet Boat - Wareham
I think that I'm right in saying that the depth offset is just a variable resistor, cleaning up its contacts by fully rotating it end stop to end stop a dozen or so times might help.
 

eddystone

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18 Aug 2013
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1,604
Location
SW Leicestershire
They will work in air. Just they report the distance out by 4. Sound travels in air approx 1/4 the speed in water.
Other problems, dirty bottom?
I am fitting new St50 and airmar in hull transducer to replace old spinning diode;as it’s in a boatyard what depth do you expect it to show if it’s hypothetically 1 metre to the ground - 4 metres or 0.25 metre?
 

eddystone

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18 Aug 2013
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1,604
Location
SW Leicestershire
I am fitting new St50 and airmar in hull transducer to replace old spinning diode;as it’s in a boatyard what depth do you expect it to show if it’s hypothetically 1 metre to the ground - 4 metres or 0.25 metre?
Sorry should have read your post properly - if signal travels in air at 25% speed in water must think it’s 4 times deeper therefore will report actual 1 metre as 4 metres?
 

laika

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6 Apr 2011
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7,319
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London / Gosport
Well it seems fine in depths greater than 12m…just not so good where I actually need it. Anchoring today based on gps and charted depth plus calculated tide which I’m not super comfortable with. Now to find where I can buy a lead line….
 

James_Calvert

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6 Oct 2001
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2,228
.... Anchoring today based on gps and charted depth plus calculated tide which I’m not super comfortable with. Now to find where I can buy a lead line….
If your anchor chain is regularly marked, that should help tell how deep it is. It gets lighter when the anchor touches the bottom. Not sure how well you can tell if you have to use a windlass...
 

oldmanofthehills

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13 Aug 2010
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Bristol / Cornwall
I have had a short leadline made with bit of scrap lead pipe, for the last 35 years. Excellent for creek crawling and in particular for checking that possible drying out positions dont have deep water too close to one side.
 

laika

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6 Apr 2011
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7,319
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London / Gosport
Curiously it seems to be ok in 12+m but below that mostly reads 1.5m. Seems few places are happy to hold me in slings for just long enough for the sealant to cure so maybe it';s lead line or KevinT1's suggestion of an in-hull. An in-hull for now would be cheaper than a lift that I don't need.

If your anchor chain is regularly marked, that should help tell how deep it is. It gets lighter when the anchor touches the bottom. Not sure how well you can tell if you have to use a windlass...
Not sure that's practical with a 20Kg anchor and 10mm chain. Plus I kinda want to know the depth *before* I drop it....
 

mike_1987

New member
Joined
28 Jun 2020
Messages
24
My brand new nasa target 2 sikaflexed to the inside of the hull has a similar issue
Was a bit of an embarrassment yesterday sat on the bottom for 2 hours when It said I had 80cm under the keel
Oddly enough it said I had 14m whilst I was sat waiting for the water to return
 
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