How accurate is your depth sounder?

[Pkewish]

I have a reasonably new Nasa Clipper Duet and transponder. The other day, while motoring in Poole harbour, I flew over 0.2M but bumped on the bottom at 0.7M. I have adjusted the units offset in line with draught and the transponder points vertically down away from any sources of air bubbles etc. how accurate are these thing supposed to be, particularly at shallow depths?

 

[Lakesailor]

I don't know about the Duet. I had a NASA depth sounder which was pretty accurate. However it only reads from directly below. I was caught out on Windermere(which has a rocky bottom with some pretty big rocks) when the sounder showed 2.6 metres (4 foot fin keel) but I hit a rock.

A fish finder is better in my opinion. It shows the weed level. On Windermere the weeds grow about 4ft from the bed of the lake so when it's showing 1 ft under your keel it could really be 5ft. The fish finder shows the weeds for what they are.

Even at 20 ft it identifies weed

 

[LittleSister]

Tell me about it - we were firm aground while our NASA sounder was insisting there was 2.5 metres below the keel (swinging the lead showed about 10cm less than keel depth all round, and from the chart we knew there was nothing like 2.5 metres within half a mile!).

That said, most of the time it gives usefully reliable info.

Going aground is part of the fun here on the East Coast and in other muddy places like Pooole Harbour. On rocky coasts you have to be much more certain where you are, and leave a greater margin of safety. (Though the advantage of rocks is they don't keep moving around and changing depths like mud and sand tends to!)

Echo sounders can get confused, especially by soft muddy bottoms (and especially when you're stirring the mud with your keel or motor!), and adjusting the gain one way or the other (see the manual) can help get a better reading.

Part of the problem is that the sounder actually often receives blurred echoes (and different echoes of echoes, and reflections from shoals of fish passing by, etc.), which the electonics tries to narrow down and interpret as a single clear answer. The digital readout of modern sounders gives a false impression of certainty and accuracy. On old fashion 'whirling dial' type sounders you could see when the echoes were getting blurred or doubling up, and interpret or adjust gain accordingly.

I imagine / would hope (but don't know) that if you bought top of the range brand instruments, rather than NASA cheapies, their electronics would be more complex and sophisticated, and better able to resolve the uncertainties and automatically adjust the gain.

I have cursed my NASA sounder more than once for misleading me or starting to produce gobbledook at a critical moment, but overall they have been great - I've sailed many thousands of miles with NASA sounders on different boats and found them generally very reliable and good value for money.

 

[Daydream believer]

Years ago i changed my seafarer echo sounder for a more modern one
On a Stella the transducer is just slightly less than 1 metre from the bottom of the keel
I found that it did not work in shallow water- when i needed it - because the return echo confused the readout as it was too close to the sea bed so i went back to the bamboo cane when crossing the Ray Sand etc
The nature of the seabed also has an effect on readings. Ie mud or hard sand or rock , so one needs to have an idea what one is sailing over. Very silty disturbed water can affect readings.

 

[Ian_Edwards]

Echo Sounder actually measure time.
They measure the time it takes for the pulse of sound to go from the transducer to the seabed and back to the transducer.
The time is then converted to distance using the speed of sound in seawater, nominally 1,500 meters per second. In the real world the speed of sound changes with both temperature and salinity of the seawater.
A further complication is the amplitude of returned signal level which triggers the detection of the seabed. Different types of seabed will give different rise times for the returning pulse, hard rocky seabeds generally give a very fast rise time (a sharp pulse) and little ambiguity in timing. In contrast a soft muddy seabed will return a slow and often indistinct pulse, which will give rise to a larger uncertainty in the measured depth.

Off North West Scotland, where the water depth often exceeds 100m, I get frequent false readings of just a few meters, which set off my minimum depth alarm (set at 4m) when the water depth is well over 100m. Theses false alarms are caused by either multiple reflection from the seabed, or a returning pulse arriving after the time gate has closed (which corresponds with the maximum depth the Sounder is designed to measure) and triggers the seabed detection circuit in the subsequent measurement cycle.

So not a real answer to the original question, but because simple echo sounders usually have a fixed speed of sound and a fixed amplitude level for detecting the seabed, they give an approximate depth and the error you get with depend on the actually operational conditions.

Hope this helps.