A strange thing

[Guest]

We were crossing the top of the Alderney race under motor on passage from Cherbourg to Alderney a couple of weekends ago in condiions of no wind and very little swell when I noticed the depth dropping.

Over about 15 seconds it dropped from 180' to 30' and kept on falling, I put the engine into reverse and came to a crash stop. At 10 ' the echo sounder alarm goes off and the depth drops to about 6'.

We are almost stationary, there was no major disturbance on the water and no signs of fish shoal. Took a quick look at our position on the chart - no obstruction within miles.

Moved to slow ahead and after about 15 seconds the depth suddenly increased to 180' or so

I guess from the first time I notice the depth falling we must have covered several hundred metres

Any ideas what caused this?

Martin Perry

 

[billmacfarlane]

The same thing happened to me a couple of years ago on passage from Guernsey to Cherbourg. I was rounding Cape de la Hague in a VERY thick fog but confident of my position when the echo sounder suddenly dipped from 50m to 3m. My first instinct was to think that I cut the corner too fine but it turned out afterwards that I was a mile offshore confirmed when the fog cleared. The explanation I was given rightly or wrongly was that there are 2 currents that meet , one cold and one warm. The echo sounders blips reflect off the boundary of the currents which is why you get such a shallow reading. I'd be interested if anybody else has an explanation.

 

[vyv_cox]

That would be my view as well, I've noticed the same thing where currents meet. We sometimes see the opposite also, in locks. As the paddles open and water enters the lock the depth increases from somewhere around 3 metres to about 30. After a few minutes it reverts to a more sensible figure.

 

[Guest]

Sounds like water temperature layering, or tidal swirls. Hiding under the temperature layer is a well-known technique for submarines to avoid detection by the opposition as the sonar waves can bounce off at the layer change. Or, so I have read in various novels. Prehaps, a real submariner can confirm.

 

[chas]

I have noticed a similar effect about three times this year entering Exmouth on an ebb. There should be about 14 metres at the harbour entrance and my sounder has suddenly gone down, once to 1.9m. Gets the collys wobbling but I suspect it is temperature inversion.

 

[Guest]

The temperature layers in water can be used for various effect on purpose and also not.
It is common for E/S to blip on the interface, but normally you would not be concerned as it blips up and down as it pentrates.... BUt occasionally the interface is so definite and defined that you get what you describe here.

A second factor that is used to commercial effect ..... a lower level layer exists normally at about 25 - 30 m and this does not normally affect E/S to much degree, but is used by Doppler speed logs when off the continental shelf etc. and in very deep water.

Layered water is like layered air .... more common than you realise. It is also one of the reasons that some commercials have different 'frequencies' on some gear - the frequency can alter whether it is affected or not.