Funny depth readings

[jimi]

Sailing around Berry Head a couple of weeks ago in c 40m of water I glanced at the depth and to my horror it read 2.8m and as I watched it went to 1.2m (I've got 1.6 offset), so I spun 180 started the engine & got the hell out of it. The depth almost immediately returned to normal. Checking the GPS chartplotter track later the spot whre I had spun was literally within feet of my track on the way out & the charts show the area as being clear of any obstruction. Could a shoal of fish done this or is it more likely to have been a submerged object of some sort?

Jim

 

[Guest]

a shoal of fish, weed, different water temperatures caused by a strong tide running....take your pick really. I have really only thought of echo sounders good for 2 things...
following a contour
telling that I have gone aground after the event

Pete

 

[vyv_cox]

Water layers at different temperatures seems the most likely off tidal headlands and suchlike. We know of various places where this invariably happens to us, setting off the depth alarm and frightening the crew!

One is the Maas entrance, dredged to a minimum depth of 20 or so metres. River water mixes with seawater and is presumably at a different temperature, there is always some turbulence and the depth alarm always goes off (2 metres) during the crossing.

The other is in almost every lock we go through. As soon as the level begins to change the depth will go either to zero or, very often, to a ridiculously high value, somewhere between 20 and 30 metres.

 

[Twister_Ken]

Apart from the factors others have mentioned, was there a large motor boat or ship nearby? Prop turbulence can last quite a long time and can confuse an echosounder. I've lost count of the number of times the Lymington-Yarmouth ferry has caused me a to do a quick double take.

 

[jimi]

No, the only other boat near me was a small sailing yacht about 50 yards away. I did notice though that he put in a quick tack in roughly the same place as I did my about turn. But being a bit short handed at the time I did'nt have the opportunity to give him a call on the VHF to ask why.

Jim

 

[Shanty]

A similar thing has happened to me several times sailing round Chanonry Point. Depth suddenly goes from about 35m to just over 2m. First time it happened, panic very nearly set in, until a dolphin surfaced alongside with a grin on its face. It had been sitting under the boat, in the beam of the echosounder.

Apparently, dolphins can hear 150kHz (the frequency at which most echosounders work), so maybe it thinks the boat is trying to communicate. I wonder what it thinks the boat is trying to say?

 

[yachtcharisma]

I'd definitely go along with the different "layers" of water at different temperatures theory, especially since you were rounding a headland. We have the same experience as Vyv Cox, again especially at a river mouth - where the river Tamar comes out into Plymouth sound it's about 30m deep, but very turbulent, with fast currents and those oily "slicks" in the water indicating upwellings. Depth sounder is best ignored in the vicinity, often deciding that we're in 1m one moment and 30m the next, and sometimes staying fixed on an apparently shallow depth for some time.

But there are perils in ignoring it too much - I managed to motor straight into the side of the (submerged) Cremyll Ferry jetty which sticks out into the river rather further than you might think through one day's rather blase overlooking of the depth sounder! Fortunately no damage done, but not a pleasant experience!

Cheers
Patrick

==================
Sailing a Corribee
vzone.virgin.net/patrick.fox
==================

 

[oldharry]

Echo sounders v Fishfinders

This thread highlights why I am sold on using a Fishfinder rather than an echo sounder. Its screen tells you exactly what is going on underneath, and it is able to discriminate between temperature layers, Dolphins, ships wakes, etc. All for much the same price as an echo sounder.

Its also a whole lot easier for following a depth contour, and with a little experience it is possible to tell the composition of the sea bed, making for safer anchoring, as mud, sand, rock or weed all give distinctive on screen patterns.

For (quite) a bit more money the screen will even give a 3d 'map' of the bottom - useful for Bilge keelers wanting to dry out over the tide.

It also finds fish.....

<P ID="edit"><FONT SIZE=-1>Edited by oldharry on 15/08/2002 09:52 (server time).</FONT></P>

 

[ponapay]

Another very simple reason can be - with through the hull transducers - is that there is no direct contact with the hull, fill the empty space with castor/olive or vegetable oil. It does wonders when the depth is fluctuating.

ponapay

 

[Chris_Robb]

Just returned from South Brittany - we had a huge scare of the Point de Penmarsh, ( the place grows rocks!) where the depth dropped suddenly to 4 mtrs and varied up an down from 3 to 6 meters for about 15 minutes, even our spare old seafarer agreed with the depths. My navigation confirmed we were in deep water- but her in doors was scarred witless by it.- so we altered course into deeper water only to see the depth drop more.

The pilot book says that 2 tide systems, one flowing south and one out of the Anse de Benodet (warmer?) meet there - so I presume the shallow readings were the Hotter water over the colder southflowing current.

Makes your pulse go a little fast - have I made a cock up in the nav etc!!!

 

[vyv_cox]

I know the place well from the land, having spent many holidays there surfing. I can confirm the coastal drift, the general trend of waves comes along the beach from the Audierne direction (NW) and piles up huge amounts of seaweed in the corner between beach and point. This is harvested by local farmers by the truckload.

Moving just around the corner to tthe south brings a very noticeable increase in water temperature, but no waves!

 

[Chris_Robb]

Re: Penmarsh

Interesting - so it looks like it was a warm water layer! Bloody scary at the time, I suddenly kept seeing shadows in the water, and though I knew we were precisley on the waypoints, you still have doubts!