dont trust your charts or plotter

dabsolute470 said:
we ran aground even though the plotter said 3.7 mtrs

i did a video to prove it....lol

luck no damaged at all as we had slowed to almost stop but could have been hard as we was doing 25 knots 2 mins earlier.


http://youtu.be/wLgpcBD1jFU



http://youtu.be/zufsVe-ZIwE
Click to expand...

Have you considered that your plotter may have been displaying depths in feet?
I avoided a grounding last summer when I disbelieved the plotter. It transpired the earlier factory reset had put depths back to feet and I hadn't realised. Where I thought I had 4m I now had 4 feet.
 
Oops, we've probably all done that sometime.

Can I ask though, you mention in the video that 'the lifeboat's been called 'cos we keep getting blown onto the sand'. Does that mean you have no anchor???
 
What did your echosounder tell you? It can be useful to compare what different instruments are saying. What were the local Nav aids (bouys, perches etc) indicating, where you in a channel or cutting across a bank?
 
I've been very very close to grounding there and I wouldnt use that short cut, just not worth it.

I will try to explain why the charts of sand banks cant be trusted following my experience in the Humber and other sandy tidal estuaries .

Sand banks are formed due to the currents, strength of which are determined by the Moon.

Admiralty charts arent useless in such areas but they need special interpretation.

Your chart shows a very long sandbank with a lower area in the middle, the size of the hollow will depend on the season/moon phase which further vary considerably between years / decades.

The Humber has charts available that are far more accurate than Admiralty, issued every 1 -3 months but even those are affected by the moon phase.

when you run out of water, shut the engine off.
everyone stand on the bow to lift the stern clear and allow the current to take you to the deepest water , be patient the current will always find the deepest water , only a strong side wind will keep you stranded.

Very interesting post :)
 
dabsolute470 said:
we ran aground even though the plotter said 3.7 mtrs. i did a video to prove it....lol luck no damaged at all as we had slowed to almost stop but could have been hard as we was doing 25 knots 2 mins earlier.
Click to expand...

Doesn't look like H thought it funny, she may even have thought it was your fault :)

What was the tide doing, you're on shafts you know!
 
The channel across Caernarfon bar changes quite frequently. A big ebb with a strong SW'ly can move it in one night, but mostly it shifts slowly over a period of years & months. As an area gets shallower, so the tidal flow is diverted accelerating the effect. But a big storm can break a new channel thro a sandbank over night & the new tidal flow can continue cutting the channel deeper & wider until the flow is slow enough to allow silt to drop out & start the silting up process again.

This is a normal & natural process, but the seabed only surveyed very occasionally in most areas and re-issues of charts are even rarer. Have you ever tried getting your electronic charts updated? Even doing it yourself can be difficult or even impossible with some plotters. However, my harbour charges help pay for the Caernarfon Harbour Trust to survet the Bar regularly & move the Nav bouys as required.

So, rocks never move & harbours change infrequently, but sandbanks are ALWAYS on the move.
 
dabsolute470 said:
the plotter was set to meters....lol

here is a link to the story told in the east coast pilot mag.

http://www.eastcoastsailing.co.uk/ECS03-low.pdf.

the story is near the last page of mag
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Thanks for the link Steve. Just added the coords to a chart here and that's exactly where i would have crossed, had i been making the same journey, as it puts you right on course for Fishermans and deeper water to Ramsgate.
 
dabsolute470 said:
we ran aground even though the plotter said 3.7 mtrs

i did a video to prove it....lol

luck no damaged at all as we had slowed to almost stop but could have been hard as we was doing 25 knots 2 mins earlier.


http://youtu.be/wLgpcBD1jFU



http://youtu.be/zufsVe-ZIwE
Click to expand...

A great post Steve and a great east coast forum, glad you have the balls to share it with us as it tells us all never trust our charts or plotters, especially on the east coast.

By the way, a contrast in heathers looks, all smiles on the first vid then Tongue out on the second. Personally if that had happened to me I'd have been ok with the mrs....... Until the wine ran out!
 
Good story. Were the depths in the immediate vicinity as per the chart? Just wondering whether this is a chart error or a GPS error?
 
those sandbars are forever on the move and if im coming out the blackwater and heading for the crouch i have to travel about 7 miles along that sandbar to get to the crouch.the seals sunbathing normally are a good sign but your yacht must be pulling a couple of meters draught so with a safety margin on top of that you were always going to be close to grounding as soon as you were within the 5m contour.luckily were on the muddy flats so just your pride gets hurt.you did not look far away from the bar but did you not have a sonar stroke depth finder alarm on your chartplotter?
 
sarabande said:
which GPS geodetic system are you using ?
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Doesn't matter - as the East Coast Sailing article makes clear, there really was a drying bank in a position where the current Admiralty chart with the latest corrections showed 3.7m below datum.

Sandbanks moving that fast are another good reason for me to head west rather than east :)

Pete
 
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