Just when you thought you'd got the hang of it.

[Peppermint]

Just when you thought you\'d got the hang of it.

I’ve always felt bad that I wasn’t certain with my tidal calculations in Southampton Water. Not any more.

I have a tidal curve for Dock Head on 1st & 2nd of February 2004. High water should have happened at around 1900hrs but it arrived a little early and about 50cm over estimate. For the next hour and a half it dropped like a stone. From a little over 4.00m to around 2.80m. In the next hour it climbed about a metre before dropping to about 10cm below the 2.00m predicted low water at about 01:30hrs The next flood had a similar profile gaining about a meter in an hour and a half before dropping back and then starting again.

When you consider that many visiting ships work with 1:00m clearance under keel you’ll appreciate that the pilots & VTS were scratching their heads a bit over this one.


<hr width=100% size=1>

 

[charles_reed]

What was

the barometric pressure doing at the same time?

<hr width=100% size=1>

 

[VicMallows]

Re: Just when you thought you\'d got the hang of it.

If you stay listening to VTS (usually the most interesting thing to listen to) the Captains/Pilots are constantly asking for the latest Dockhead or Calshot readings ... they obviously know the predictions are not to be relied upon.

Vic

<hr width=100% size=1>

 

[boatmike]

Re: What was

Not only Barometric Pressure but wind direction. Both can have a lot of bearing on the actual height experienced in Southampton Water. When launching anything at Vospers we always checked actual heights on our own tidal marker beforehand. Often it was either late or early by 1/2 hour or more but the actual height seldom varied by more than 0.2 metre (although that can be enough if you are squeaky on draught......

<hr width=100% size=1>

 

[Peppermint]

Re: It was to sudden and profound

to be caused by pressure at Southampton.

The theory is that an event out in the channel caused a pulse on the tide.

<hr width=100% size=1>

 

[alant]

Re: Just when you thought you\'d got the hang of it.

Arn't these discrepancies expected - that's why the tidal curve for Southampton is based upon Low Water. But I'm sure you already knew that.

<hr width=100% size=1>

 

[Peppermint]

Re: Just when you thought you\'d got the hang of it.

Some are expected. But you don't expect to loose 1.2m in an hour or so and then regain it. As to working on low water. The following low water followed the same profile with a damn great spike in the middle of the curve. So, so much for working from low water as being fool proof.

What it really shows is that events many miles away, and undetectable to sailors, can profoundly screw up the most careful calculations. Particularly in these channeled waters like The Solent & Poole.

<hr width=100% size=1>