Stupid Question #1

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CMJ

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I went to Beaulieu on the boat at the weekend and having asked the harbour master on the ‘phone before-hand about the tides, I was surprised to see there was a big difference between Beaulieu’s published tides, and the predicted ones for Beaulieu on my Navionics charts (and repeated on the iPhone’s Navionics app). If I remember there was about a two hour difference…..and I doubled checked that the plotters / phone were on the right time-zone and dates.

So what’s going on and what source should I trust?
 
Could it have something to do with the time it takes water to surge up a narrow channel.

Navtronics will be based on earths rotation in relation to the moon however the water will be late due to the resistance it encounters getting up the narrow channel.

At the end of a long tidal river the water can flood for 2 hours and then ebb for over 10 hours.
In effect a delay in predicted high water of 4 hours.
 
View it all with extreme suspicion until you have checked it out for yourself !!!

Gismology is all very well until you accept that if you put garbage in you will get garbage out and harbourmaster office staff have been known to misread their own tide tables.

Tom
 
Re: NOT a stupid question

Very interesting. Please state HM's times AND your Navionics prediction. Admiralty Easytide Bucklers Hard HW was 1516 on Sat 13th, 1602 on Sun 14th; how does that compare?
 
Re: Beaulieu tides

Beaulieu River is hardly very long - it's only 3 miles from the Bar to Bucklers Hard, another mile to the millpond in Beaulieu where the stream trickles over a weir; the current is quite weak, allowing a 2-hour stand at HW.
 
I think in an area as complex as the Solent you need to factor a bunch of local knowledge on top of any tidal predictions you might read about.

Solent tides can be affected up to a foot or so by low or high air pressure and a strong SW breeze can have a huge effect.

I've seen a 4.8 mtr spring HW surge to 5.6 in the middle of a deep depression blowing hard from the SW.
 
coincidently this weekend I had a similar experience with the Navionics charts.

Last saturday we planned 2 dives in the Northsee
on Birkenfels and close to Westhinder
(as a reference, approx half way between Ramsgate and Zeebrugge)

as usual I used this site
http://www.mumm.ac.be/EN/Models/Operational/Currents/series.php?station=birkenfels&gs=1070
to predict the moment of lowest current.

just as a confirmation I checked the current figures on my chart plotter with Navionics Gold card,

on some points in the neighberhood predictions where exactly the same as mine, but on one point, very close to our diving spot, there was a 2 hour difference in the lowest current point.
Our dive prooved that the Navionics prediction was NOT correct.

I am wondering where we could asc / discuss about eventual mistakes on the Navionics charts ???
 
Re: NOT a stupid question

[ QUOTE ]
Very interesting. Please state HM's times AND your Navionics prediction. Admiralty Easytide Bucklers Hard HW was 1516 on Sat 13th, 1602 on Sun 14th; how does that compare?

[/ QUOTE ]


Hi,

Here's the difference I saw last Friday (12th June).......

Source: 2009 Beaulieu Tides and Information Booklet

LW @ 19:27hrs, 1.3m


Source: Navionics Gold - Buckler's Hard (on iPhone - matches the Platinum charts on the boat if I remember correctly)

LW @ 20:40hrs, 1.4m

Ok, in this case difference is just over an hour, but with entry in the River considered around anything but +/_ 2hrs LW, it's obviously quite important to know the facts.
/forums/images/graemlins/confused.gif
 
Re: NOT a stupid question

Grannies and sucking eggs aside,

As there is almost exactly an hour difference have you checked that either Beaulieu is printed in UTC and so an hour has to be added in the summer or that the time input on the Navionics gear has the correct offset applied.

I am going to duck for cover now.

Tom
 
Re: NOT a stupid question

Beaulieu tide tables are definitely all UTC, with a shaded area to indicate where an hour should be added for BST. It may be that further confusion is being caused because when there is a stand the HW time given is approximately in the middle of the two hour period, whereas at springs, when there are two HWs, the time given is that of the first HW. Or something like that.

Also the Beaulieu tide tables are as at Buckler's Hard - I don't know what your computer programme counts as Beaulieu but I suppose it could be anyhere from Stansore Point to Beaulieu itself.

Plenty of scope for confusion /forums/images/graemlins/smile.gif
 
Re: NOT a stupid question

Does B'Lee have a young flood stand? In Hamble on Springs after about 1hr of flood the tide just stops dead for about 30 mins then it starts to flood again, its very distinct.

In Hamble also, the flood and stand on springs can be 7 hours so the ebb is all done in five, I have been stood on the pontoon in the middle of a spring ebb and I swear you can feel it falling.
 
Re: NOT a stupid question

[ QUOTE ]
Beaulieu tide tables are definitely all UTC, with a shaded area to indicate where an hour should be added for BST.


[/ QUOTE ]

Ah – yep, it was a stupid question from me! Having a proper look at the tide table now I can clearly see the shading and the comment to add 1 hour for summer time – doh!

I think what confused me was the harbour master quoting the UTC time on the ‘phone when I asked ‘what time is low-water today’?...anyway, my fault as I should have checked!

Thanks for all of your help and I now have restored faith in my Navionics tide predictions /forums/images/graemlins/smile.gif
 
Shifting the thread slightly, I noted you referred to having the Navionics Iphone app. I was thinking of getting and Iphone and wondered how you found the Navionics app. What do the charts cost?
Thanks
 
Re: NOT a stupid question

There's a bit of a slowdown in the flood at springs, about 1.5 hours after LW, but nothing like as pronounced as the Hamble. The ebb, however, is more like the Hamble - it takes about 5 hours and at equinoctial springs it can flow at quite a rate: I've heard five knots quoted though I've never experienced it myself.
 
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