Tides Question?

[Dougal]

If anyone here as a little time to spare, please help me out....
Can you please look up morning H/W tide times for St Nazaire (France) on 21st & 22nd Sept 2015

Either I'm doing something very wrong, or something bizarre happens over that 24hr period.

Or of course, I could just be having a VERY long 'senior moment' ;-)

Thanks

 

[Kwik Decision]

http://www.ukho.gov.uk/easytide/easytide/ShowPrediction.aspx?PortID=1664&PredictionLength=7

I agree, that does look odd. HW at 0842, and 2108 on 21st, and then the next one is 14.5 hrs later at 1140 on 22nd. No idea why though.

 

[Dougal]

I have three different tide books here. They all give a THREE HOUR tide difference between morning H/W on the 21st to the 22nd!!!

http://www.ukho.gov.uk/easytide/easytide/ShowPrediction.aspx?PortID=1664&PredictionLength=7

I agree, that does look odd. HW at 0842, and 2108 on 21st, and then the next one is 14.5 hrs later at 1140 on 22nd. No idea why though.

 

[MM5AHO]

What time system in use?
UTC?
GMT (not the same as UTC)
BST?
FST?

 

[ProDave]

Look at the SHAPE of the tide graph. The actual highest point occurs earlier or later due to the shape of the graph.

If instead you computed the high tide time as half way between the adjacent low tides, all would appear normal.

(wasn't this same question answered just a few days ago?)

 

[Lucky Duck]

(wasn't this same question answered just a few days ago?)

The issue of high water stands producing some 'odd' results for predicted high waters was certainly discussed not that long ago...

 

[Dougal]

I'll search to see. It seemed such an obscure question, I never thought for a minute it might be here.

Look at the SHAPE of the tide graph. The actual highest point occurs earlier or later due to the shape of the graph.

If instead you computed the high tide time as half way between the adjacent low tides, all would appear normal.

(wasn't this same question answered just a few days ago?)

 

[Dougal]

What time system in use?
UTC?
GMT (not the same as UTC)
BST?
FST?

The books I have are all UTC+2 (Local)

 

[Dougal]

(wasn't this same question answered just a few days ago?)

If you mean this: http://www.ybw.com/forums/showthread.php?439392-Tides-and-slack-water-help-please&highlight=tides
It still doesn't answer my question. Nor do all the books I have, or even the bits of paper I have from the RYA;-)

I guess what I need then, is indeed, a better understanding the graphs. The idea of such a time difference between H/W over 24hrs is new to me.

 

[dedwards]

Looking at the graph, it looks like HW neaps at St Nazaire lasts for over an hour and on the 21st the highest point of the HW stand is at the beginning of HW whilst its at the end of HW on the 22nd.

In other words, the midpoints of HW @ St Nazaire are still 12.5ish hours apart but the absolute highest times are further apart due to minor differences in the shapes of the curves.

I'm no expert on French ports but I've seen the effect on a number of uk ports. The advice from UKHO is to always look at the curve because sometimes a height difference of a few cm's can drastically change the time of the highest water.