Tidal stream calculation question

[dokondr]

Hi all,
Reeds almanac at the beginning of Dielette entry (Area 18 - Central France) has the following line:

TIDES HW -0430 on Dover(UT); ML 5.4 m

Please explain how to use -0430 time difference ?
From my understanding:
Almanac has tidal stream chartles showing tidal stream set and speed from HW - 6 to HW + 5 at Dover. In case I need to find tides at Dielette at let's say at 1300 (BST) 5 Sep daytime I would do the following:
1) Find HW time at Dover Sept 5, let's say it is 1558 (BST)
2) Find how many hours after or before HW at Dover time of 1300 (BST) is
3) Find corresponding Dover tidal stream chartlet, let's say it woul be HW - 3

Where -0430 time difference on Dover comes into play?

Thanks!

[nigel1]

Hi

You are correct in the first part, use the difference in HW Dover to find the correct tidal stream chart to use.
The tidal stream charts in Reeds are taken from the Admiralty Tidal Stream Atlas's.
These also use HW Dover as the base point.
But, they also show the time difference from HW Dover to HW at the most convenient local standard port for the area of the tidal stream atlas.
I'm looking at NP 253 Tidal Stream Atlas North Sea Eastern Part.
It shows the HW difference between Dover and Helgoland (maybe Helgoland is Germany's equivalent of Dover)
So, you could have Tidal Stream Atlas, but not the times for HW Dover, but you may have the HW times for Helgoland (because it might be assumed you have the tide data for the area you are in)
So, knowing the time difference, you can find the correct page.


In you case, HW at Dielette is 4h 30m before HW Dover, so the Chartlet which is labeled HIGH WATER DOVER, can also be labeled as 4h 30m before HW Dielette.

Only really useful if you dont have HW times for Dover, but if you have Reeds, you should have those times, but who knows, you might have only taken Area 18 with you for the trip.

[dokondr]

Originally Posted by nigel1

Hi

You are correct in the first part, use the difference in HW Dover to find the correct tidal stream chart to use.
The tidal stream charts in Reeds are taken from the Admiralty Tidal Stream Atlas's.
These also use HW Dover as the base point.
But, they also show the time difference from HW Dover to HW at the most convenient local standard port for the area of the tidal stream atlas.
I'm looking at NP 253 Tidal Stream Atlas North Sea Eastern Part.
It shows the HW difference between Dover and Helgoland (maybe Helgoland is Germany's equivalent of Dover)
So, you could have Tidal Stream Atlas, but not the times for HW Dover, but you may have the HW times for Helgoland (because it might be assumed you have the tide data for the area you are in)
So, knowing the time difference, you can find the correct page.


In you case, HW at Dielette is 4h 30m before HW Dover, so the Chartlet which is labeled HIGH WATER DOVER, can also be labeled as 4h 30m before HW Dielette.

Only really useful if you dont have HW times for Dover, but if you have Reeds, you should have those times, but who knows, you might have only taken Area 18 with you for the trip.

Thanks!
To summarize:
Providing I have only tide tables for Dillete standard port and don't have the ones for Dover, how can I use the fact that HW at Dielette is 4h 30m before HW Dover?
Please correct my reasoning:
1) I can find HW at Dillete
2) I can find HW Dover = HW Dillete + 0430
3) Next? There are tens of of ports that have the same tidal stream patterns as Dover. Does it mean that every tidal diagram in the atlas for this area will have tens of labels like "4h 30m before HW Dielette", "4h 40m before Carteret", ... etc?

[Hoofsmit]

Please correct my reasoning:


1) I can find HW at Dillete


2) I can find HW Dover = HW Dillete + 0430


3) Next? There are tens of of ports that have the same tidal stream patterns as Dover. Does it mean that every tidal diagram in the atlas for this area will have tens of labels like "4h 30m before HW Dielette", "4h 40m before Carteret", ... etc?

1: ok
2: o
3: I think you mean there are tens of ports that are referenced to HW Dover ( they won't have the same tidal stream patterns) so the other areas will be referenced to there standards ports , so each area will have something like " 3h 50m before Calais" ( or it's standard port)
I think that's what your saying ?

[Hoofsmit]

Or possibly

Do you mean

the hourly chartlets for each area.....
In which case you need to do the same for each one as they are all referenced to the same standard port

[dokondr]

Ok, let's say I have only tide tables for Dillete and don't have the ones for Dover.
Please give an example how the fact that "HW at Dielette is 4h 30m before HW Dover" will help me to find tidal streams at Dillete at 1500 Sept 5.

[pupuce]

Don't waste your time: get on-line tide-tables as:

Prédictions en ligne


Select Port - EasyTide - on-line tidal predictions from the UKHO


maree.info - Annuaire des marées - horaire, hauteur, coefficient de marée en Atlantique Manche Mer du Nord


Tidal Site Selection: All Sites Listed Alphabetically


http://www.fisica.uniud.it:8080/zones/

and you'll certainly find a lot of others...

[dokondr]

Originally Posted by pupuce

Don't waste your time: get on-line tide-tables as:
...
...
and you'll certainly find a lot of others...

Thanks for the links. They are very helpful but I also need to be able to find info from the almanac without electronic aids.

[Hydra]

In fact, the Reeds Nautical Almanac uses St Malo as the standard port for Diélette. And by sheer luck, the tidal stream chartlets for Central N France also use St Malo as a (secondary) reference.

Then, when one only has the tide tables for Diélette, it's a very simple business to estimate the time of St Malo HW.

Alain

[dokondr]

Originally Posted by Hydra

In fact, the Reeds Nautical Almanac uses St Malo as the standard port for Diélette. And by sheer luck, the tidal stream chartlets for Central N France also use St Malo as a (secondary) reference.

Then, when one only has the tide tables for Diélette, it's a very simple business to estimate the time of St Malo HW.

Alain

Ok, but what is the reason for Reeds almanac at the beginning of Dielette entry (Area 18 - Central France) to give the following line:

TIDES HW -0430 on Dover(UT); ML 5.4 m

How this info can help?

[Hydra]

You don't need to use the time difference between Diélette and Dover.

I suppose this information is given because Dover is the main reference port used by UKHO for the British Isles and the surrounding areas, in the same way as Brest is the main reference port used by SHOM (the French Navy hydrographic office).

Alain

[goboatingnow]

The purpose of providing you with Dover offsets is to allow the use of UKHO tidal stream atlas etc.


dave

[Idlegreg]

I suspect it is just a hangover from the days when we did not have so much information at our fingertips. I think the Reeds port entries have had this line for as long as I can rememebr and I don't ever use it for calculations. I suppose it gives a quick idea of how the port HW compares to those around it as the tide comes up the channel.



Maybe you could find yourself in a position where you have last years Reeds and this years Dover HW table. If so it could help you figure an approximate HW at Dielette but this would not be relevant to Tidal STREAM in any case as your stream atlas in the Reeds would only need the current Dover HW time (or St Malo). Maybe if you only had a Dielette tide table aquired locally you could then use the difference information to calculate Dover HW and therefore use the stream atlas from a previous years Reeds.



But I am clutching at straws looking for uses and I think the best thing to do is ignore it - they probably leave it there becasue someone is bound to complain if they remove something that has been there for so long :-)


Mind you I have not given this much thought so now I wait for someone to give a good reason and I will feel a bit stupid...

[Idlegreg]

Well, when I engage brain a bit more it does mean you can more easily see which of the tidal stream chartlets will be in play when you leave/arrive when doing your initial planning.



So If I am at a port that required leaving within (for example) 1 hr of HW then I can easily see what the stream might be doing when I leave.