Tides - maybe daft question

T

TimLamb

Well-Known Member
Joined
30 Sep 2001
Messages
75
Location
Northumberland UK
Visit site
Is the difference between high water at two different places always the same ?
Specifically, Amble (NE coast) and Dover.
High tide (this week anyway) sems to be a consistant 7hr 25 mins apart - give or take about 3 minutes.
Is that constant ?

Thanks in advance for any replies.

Tim
 
The "give or take" is usually a bit more than three minutes, but in principle you are right. In fact the Admiralty publish a chart (5058) which shows these differences all round the coast of the British Isles, and the European coast from Brest to Kristiansund, Norway. 5058 also shows how the range of tide varies between different places, and the way in which tide affects places not just on the coast but also out at sea.

Maybe a suitable topic for a PBO or YM article?
 
The difference between high tides will be almost the same, but beware of the "range difference" between the two places
 
An important fact to remember that this does not hold true between Chichester and Swanage as they are effected by a double HW between each LW.


The only way to go is by water
 
Also remember that atmospheric conditions - pressure, wind speed and direction - can cause the actual HW time to vary by as much as an hour and a half, or more in extreme conditions.

This of course can not be predicted in the tide tables!
 
Not quite the same. If you look at the secondary port calcs in an almanac you will see that there can be a reasonable difference depending on what time of day the high tide at the standard port is; and this varies throughout the month.
 
Port Ellen

The variation in tidal times is shown most clearly by looking at the high and low water corrections. Usually the corrections given relate roughly to springs or neaps, and it's very unusual for the springs and neaps correction to differ by as much as an hour.

But if you want to see the extreme variation of times, look at the high and low water corrections for Port Ellen (on Islay). The tides are based on Oban. The correction at spring tides is -0050; at neaps it is -0530. Fortunately the spring range at Port Ellen is only 0.6m (2 ft), so if you take it as mean level the worst you can be out by is 1 ft.
 
So, when it comes down to it and to answer Tims question - Sometimes, Maybe, Almost, Occasionally, From time to time, Every so often, Depends and Now & again.

I think .....

Dallas
 
Re: Tides - Slack Water

Another point worth keeping in mind is that HW and LW is often not SLACK water, when the tide is actually on the turn. The current can continue to flow in the same direction after HW / LW has occured.
ie Portland Bill - Local HW is about HW Dover +4hr 20
Slack Water Portland Bill when the current changes from W to E direction to E to W direction occurs at about HW Dover.

God - I wish it would stop raining!
 
Re: Tides - Slack Water

Nigel,

How does this translate into calculating the tidal window for Portland (ie the times when the race is safest to pass through)? Most Almanacs and pilots talk about taking advantage of tide for sailing boats, so timings for east and west passages differ. Assuming you are not interested in tidal flow (eg powerboat) when is the best time to go through?

Brendan
 
Re: Tides - Slack Water

Given moderate winds it doesn't make much difference what time you go around the Bill's inner passage in a planing boat. You get over falls, which change their location dependant on the time + or - on slack. In good conditions they are easy to see and you can avoid that patch of water. At slack water the over falls are moderate, I've never seen an occasion when there hasn’t been disturbed water, you would think that if there were no current there should be no disturbed water. The best I've seen it is just swirling whirlpools.
Having said all that, slack water is always the best time, so that’s about HW Dover +/- an hour or HW Dover +6 (to go East for a displacement boat) are the times to aim for.
As has been said before, if you are out of stone throwing distance, you are too far out! I've dived off the bill, and can confirm that it drops off to 20mtrs in spitting distance.

P.S. I made a mistake in my previous post.
I said that HW Portland was HW Dover + 4hr 20 that should be - 4hr 20.


God - I wish it would stop raining!
 
Re: Tides - Slack Water

Nigel,
Thanks, that's far clearer than any guide I've read so far.

Are there similar rules of thumb for other well known overfalls such as St Albans, St Catherines, Start Point etc, and even North Channel and Needles.

Reeds Nautical is a very dry read, and tends to reference lots of different HW ports which makes it a tedious calculation, and even the Yachtsman's tidal atlas, while being workable, refers to HW Cherbourg, and from the tidal flow diagrams it still isn't clear what the best window is. Being able to work a trip by one reference (Dover?) is much easier, and a few simple rules of thumb that are easy to rember while on route when trying to do calculations in your head are far less likely to result in errors

Brendan
 
You must log in or register to reply here.

Share this page

Top