Portsmouth tides

[Kochi]

I stumbled on this site this morning http://www.isleofwightweather.co.uk/isle_of_wight_tide_times_july2010.htm
and as my tables for Portsmouth are on the boat and this site offers the entire month not just 7 days I printed the table for July. Thought I should check wether UTC or BST so I went to
http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/ShowPrediction.aspx?PortID=0065&PredictionLength=7 and found that the predictions are slightly different. For example:
Wed 14
01:44 4.8m
07:05 0.4m
14:22 4.9m
19:29 0.7m
for the IOW prediction and
Wed 14 Jul

00:32 4.8m
06:14 0.5m
13:09 4.9m
18:35 0.7m
for EasyTide (and the BBC too)
so even adding an hour for BST they are different. Am I being dim here expecting them to agree?

 

[VicS]

They agree as closely as I would expect them to.

Go to the Proudman Oceanographic Laboratory site and get a third set

http://www.pol.ac.uk/ntslf/tides/?port=0008

Or here for two more

 

[capnsensible]

Would it depend on where in the harbour the table datum is/ Big place??

 

[l'escargot]

Would it depend on where in the harbour the table datum is/ Big place??

Not that big. The difference with other parts of the Solent is only minutes, the difference in the harbour would be negligible.

Different sites use different prediction methods to come up with times and they all vary to some degree - especially with Portsmouth where the tide "stands" for a period of time. Some predictions may give when it first reaches peak height, others may give when it last remains at maximum height.

 

[Kochi]

So all these years I've struggled to get my secondary port calculations exact when the standard port is only approximate then?

 

[l'escargot]

So all these years I've struggled to get my secondary port calculations exact when the standard port is only approximate then?

There is nothing exact about tides, they are only ever a "prediction". Anything within about one foot and one hour is pretty good.

 

[mcframe]

There is nothing exact about tides, they are only ever a "prediction". Anything within about one foot and one hour is pretty good.

Portsmouth you say?

"High water: springs at noon, neaps at dawn; extrapolate those in-between". ;->

 

[l'escargot]

Portsmouth you say?

"High water: springs at noon, neaps at dawn; extrapolate those in-between". ;->

Sounds good, but like the "Rule of Twelfths", doen't work in the Solent.

 

[LadyInBed]

Portsmouth you say?

"High water: springs at noon, neaps at dawn; extrapolate those in-between". ;->

As is Dover:
High water springs at noon
Low water neaps at noon

 

[richardbrennan]

Different sources use different algorythms (hope I spelt that right!), the ones used bythe UKHO are generally more detailed and therefore more accurate within the constraints already mentioned. You will find the tide data on most plotters also does not exactly match that from the UKHO/Reeds either.

 

[capnsensible]

Sounds good, but like the "Rule of Twelfths", doen't work in the Solent.

Are you sure? Not quite trusting my 20 odd years of sailing out of there, I checked a few of this years tides and it certainly is just about right.

 

[l'escargot]

Are you sure? Not quite trusting my 20 odd years of sailing out of there, I checked a few of this years tides and it certainly is just about right.

Tidal curve for Portsmouth today:

1/12 in the first hour, 2 in the second, you would expect the tide to be a quarter of the way down the back of the curve after 2 hours. You can see it is quite a way short because of the stand. The error is greater in the first half of the tide. Similar error in the rising tide. You need a nice even curve for the Rule of Twelths. The curve also changes shape across springs and neaps making the error variable.

Southampton is even worse:

The article on this page: http://www.bristolnomads.org.uk/stuff/double_tides.htm gives a nice explanation of Solent tides.

 

[capnsensible]

Well that seems to prove that high water springs are just after noon....as in the post I was commenting on!

When duty watch at HMS Dolphin many tides ago, used to go down to the end of Fort Blockhouse on late Sunday afternoons, spring tides and watch the masses of boats trying to bash in against the ebb. non duty weekends....would be waving from the boat to the goofers watching me! Spring HW mid day.

 

[l'escargot]

Well that seems to prove that high water springs are just after noon....as in the post I was commenting on!

When duty watch at HMS Dolphin many tides ago, used to go down to the end of Fort Blockhouse on late Sunday afternoons, spring tides and watch the masses of boats trying to bash in against the ebb. non duty weekends....would be waving from the boat to the goofers watching me! Spring HW mid day.

At crossed purposes I think - Not disputing the timing of Springs, it's the extrapolation which is difficult because of the inconsistent tidal curve.

I sometimes sit down at Sconce Point at Hurst and watch the yachts sailing backwards when they have left it a bit late getting back and hit the start of the ebb.

 

[LadyInBed]

I sometimes sit down at Sconce Point at Hurst and watch the yachts sailing backwards when they have left it a bit late getting back and hit the start of the ebb.

The direction of current flow doesn't necessarily coincide with HW & LW

 

[capnsensible]

I'd wondered, think we now sing from same songsheet!

People may drip about the Solent but it was an apprentiship in sailing for me around there and have many happy memories.

Don't miss it though!!!!

 

[mcframe]

At crossed purposes I think - Not disputing the timing of Springs, it's the extrapolation which is difficult because of the inconsistent tidal curve.

Ah - I meant extrapolate the *times*, based on the phase of the moon - that rule of thumb is handy for a zeroeth approximation of passage planning & ETD(home) when you're on a drying mooring.

 

[l'escargot]

The direction of current flow doesn't necessarily coincide with HW & LW

Not many places where the tide can change direction and start going "out" nearly two hours before it has finished coming "in". But that's what makes the place interesting. The tides around here always interest me and still sometimes do their best to catch me out.