Depth calculations near secondary ports

[BlueSkyNick]

At the risk of being publicly villified, I need to clear a mental block about this.

Simple nav theory No.1: from the range of the tide at a standard port and using either the Rule of Twelths or a tidal curve, the height of tide can be calculated. This is simply added to a charted depth to calculate the actual depth at a given time. (ignoring barometric pressure etc)

Simple nav theory No.2: Range of tide at a secondary port is not the same as its nearest standard port. Differences between the two are given in an almanac, and vary between springs and neaps.

Looking at a chart which includes one standard port and several secondary ports, depth at a specific position is calculated according to the charted depth plus the height of tide at the time, at the port nearest to the position.

If a position is somewhere between 2, 3 or n ports, the depth at that position is calculated by averaging the tidal heights at those ports. True or False?

 

[aquaplane]

Sail somewhere where it don't matter then it don't worry your head.

 

[VicS]

[ QUOTE ]
True or False?

[/ QUOTE ] Probably true but always take the worst case scenario. Calculate the heights at the nearby ports and if there is a significant difference take the lowest and add that to the charted depth. Good chance then that the actual depth will be more than calculated.
Except in areas with a big tidal range the meteorological conditions (not just baro pressure) will have greater effect than the variations between various nearby ports.

 

[peterb]

False.

As a Lymington man, you won't often have to worry about it. But for anyone who sails in the Thames Estuary, tidal heights over the banks can be critical. So the Admiralty Tidal Stream Atlas for the Thames Estuary includes co-tidal and co-range curves for the area.

The co-tidal curves are lines along which the tide has the same timing; four sets are given for low water springs, low water neaps, high water neaps and high water springs. Over the East Barrow Bank, for example, high water springs is 20 minutes later than at the Standard Port, Walton-on-the-Naze; Colne Bar, on the same line, is also 20 minutes later

The co-range curves show how the range varies compared with the Standard Port. East Barrow has a range 12% higher than the Standard Port, while Colne Bar is about 15% higher.

If you're really interested in the subject, get a look at Admiralty Chart 5058, which gives co-tidal and co-range curves for the British Isles and adjacent waters, including the North Sea. Did you know that there are three spots in the North Sea that have no tidal rise and fall at all?

 

[ebbtide]

"As a Lymington man" - AND Cruiser Captain organising rallies for his local club.

To be seen aground on the Inner Swash en route to Haslar no doubt!

(Been there, done that, got the T-shirt . . .)

 

[l'escargot]

You can forget the rule of twelths in the Solent - it doesn't allow for the "sit" after high water.

 

[CPD]

[ QUOTE ]
[ QUOTE ]
True or False?

[/ QUOTE ] Probably true but always take the worst case scenario. Calculate the heights at the nearby ports and if there is a significant difference take the lowest and add that to the charted depth. Good chance then that the actual depth will be more than calculated.
Except in areas with a big tidal range the meteorological conditions (not just baro pressure) will have greater effect than the variations between various nearby ports.

[/ QUOTE ]

Agreed, and add a substantial margin of error.

 

[DJE]

All good stuff but you're still not going to get in to Christchurch!

 

[Skyva_2]

Just to add to the above, the standard port is not necessarily the closest, its the one with the most similar shape of tidal curve to the secondary port. I don't have an almanac to hand, but from memory, for Braye Harbour it is St Helier, not St Peter Port. For Helford River it is Plymouth, not Falmouth.

I don't think averaging would necessarily work, due to the effect of local geography on local tide heights.

 

[Pye_End]

Peterb has it.

Look in the Admiralty Tidal Stream Atlas and you will get all your answers.

Mind you, for Thames Esturay there is little point in calculating to nth degree as the bottom changes so much anyway!

 

[BlueSkyNick]

I knew I would get at least one obnoxious answer, should have guessed it would be from you.

 

[BlueSkyNick]

[ QUOTE ]
Just to add to the above, the standard port is not necessarily the closest, its the one with the most similar shape of tidal curve to the secondary port. I don't have an almanac to hand, but from memory, for Braye Harbour it is St Helier, not St Peter Port. For Helford River it is Plymouth, not Falmouth.


[/ QUOTE ]This is partly what got me going on this subject, as Portsmouth is the Standard Port for every where from Christchurch (which is closer to Poole) to Chichester, except Southampton. However this is a seperate matter as long as one has looked up the correct standard port.

[ QUOTE ]

I don't think averaging would necessarily work, due to the effect of local geography on local tide heights.

[/ QUOTE ]So suppose we are sailing blind (eg thick fog, no GPS) from one secondary port to another, who share the same standard port, following depth contours. Lymington to Cowes is a good example, as their differences to Portsmouth are ...erm.... different to each other. In the middle is Beaulieu which is different again.

At what point do we calculate the depth on Beaulieu instead of Lymington, then on Cowes instead of Beaulieu?

In practice, it wouldnt matter very much because there is only a 1.0m change across the passage, but in other areas of greater depth variations it could become more relevant.

Just wondering, in case I happen to be in an exam situation possibly sometime in the future. /forums/images/graemlins/wink.gif

 

[BlueSkyNick]

[ QUOTE ]
You can forget the rule of twelths in the Solent - it doesn't allow for the "sit" after high water.

[/ QUOTE ]Very good point - I was trying to keep the question to be of more general interest to all. In my other response to Skysail, I have gone into an example in the Solent, though.

 

[Twister_Ken]

"So suppose we are sailing blind (eg thick fog, no GPS) from one secondary port to another, who share the same standard port, following depth contours."

Pragmatic answer: Pick a contour that doesn't offer a ploughing match.

 

[DJE]

The thing that always baffles me is that chart datum varies from place to place. At Hurst it is 1.83m below Ordnance Datum and at Portsmouth it is 2.73m below Ordnance Datum. So the depth contours are not contour lines as you might find on a land map; they rise 0.9m along the length of the Solent! So the soundings on a chart of the whole Solent can't all be reduced to one common datum. (The levels of the Various "Datums of Soundings" are on the back of chart 5600.18 in the small craft folio.) I suppose soundings to local LAT date back a lot further than accurate land surveying to a common datum and of course it's local tide levels that matter to mariners.

 

[Skyva_2]

Errm - think I need more notice of that question /forums/images/graemlins/blush.gif

 

[BlueSkyNick]

I think you are making the same point in a different way.

If the whole solent was on one chart datum, and therefore the range at any charted depth is relative to that datum, many places would be underwater at high tide!

I think the real answer is that the sea is not level but on a slope! /forums/images/graemlins/shocked.gif

 

[peterb]

[ QUOTE ]
The thing that always baffles me is that chart datum varies from place to place. At Hurst it is 1.83m below Ordnance Datum and at Portsmouth it is 2.73m below Ordnance Datum. So the depth contours are not contour lines as you might find on a land map; they rise 0.9m along the length of the Solent! So the soundings on a chart of the whole Solent can't all be reduced to one common datum. (The levels of the Various "Datums of Soundings" are on the back of chart 5600.18 in the small craft folio.) I suppose soundings to local LAT date back a lot further than accurate land surveying to a common datum and of course it's local tide levels that matter to mariners.

[/ QUOTE ]

It's very simple, really. OS datum is mean sea level (ML) at Newlyn, Cornwall. ML stays roughly constant round our coast, with the tides going up and down round it. But the chart datum (CD) is lowest astronomical tide (LAT), and is therefore always lower than ML. How much lower depends on the range of tide; at Avonmouth, for instance, ML is 7.1 m above LAT (the total range is about 14 m) while at Poole the equivalent figures are 1.5 m above LAT and a range of about 3m.

 

[MARGETTS]

So suppose we are sailing blind (eg thick fog, no GPS) from one secondary port to another, who share the same standard port, following depth contours. Lymington to Cowes is a good example, as their differences to Portsmouth are ...erm.... different to each other. In the middle is Beaulieu which is different again.

At what point do we calculate the depth on Beaulieu instead of Lymington, then on Cowes instead of Beaulieu?

In practice, it wouldnt matter very much because there is only a 1.0m change across the passage, but in other areas of greater depth variations it could become more relevant.

If you maintain say 2 metres under the keel surely you are sailing your own defined contour and secondary differences will only matter as to whether there is enough water for where you want to end up.

 

[lw395]

I think the real answer is that secondary port calculations only offer a prediction at one point in that secondary port, and very limited information should be extrapolated to anywhere outside.
Just because the rise of tide is 3m in Portsmouth and 2m at Lymington town quay, you cannot say with certainty what the rise of tide is 1M outside Lymington.
You should not really be navigating in such a way that fractions of a metre in depth are crucial. The soundings are samples at points, the seabed is not a perfectly smooth surface and it may have moved since the chart was drawn.
Also if you follow www.pol.ac.uk/ntslf/sadata_tgi_ntslf_v2.php?code=Portsmouth&span=1 from time to time you will find that the sea level at Portsmouth is significantly off prediction surprisingly often, even in unexceptional weather.
To do the blind nav, the trick is to find a set of contours with a good slope, contour lines that follow the adjacent one. It should be sufficient to keep between the 5 and 10m lines, by sailing to allow say a 1m error in tidal prediction. If you try to use anythig finer than that, then it will probably fail as the charted contour will be lost in the random ups and downs of the sea bed.
For the west solent passage discussed, you could probably do worse than do the secondary port calculation for Newtown (!) becuase the almanac is actually giving difference for Solent Bank, which is off Newtown.
The point of all this is to show that you can do secondary calculations, but in passage planning or blind nav I think it pays to accept that they only offer a limited view of other points on the chart, and have a strategy that is not dependent on tidal height to the nearest mm.
Flak Jacket on. Tin Hat..