Depth calculations near secondary ports

[MARGETTS]

Hi Lw395. I was replying to the post from MoodyNick, but did not snip enough of his post ( will get the hang of this soon ). My reply was the last 3 lines.
Basically this simply meant that on a traditional blind nav passage, the charted depths / rise of tide are irrelevant (excluding rocks, isolated dangers etc.) as you are sailing to maintain a constant depth. Hence creating your own depth contour. The secondary port information ie required only to know if there is enough water at your destination to access the marina, cross the bar etc.

 

[Refueler]

[ 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.

[/ QUOTE ]

Generally true that LAT is CD, but there are areas that MLLS is used. Don't ask me why !! MLLS = Mean LOW LOW Springs. It is reckoned to be the lowest spring tide achievable. LAT is lowest tide possible by action of all astronomical bodies in unison ... rough explanation.

I only remember vaguely the MLLS as it was brought up while I was a Cadet in Merch College ... what they call it now or reference - I've lost track.

 

[BlueSkyNick]

[ QUOTE ]
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.


[/ QUOTE ] Not sure why it would be 1metre, surely it would be 2m plus a little bit close to Lymington, increasing to 3 metres near Portsmouth.

If it was 2m at Lymington Quay and 3 metres at the river entrance, there would be a big step or slope somewhere between the two!!

I take you point about not navigating too precisely, and your advice to stay between two contours makes a lot of sense, thank you.

 

[lw395]

I was using the notation 1M= 1 nautical mile
2m=2 metres
Sorry that wasn't clear or if I didn't fully read anyone else post. I'm not here to tick people off mostly just trying to see how other people do these things, and see if anyone has better or more succint explanations.
As an aside the gradient on the sea might be bigger than you think in places, that's why its often flowing one way or the other?
Next time you're in Bembridge compare the range on the tide gauge outside to the pontoon piles....
And last time I sailed into Lymington there were some quite big steps in the water!

 

[BlueSkyNick]

ah sorry, I now realise I read your post too quickly ! Thanks, anyway. /forums/images/graemlins/wink.gif

 

[peterb]

[ QUOTE ]
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.

[/ QUOTE ]

If you look at chart 5058 you'll see that the co-tidal and co-range lines near Poole look a bit like half a spider's web, with the middle of the web about 20 miles inland of Poole. If this happens in open sea, then at the middle of the web there is no tidal rise and fall; in technical terms it is an 'amphidromic point'. In such a situation the tidal range increases as you move away from the point. The mean spring range at Poole Entrance is about 1.7m, increasing to about 2.2m at Hurst, 2.5 at Lymington and Yarmouth, 3.2 at Stansore Point, 4.1 at Portsmouth and 4.2 at Chichester Bar. A steady climb as you travel east. But across the Solent there is very little change in range, hence Lymington and Yarmouth having the same range.

So if you really want to make the best possible estimate (repeat, estimate) of the tide in the Western Solent then take the nearest secondary ports east and west of your position, then interpolate, taking no account of north or south. But remember that it's an estimate, and that other factors, particularly wind and pressure, can easily put you out by a metre or more. And near Southampton Water the progression turns into a north-south one, with a 4m range at Southampton compared with the 3.2 at Stansore Point.

Come to think of it, perhaps someone should publish co-tidal and co-range curves for the Solent. They'd certainly be interesting.

 

[BlueSkyNick]

that's excellent, Peter, thank you.

I think I might be overcomplicating the topic, if it is not part of YM syllabus already!

 

[peterb]

It's not! Just that I have an interest.

 

[lw395]

I'd not heard of chart 5058 before, thank you!

 

[matelot]

neither had I !

my guess is that the co-tidal diagrams like the tidal atlasses will be extrapolations based on observations at a number of specific points - the tidal diamonds plus others not published.

now I dont know how reliable you find the tidal atlasses but here in the Bristol channel they arent IMHO much better than a rough indication - so I wouldnt attach any great accuracy to 5058 either. add into that the moveable feast of depth to CD and I will not be basing any decision on theortetical depths if I can avoid doing so.

does anybody know how 5058 is actually derived?

 

[BlueSkyNick]

I am sure you are right about the inaccuracies and variables, affecting depth and measurement of it.

I assure you I have no intention of navigating to into shallow water based on this thread! I just wanted to be certain of my own understanding, and able to explain it to others ..... an examiner for example.

Learned quite a bit, and I think others have too, thanks to all.

 

[alant]

Do you use the correct Tidal Curve for calculations?

With regard to calculating eg Lymington or Beaulieu, how many people continue to use the Tidal Curve for Portsmouth (Std Port) which is a HW curve, ignoring the fact that they should be using the LW Tidal Curves as published in the Almanac "Special Tidal Problems between Swanage and Selsey"?

Also, if you get stuck with your calculations, a shortcut you can use, is to circle a Green/Red lateral mark several times, these usually on a 5m contour, checking your actual depth on the sounder. You can then use this measured height of tide to check calculations.