Calculating secondary port heights with drying harbours

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CraigB

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Anyone care to help solve this little question for me?

I'm studying ym theory and trying to work out how to determine the lowest point of tide at which I would still have clearance in a secondary port where, rather than a + or - value in metres, the almanac just states 'dries' (such as Rye Hbr). I seem to remember the charted depth outside the entrance is -1.5.

When plotting the daily range on the tidal curve for this secondary port, you need to mark the adjusted HW and LW heights at the top and bottom on the left-hand scale in order to draw a line etc. But when the almanac doesn't have a height difference for LW, it just says 'dries' - how do you plot this line to work out the time of a certain height?

Hope this makes sense..
Thanks
 
Read the pilot carefully!!! Reed's says the bar dries 2.75m so I'd use that plus a bit for safety and go in on a rising tide!
 
Okay, I'm sure you're right, I was just going from (poor) memory as the almanac's down on the boat. I'm still not sure how you work out the time - say if the drying height is 2.75m, and you need 2m clearance, how do you work out at what time the height is 4.75 above chart datum ..
 
Dover is the standard port. Use the curve starting at say -3.0 to say 7.8 at MHWS, yo might have to extend the scale leftwards for the negatives at LW
 
Cheat a little. Chart 5058 shows that the range at Rye is about 10% higher than at Dover, while the timing is virtually the same. Take Dover times, add 10% to the heights, and you will have the high and low water heights and times for Rye (or just outside, where it doesn't dry). From that, if you know the height of the bar, you can work out when you can get in.
 
Strictly speaking you need to work from the drying height which you obtain from the largest scale chart of the area. However, in the case of Ryde, AFAIAA there is no Admiralty chart that shows this info which is of course why there is no figure in the Almanac.

For your YM exam you will not be required to do these calculations for ports that information is not available for, or at least invented for the purpose of the exam. For practice purposes you should therefore concentrate on ports for which the info is available.

However I imagine you have a personal interest in doing the calculation for Ryde.

To that end you will find that the (unofficial) Ryde Harbour website gives figures of 1.45m above chart datum for the entrance between the piles and 2.2m above cd in the centre of the harbour.

So to get on the pontoon you need a height of tide 2.2 + your draft + a safety margin above chart datum. Say 3.3m for the sake of argument

The standard port is Portsmouth and the height differences are -0.2, -0.1, 0.0 & -0.1 for MHWS,MHWN, MLWN & MLWS respectivley so so take -0.2 (you will probably going in near HWS anyway. That means you will need a height at Portsmouth of 3.3 + 0.2 = 3.5m.
Now you can determine the times before (or after HW) that correspond to that height from the tidal curve for Portsmouth. Use the range for the tide you are interested in but taking 4.1m which is the mean range for a spring tide and the height figure for the relevant LW which for this purpose we will take as 0.6m. That means you will need a rise of 3.5-0.6=2.9m above LW. That gives a factor of 2.9 ÷ 4.1 = 0.71 with which to enter the tidal curve. That will give you an earliest entry time of 2 hours before HW. The time difference for Ryde for a Spring tide (ie 00.00hrs and 12.00 hrs) is -0010 which you can now deduct if you must!

If you have the tidal curve for Ryde you can use that but you still have to use the tide times and height for portsmouth and apply the time and height differences.

A quicker but less accurate method for ports between Swanage and the nab is to use the table for that area.

In all cases though you need that first vital bit of info : The drying height!

I hope this helps and I hope it is correct. It has quite exhausted me so I'm off to bed, well it is well gone midnight.
 
If you are trying to answer a question in the RYA papers use the practice navigation tables supplied for your course as thes are fictitious and the questions are designed with this in mind.If it is real life example check the other posts.
 
Yes, although we're studying YM theory this was a practical exercise from a real life situation to see if the theory had sunk in. Still think I'll keep my 1.8m draft away from Rye though!

Thanks for the tips on working out the heights via other methods, and thanks Vic for the whole workings for Ryde, it is very reassuring that it all made sense, whereas this time last year I would have been left scratching my head..

So I guess the answer is, if the secondary port information says 'dries' instead of MLWN/MLWS differences, then you have to look around for other information sources, and do some guesstimating. I would have thought that theoretical LW differences would have been calculable to make things easier.
 
[ QUOTE ]
Dover is the standard port. Use the curve starting at say -3.0 to say 7.8 at MHWS, yo might have to extend the scale leftwards for the negatives at LW

[/ QUOTE ]

By the way Jimi, where did you get the value -3.0 from? Was it a guess based on the unlikeliness of the real figure being anything beyond that? LW at Dover looks like it stays in a range of approximately +0.5 to +2.5 - how do you get -3.0 from this for Rye?
 
[ QUOTE ]
erm the original poster was asking about Rye not Ryde

[/ QUOTE ] So he was. funny what tricks the eyes play on you after midnight.

All the theory holds though for any drying secondary port, the vital thing being that you need a figure for the drying height before you can do the calculations. The large scale chart for Rye is 1991 which should give the all the figures. SC folio 5605 covers Rye also. There is quite a bit of info about depths etc on the web site but it does not give the actual soundings and drying heights.
 
[ QUOTE ]
All the theory holds though for any drying secondary port, the vital thing being that you need a figure for the drying height before you can do the calculations. The large scale chart for Rye is 1991 which should give the all the figures. SC folio 5605 covers Rye also. There is quite a bit of info about depths etc on the web site but it does not give the actual soundings and drying heights.

[/ QUOTE ]
It's not quite that simple. The tidal data for Ryde includes the height differences at high and low waters, springs and neaps. From these you can use the standard methods to get high and low water heights and times for your particular tide, and hence work out a tidal curve. Yes, you have to make allowance for drying heights, keel clearance, etc., but this is (or should be) covered in the normal shorebased course.

Not so for Rye. The high water data is there, for both neaps and springs. But look for the low water data and all you see is "Dries". No low water differences means that you can't work out a low water height to use for your tidal curve, hence no curve and no ability to work out any times at which one might enter. It's not uncommon, Maldon and ports round the Wash have the same problem.
 
That's where a decent pilot book, or a quick chat with the HM, pays dividends.

"Flood does not enter the river until HW-3, and then sets rapidly reaching 6kn at springs. At Springs HW -1 it flows over E Pier and W training wall when stream slackens in fairway. This is the best time to enter"

And not a pencil in sight.
 
[ QUOTE ]
[ QUOTE ]
Dover is the standard port. Use the curve starting at say -3.0 to say 7.8 at MHWS, yo might have to extend the scale leftwards for the negatives at LW

[/ QUOTE ]

By the way Jimi, where did you get the value -3.0 from? Was it a guess based on the unlikeliness of the real figure being anything beyond that? LW at Dover looks like it stays in a range of approximately +0.5 to +2.5 - how do you get -3.0 from this for Rye?

[/ QUOTE ]

Reed's says the bar dries 2.75m so I'd use that plus a bit = 3.0 for safety and then go in on a rising tide!
 
[ QUOTE ]
"Flood does not enter the river until HW-3, and then sets rapidly reaching 6kn at springs. At Springs HW -1 it flows over E Pier and W training wall when stream slackens in fairway. This is the best time to enter"

[/ QUOTE ]

CA Almanac.
 
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