LadyInBed
Well-known member
If you think any method will give you anything exact or accurate you have forgotten what it says at the top of the page - Predictions
Anything give or take fifteen minutes or about 10% of the range is close enough for me.
Secondary ports .. what do you use to calculate?
- Thread starter Nostrodamus
- Start date
If you think any method will give you anything exact or accurate you have forgotten what it says at the top of the page - Predictions
Anything give or take fifteen minutes or about 10% of the range is close enough for me.
Bosun Higgs
N/A
Bit of quick mental arithmetic. Its easy enough.
Skylark
Well-known member
Back of the proverbial fag packet if mental arithmetic fails me (it's an age thing). Chart plotter is perfectly legit but is lazy. Using a computer seems over-the-top, IMHO
Fantasie 19
Well-known member
OK - so what is the mental arithmetic you're all doing?? What's the calculation?
(I'm doing RYA Coastal Skipper at the moment, and am doing graphs, but it does take a while!
)
(I'm doing RYA Coastal Skipper at the moment, and am doing graphs, but it does take a while!
)
D
Deleted member 36384
Guest
OK - so what is the mental arithmetic you're all doing?? What's the calculation?
(I'm doing RYA Coastal Skipper at the moment, and am doing graphs, but it does take a while!
Rule of Twelfths on the Range is one way: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12. If the range is 3 m, then 3 hours after LW the tide will be 1/12 + 2/12 + 3/12 = 6/12 or in easy speak a half, so 3 m x 0.5 = 1.5 m. If LW was at 03:45, and 2m then at 06:45, the depth would be 2m + 1.5 m = 3.5 m depth. Rule of Twelfths is not applicable for all areas i.e. where double high tides occur.
For secondary ports its just simple linear interpretation. Standard Port MHWS / MHWN is 18:00 or 06:00, actual time on the day of HW is 24:00
Secondary port time differences are MHWS -00:40 mins and MLWS +20 minutes
By simple linear interpolation 24:00 is half way between 18:00 and 06:00
Therefore the same applies to the secondary port. Between -40 and +20 is 60 minutes, therefore half way is 30 minutes, working from the -40 minute side, we get -40+30 = -10 minutes. This time can now be applied to the standard port HW time to get the secondary port HW time i.e. 24:00 - 00.10 = 23:50
Its the same principle for finding the difference in depths to be applied at the secondary port.
The issue I have with mental arithmetic is that I have made stupid mistakes when tired. Hence graphing could be considered a more fool proof way.
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Bosun Higgs
N/A
You just pro rata things. Using the Helford river info for 1300 ( which is 1/6th of the way from, 1200 to 1800) the time difference is 1/6th of the way from -0030 to - 0035 which is -0031 near enough. Not that I would bother to a minute since the tides arent that reliable.
And if the high water at plym is 4.7m say then the adjustment to hw helford is 3/11 from the bottom one to the top ( they are in fact the same at -0.2 so you wouldnt bother) but 3/11 is near enough 0.27 so you would multiply the difference by 0.27 and add it onto the min.
If I'm wanting to know the depth at any point in time I use the rule of twelvths and make sure I have a decent safety margin.
And safety margin is the key. The tidal data in your almanac are predictions not fact. They are affected by weather and baro pressure and even previous rainfall. So no one but a fool relies on the timing to within 10 mins or in my area ( bristol channel) to within 0.5 metre
And if the high water at plym is 4.7m say then the adjustment to hw helford is 3/11 from the bottom one to the top ( they are in fact the same at -0.2 so you wouldnt bother) but 3/11 is near enough 0.27 so you would multiply the difference by 0.27 and add it onto the min.
If I'm wanting to know the depth at any point in time I use the rule of twelvths and make sure I have a decent safety margin.
And safety margin is the key. The tidal data in your almanac are predictions not fact. They are affected by weather and baro pressure and even previous rainfall. So no one but a fool relies on the timing to within 10 mins or in my area ( bristol channel) to within 0.5 metre
onesea
Well-known member
OK - so what is the mental arithmetic you're all doing?? What's the calculation?
(I'm doing RYA Coastal Skipper at the moment, and am doing graphs, but it does take a while!
The more you understand the easier it is to switch to doing it in your head...
Oh my answer is in my head, or Easytide on my phone (its free).
On paper? urm yeah I would have to think...
Skysail
Well-known member
Something like this:
HW St Helier is at 0300, the HW at BRAYE is 50 minutes later.
HW St Helier is at 0900, the HW at BRAYE is 40 minutes later.
Between 0300 and 0900, interpolate the difference at Braye, so at 0600 the difference is +0045 minutes, and HW Braye is at 0645. If required you can now add the hour for Summer Time - 0745.
Similarly, from 0900 to 1500, the difference varies from +40 to +50 minutes, from 1500 to 2100 it is +50 to +40, and from 2100 to 0300 it is +40 to + 50.
One way of applying the differences is as follows:
The difference at Braye varies by 10 minutes over 6 hours, so each hour applies 1.66 minutes of difference between 0040 and 0050 minutes:
0300 50.0 mins
0400 48.3
0500 46.6
0600 44.9
0700 43.2
0800 41.6
0900 40.0 mins
But anywhere between 40 and 50 minutes you can guesstimate reasonably well. Or you do the whole thing with a calculator.
HW St Helier is at 0300, the HW at BRAYE is 50 minutes later.
HW St Helier is at 0900, the HW at BRAYE is 40 minutes later.
Between 0300 and 0900, interpolate the difference at Braye, so at 0600 the difference is +0045 minutes, and HW Braye is at 0645. If required you can now add the hour for Summer Time - 0745.
Similarly, from 0900 to 1500, the difference varies from +40 to +50 minutes, from 1500 to 2100 it is +50 to +40, and from 2100 to 0300 it is +40 to + 50.
One way of applying the differences is as follows:
The difference at Braye varies by 10 minutes over 6 hours, so each hour applies 1.66 minutes of difference between 0040 and 0050 minutes:
0300 50.0 mins
0400 48.3
0500 46.6
0600 44.9
0700 43.2
0800 41.6
0900 40.0 mins
But anywhere between 40 and 50 minutes you can guesstimate reasonably well. Or you do the whole thing with a calculator.
Poignard
Well-known member
I would just add 45 minutes.
Skysail
Well-known member
Me too!
Fantasie 19
Well-known member
Cheers guys... appreciated!
The more I did of them in the practice papers the more I realised it was just a matter of percentages... which is what you've confirmed.. simplistically - figure out what the percentage difference is in time to the standard port, and then apply the same percentage to the tide difference.... i think...
The more I did of them in the practice papers the more I realised it was just a matter of percentages... which is what you've confirmed.. simplistically - figure out what the percentage difference is in time to the standard port, and then apply the same percentage to the tide difference.... i think...
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Athene V30
Active member
When I am teaching tides to the URNU students some like doing the sums in their head others like 'crocodile teeth' graphs. Certainly easier to check their calculations if there is a graph to look at. Got a aide memoire somewhere.
When I am on my boat I tend to do it in my head (unless I am very tired when I will draw it out) and add in a safety margin then use the echo sounder to creep in if on a rising tide.
When I am on my boat I tend to do it in my head (unless I am very tired when I will draw it out) and add in a safety margin then use the echo sounder to creep in if on a rising tide.
snooks
Active member
I just work them out in my head. Nothing is that precise when sailing and if I have to be somewhere with a 10 minute margin for error then I shouldn't be going there.
It only has to be a rough educated guess and doesn't have to be worked out to the minute or centimetre. Nearest .3 of a metre and 20 mins suits me fine
It only has to be a rough educated guess and doesn't have to be worked out to the minute or centimetre. Nearest .3 of a metre and 20 mins suits me fine
Fantasie 19
Well-known member
I just work them out in my head. Nothing is that precise when sailing and if I have to be somewhere with a 10 minute margin for error then I shouldn't be going there.
It only has to be a rough educated guess and doesn't have to be worked out to the minute or centimetre. Nearest .3 of a metre and 20 mins suits me fine
Normally I would agree...
I suspect that an RYA examinor looking at my exam paper may feel slighty differently...
snooks
Active member
Normally I would agree...
I suspect that an RYA examinor looking at my exam paper may feel slighty differently...
Shore based is one thing, you are proving you know the theory. But in practice it's often very different. If you were doing your practical you would probably have just demonstrated you know what you are doing and they would move onto the next task.
Then again I only have my practical qualifications not my theory
I've never been marked on paper for the way I sail or anchor
Nostrodamus
New member
Thank you for all your replies.
I don't know if it is important to be taught the RYA way of working secondary ports out as how often do we need to be that acurate. As pointed out by other members the rule of 12th and some rough approximations is usually good enough in the real world.
I know it is a slght drift but it might be a good idea for the RYA to do a course on chart plotters, radar etc which I would like to see.
I don't know if it is important to be taught the RYA way of working secondary ports out as how often do we need to be that acurate. As pointed out by other members the rule of 12th and some rough approximations is usually good enough in the real world.
I know it is a slght drift but it might be a good idea for the RYA to do a course on chart plotters, radar etc which I would like to see.
Bosun Higgs
N/A
Normally I would agree...
I suspect that an RYA examinor looking at my exam paper may feel slighty differently...
Not necessarily. I would be more worried about a sailor who disappeared down below approaching port to work out the tide times to the nearest minute and cm.
I failed one woman because ( amongst many other issues it has to be said) she could only do the rule of twelvths if the tides were in feet and inches. I kid you not.
webcraft
Well-known member
.
Cockpit Cards have one of the clearest explanations of secondary port calculations that I have come across, and the simple graphic method of working out differences is easily grasped by most people.
- W
Cockpit Cards have one of the clearest explanations of secondary port calculations that I have come across, and the simple graphic method of working out differences is easily grasped by most people.
- W
GrumpyOldGit
Member
press buttons on my htc desire, downloaded the charts for £20 with android and now have instant accurate secondary port data in seconds. oh, and yes, I can use the almanac but this way is more accurate and quicker
Blue5
New member
And I guess with a mobile phone you could always phone a friend..........................