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Old 28-03-2014, 09:39   #1
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Need help with tidal calculation and passage planning

Hello all!
In case this is not the right forum for the above subject, please bear with me and direct to the right one.
I am getting ready for RYA Yachtmaster Offshore exam and have some _theoretical_ questions regarding tides and passage plan.
My first question is about the right way to do tide height interpolation. The following example is taken from RYA Training Almanac (fictional waters):

Passage plan: from Bramhope Creek to Farlow River, Tuesday 28th May

Standard Port for Bramhope Creek is Victoria: time Zone UT, add 1 hour for summer time
Tide Mean Ranges: Springs 4.9 m, Neaps 2.4 m
Tide Heights at 28 May Victoria:
LW 0509 0.6
HW 1125 5.5
LW 1731 0.8
HW 2343 5.6

We are almost at Springs, range 4.9m, 4.8m

Time diffrences for Bramhope Creek:
Victoria High Water:
0000 0600
1200 1800
Differences Bramhope Creek
+0006 -0006

Questions:
1) In our case HW at Victoria is at 1125 while difference is given for 1200 and amounts to +0006. Though time difference is very small, yet for my understanding it would be intresting how to interpolate time difference in this case. So we have 1125 between 0600 and 1200 times:

Times:
0600 < 1125 < 1200
Diffs:
-00006 ? +0006

Time intervals in minutes:
0600 --325min-- 1125 --35min-- 1200
So 1125 splits time interval in two parts in ratio 11/12 : 1/12 approximately.

Now looking at time differences I have questions:
2) What is the length of [-00006, +0006] interval, is it 12 hours?
3) In what ratio should I split [-00006, +0006] interval?

Thanks for your help!
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Old 28-03-2014, 15:08   #2
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Re: Need help with tidal calculation and passage planning

dokondr,
It is not correct to interpolate the time difference with respect to the tide range. In fact, the time difference changes with the "age of the moon". Springs occur roughly 2 days after new moon and full moon, and neaps 2 days after the quarters.

If you are close to spring or neaps, you chose the time differences corresponding to one of these situations. In other cases, you just use an average value. In the example, 0000 and 1200 are the time of HW at springs (difference +0006), 0600 and 1800 are the time of HW at neaps (difference -0006). If HW is at 1125, you are just one day before "max springs". Then, it's OK to use +0006.

Don't try to be really accurate in tide calculations: the corrections for secondary ports are approximate, the tide doesn't follow exactly the rule of twelfths and the weather conditions (not only the atmospheric pressure) have a big influence. It is much safer to include a substantial margin: going aground is neither pleasant nor safe.

Alain
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Old 30-03-2014, 04:41   #3
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Re: Need help with tidal calculation and passage planning

Quote:
Originally Posted by Hydra View Post
dokondr,
It is not correct to interpolate the time difference with respect to the tide range. In fact, the time difference changes with the "age of the moon". Springs occur roughly 2 days after new moon and full moon, and neaps 2 days after the quarters.

If you are close to spring or neaps, you chose the time differences corresponding to one of these situations. In other cases, you just use an average value. In the example, 0000 and 1200 are the time of HW at springs (difference +0006), 0600 and 1800 are the time of HW at neaps (difference -0006). If HW is at 1125, you are just one day before "max springs". Then, it's OK to use +0006.

Don't try to be really accurate in tide calculations: the corrections for secondary ports are approximate, the tide doesn't follow exactly the rule of twelfths and the weather conditions (not only the atmospheric pressure) have a big influence. It is much safer to include a substantial margin: going aground is neither pleasant nor safe.

Alain
Alain, thanks for your 'in depth' explanation. I agree with all you wrote. In real life situation for this particular case I would not interpolate and would use +0006 difference for 1125. I would also include a substantial margin to have enough water below the keel no matter what weather it is. These are all very good reasons to take into account that you write about.

With all this said, at RYA exam I still have to demonstrate my ability to interpolate for the secondary port calculations and make examiner happy. That's why I asked how to interpolate time diff. on +0006, -0006 interval (see my first message).
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Old 30-03-2014, 14:11   #4
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Re: Need help with tidal calculation and passage planning

I don't own the RYA book, so I have no idea of the method they recommend. The instructions in the French tide almanac say (my translation):
Computation of HW and LW time
For the time, the linear interpolation between the springs correction and the neaps correction has no meaning. One should use for HW:
- either the springs or neaps correction if the time of HW in the standard port is less than 2h from the time of springs HW or neaps HW in this port;
- or the average of the corrections in other cases.
One should do the same for LW. The correction is less and less accurate as one is farther from springs or neaps.
Maybe the correct interpolation is sinusoidal with respect to the age of the moon, approximated by the rule of twelfths?

Anyway, this whole business is about coastal navigation, not offshore!

Alain
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Old 31-03-2014, 01:42   #5
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Re: Need help with tidal calculation and passage planning

Quote:
Originally Posted by Hydra View Post
One should use for HW:
- either the springs or neaps correction if the time of HW in the standard port is less than 2h from the time of springs HW or neaps HW in this port;
- or the average of the corrections in other cases.
Alain
How exactly "average of of the corrections in other cases" is calculated? Like this:

(MHWS + MHWN) / 2 ?

where:
MHWS - Mean High Water Springs
MHWN - Mean High Water Neaps
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Old 31-03-2014, 03:04   #6
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Re: Need help with tidal calculation and passage planning

The RYA textbook way is to use the "computation of rates " Graph

Plot you springs and neaps then do a linear interpolation based on the required range at the date you need. Heres an example ( not your data) . With secondary ports just compute the time and range differences from the primary port and plug it in.



I can walk you through it if you wish

dave
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Old 31-03-2014, 03:58   #7
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Re: Need help with tidal calculation and passage planning

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Originally Posted by goboatingnow View Post
The RYA textbook way is to use the "computation of rates " Graph

Plot you springs and neaps then do a linear interpolation based on the required range at the date you need. Heres an example ( not your data)
Dave, thanks! My question that started this thread was not about tidal stream rate, but about finding tide times and tide heights at secondary ports. And in particular case when times and heights differ from those given in the difference table.

BTW, are there any web sites / ipad apps that calculate times and heights of tide at secondary ports?
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Old 31-03-2014, 06:11   #8
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Re: Need help with tidal calculation and passage planning

OK sorry , should have read more closely

TO find secondary ports, you compute the range and time offset from the reference port, Then you use either a specific tidal curve or the standard sinusoid curve and plug in the new times and ranges. extract the height of tide.

Alternatively use rule of twelfths


heres a work through using simple interpolation

Sailtrain: Navigation and Chart work, Chart information, Secondary Port calculations.



hers one with a walkthrough using the primary port tidal curve

http://www.penguinsailing.com/1%20-%...ry%20ports.pdf

Note this site has walkthrough for all the standard tidal height questions, well worth going through see http://www.penguinsailing.com/tidalcalculations.html


NOTE: I suggest a walk through the penguinsailing notes on "advanced height of tide" calculating safe anchoring heights, The RYA exams loves these types of questions and some people get confused by the using the amount of tide to fall, rather then computing absolute heights at a location.

Dave
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