How to Calculate Tidal Heights at Secondary Ports
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- Опубликовано: 11 фев 2021
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Хобби
When you make your first triangle beginning about 6:30, you correctly mark off the hour marks, but on the x-axis, which are supposed to be every 6 minutes, your "42" mark should be 44.
This man is indeed correct! Thanks for highlighting the error Mike.👍 It's sod's law that that would be the figure we come to!
Interestingly linear interpolation gives an answer of 44.3 so OP's rough calc would have been very close if they had correctly marked 44 on the scale :)
@@timrolph1352 how did u get that result exactly, I keep getting 47.305 after interpolation
Very nicely explained, thank you kindly for helping me brush up on that knowledge from distant past. The interpolation is quite simple, and a mariner should be able to do that mentally.
My pleasure.
❤ Thank You Sir! I am from Latvia and I will have my Costal Skipper's exam soon. This video was very helpful!
That's great to hear. Good luck Margarita!
Fantastic - I am first timer and you helped me to understand the REEDS 2022 where tides at secondary port are concerned. Thank you.
Thanks, nice tutorial.
Thank you very much for this examples 😎
Fantastic thanks mate this is helping me with my Yacht master theory
Glad to hear it.
if the tide is higher than the HW spring figures for the secondary port what is the procedure in that instance. Thanks. Love your work
If I understand your question correctly (forgive me, it's been a while since I posted the video) then you'd use the actual HW height on the scale and draw the line between the low water and High water to get the gradient line. I'd then use the HW Spring curve to get the best prediction of the tidal movement on that date. Thanks for watching.
Hello & thank you for the tutorials!🙏 can you please tell why did you choose the 2nd HW ?
HI. We assumed our passage was in the period between the low water and the second high water. You want to choose the tides relevant to when you are on passage. Hope that helps. : ) ruclips.net/video/1x8UzIIZQEs/видео.html
Thank you sir, however, I was wondering, to calculate the times and heights of water between HW & LW, when do we use the pecked line of the curve (the neap water line) and when do we use the solid line of the curve (the spring water line) ?!
Thank you very much in advance.
We use the neap line at neaps. The spring line at springs and interpolate (estimate between) for those days between each.
Hello and thanks for a great tutorial. I have a question regarding using the tidal curve chart for Plymouth for Teignmouth by simply crossing out the name. The curve profiles are different for each port so using Plymouth is surely not accurate and could land you in trouble?
Hi K,
Teignmouth is a secondary port. The associated primary port for Teignmouth is Plymouth (defined in the almanac). If Teignmouth had its own tidal curve, you would use that, but in this case the almanac sends you to Plymouth to use that curve.
Once you have changed the times and heights using the differences table for Teignmouth, you can plot those on the Plymouth curve and it is deemed good enough.
It is always good seamanship to allow room for error or, for instance, for when there is a strong onshore or offshore wind, or a particularly high pressure or low pressure system over the location, all of which can effect tidal height or times.
@@FreeSailingTutorials Hi, thank you for making the time and effort to reply, really appreciate it. Regarding Teignmouth being a secondary port, yes that makes sense and im aware of the process of using the primary port's data for it , its just the actual flow profile that my question was really regarding because flow patterns will be different for every port so one cannot really be used for another accurately. sure the beginning and end differences in hw/lw times and heights and ranges can be easily recorded for different points in the tidal stream cycle but the flow rates inbetween will be different in every location . For example if you took the tidal curve of southampton compared to portsmouth they are right next to each other yet completely different in profile. Im referring to the actual graphical shape of the curve graph not the secondary data. Therefore using portsmouths curve for southampton would not be accurate in terms of flow rates vs time. Anyway if using the primary ports curve is deemed accurate enough then so be it.
@@justsayin644 That is correct, and you use two perfect examples because Southampton's flow is very different, having a HW stand. Therefore, its tidal flow is based on its LW time as that is more accurate. Both Southampton and Portsmouth are Primary Ports for this reason (and the fact that they are both major ports deserving of detailed surveys). Different secondary ports in the Solent area use either Soton or Portsmouth as their primary port graph as a result, dependent on their location. Cowes being an obvious example.
In the Solent, even secondary ports like Hamble have their own mini curves in the almanacs, but this is fairly unusual and is likely precisely because it is a complicated tidal area and is heavily used by various craft year-round.
Where these chartlets do not exist, the view is that a secondary port's tidal characteristics will be sufficiently similar for the allocated primary port's curve to be used. Of course, the secondary port's tidal heights and the times of low and high water relative to the primary port will vary because they are closer or further away from the largest body of water (in this case the North Atlantic).
All the best.
@@FreeSailingTutorials Thank you for explaining this in more depth. I was on the right lines i think just good to have it confirmed because as you say the Solent is fairly unique and a complex and busy tidal region. Great channel and content!
@@justsayin644 My pleasure. Glad you find it useful.
Sir do you also need to take into account the seasonal changes?
Do you mean atmospheric pressure?
@@FreeSailingTutorials Sorry for the late reply, I mean the seasonal changes found at the bottom of the page of secondary ports. I've seen other tutorials including those in their calculation
@@Leo-td4tj No problem. I'm reluctant to say 'no' because there may well be port or port information in various places that has further specific adjustments, but I suspect that would be something to do with the way the tidal information is set out. The Spring & neap tidal heights reflect the seasonal changes already. Do you have the link? I'd be happy to take a look and clarify? All the best.
i am stumped as to why the time of high tide in Plymouth affects the difference for tigenmouth
I'm replying on this without looking back at the video, Paddy, so forgive me if I miss your point. The tidal difference at Teignmouth (i.e. it's time) varies dependent on when the time of HW is in Plymouth. So we interpolate based on the measured difference between HW at both ports. We only have detailed HW times and heights for the Standard Port in the Almanac (in this case, Plymouth) and so we have to interpolate based on the variance of shown in the secondary port info in the almanac. I hope that helps clarify.
@@FreeSailingTutorials I am picking up this question as I believe Paddy & I have the same dilemma with this subject. To be clear, the process and maths to calculate times and heights at a secondary port are understood. And very nicely explained in the video, thanks. But.... I'll try to phrase Paddy's question in a different way; How does the Tide know to behave differently at different times of day, it's a force of nature not a clock watcher!! Its simple enough to understand that times and heights will vary from location to location but why isn't it a constant irrespective of time of day?
@@nickwoodward2234 Aha. I think I get what you are asking. Why does it know to be a different variance dependent on what time HW is in the day?
This has led me on a 30 minute wild goose chase. It seems many people know HOW to calculate the difference but few can answer WHY the times 0000/0600/1200/1800 are used. I think I have a sort of answer.
First of all, the times are taken from estimates of when the max and min differences are surveyed. Second, In many cases this has been shown to be around the times shown.
I suspect (this is an educated guess), that the variance is greater or lesser dependent on influence from the sun as well as, of course, the moon. In other words, at local midday, the sun is as high as it will be and above us. 12 hours later, the opposite occurs. The lesser effect of the sun, combined with the moon, might change the strength and size of the variance on any one day throughout the lunar month.
It should be born in mind that this is very much an estimate based on a simplified variance.
I found this from a fellow instructor, which attempts to explain further. It is from the UK Hydropgraphic office;
"...Time Differences
On looking at the Time Difference curve diagrams it can be seen that there is one ‘peak’ and one ‘trough’ along their length, which effectively refer to the maximum and minimum time differences tabulated for the Secondary Port in the Admiralty Tide Tables. The times at which these maxima and minima occur at the Standard Port being the headers used in bold in the Standard Port header information (e.g. 0000 & 1200, 0600 & 1800 etc. etc.)
The above explanations of how the time differences have been originally derived at the Secondary Ports also account for the variations seen in the Standard Port header information, for example as follows with Plymouth.
and
Essentially the differences are caused by the specific relationships between the Standard and Secondary Port(s) during the period of observed tides collected at both locations, i.e. the specific turning point of the maxima and minima has shifted slightly along the length of the time difference curve referenced above.
Reference to Spring and Neap Tidal Conditions
The ‘maxima’ and ‘minima’ referred to above could also be thought of in terms of Spring and Neap tidal conditions.
For example, looking at the Plymouth predictions for a Spring High Tide on a Spring gives the following prediction:-
So High Water Springs on this day occur at 06:37 and 18:57, which is relatively close to the columns shown above at 0600 and 1800 (remember this is only one day’s Spring Tide in isolation).
Similarly, Low Water Springs occur at 00:29 and 12:50, again relatively close to the Low Water column of 0000 and 1200 (and the ‘alternative’ column of 0100 and 1300).
The Neap Tide gives similar confirmation:-
High Water Neaps on this day occur at 01:19 and 13:42, which is relatively close to the columns shown above at 0100 and 1300 (and quite close to the ‘alternative’ column of 0000 and 1200).
Similarly, Low Water Neaps occur at 07:21 and 20:04, again relatively close to the Low Water column of 0600 and 1800.
Again this is one Neap tide in isolation. The non-harmonic ‘time stamps’ were derived from a much longer period of tidal observation.
Conclusion
The above information gives a general feel for the way in which the time and height differences have been established initially and how the Secondary Port data relates to the Standard Port data accordingly.
It is not an exact science!"
Christopher Jones
Head of Tides
United Kingdom Hydrographic Office
TO CONCLUDE:
This seems to boil down to approximations and I can only theorise that the times of max variance must be governed by local noon and midnight. Here is a link to a very long thread on lots of people trying to work out why. Either way, rest assured, if you can do the calc, and allow the usual 0.5m min margin of safety, you should be fine!
forums.ybw.com/index.php?threads/secondary-port-time-differences.355563/page-2
You say Teignmouth doesn't have tide times, and yet when you turn to the page it clearly shows the high water and low water times, above those 'differences'??
It shows the differences between the standard port and Teignmouth at specific times of the day. If HW at the standard port is between two of those times, say 0600 and 1200 then you have to interpolate to get the correct time adjusted for Teignmouth. 😃
Pronounced 'Tin-muth'
And I know that too! Thanks Phil.
Very useful explanation of secondary ports as well 😀
Holy crap this seems complicated. I'm just completely lost here. The book shows the high water times and the low water times and the depth. So what else do you need? If they are different from Plymouth, and the book knows what the differences ARE, why the bloody hell don't they just put that, instead of all this messing about with graphs and stuff? This just seems bizarre to me.
See my reply to your other question. Cheers.
Also, if you are entirely new, check out tidal times for standard ports first.
Crikey. You blokes all must read the same book.
It's almost like there's a method isn't it. 🤣