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today on quick tips we're going to look
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at how to calculate tidal streams
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using a tidal atlas and an almanac
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okay so first of all um it's just worth
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pointing out that we're looking at
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two different um an almanac and a
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an atlas uh the solent and adjacent
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waters are the area that this particular
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stream
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tidal stream atlas covers and i'm using
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the channel
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almanac the reach channel almanac so
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those are the two documents that i'm
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using
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in order to calculate title
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flow information uh in this area
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right so that's what a tidal flow of
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tidal stream atlas
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looks like and you can see we've got
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a page for every hour
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of the tidal
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title flood and title ebb based around
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high water
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and it's high water portsmouth
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in this instance so always check what
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the
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title atlas refers to you can see it
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also refers to 1 hour 20 meters
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sorry when i had 20 minutes i think it
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is after dover high water so you could
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use dover or portsmouth
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dependent on whether or not you've got
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the high water
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times for dover or portsmouth but the
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almanac has both so
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let's use portsmouth it's easier as you
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can see there are arrows
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so those arrows that the bigger the
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arrow the darker and deeper the arrow
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uh the more tidal flow there is the
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smaller the arrow
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the less uh tidal flow there is and then
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you can see there
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are numbers adjacent to the arrows
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adjacent to some of the arrows
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and those numbers for instance you've
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got there 16
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comma 32 it's actually
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one decimal six and three decimal two
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we'll go into that
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in a little bit but you can see
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basically there are
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parts of the um terrain which have
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more tidal flow at specific times
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of the um of the day depending on how
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far before or after high water we are uh
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you can also see that we've got a
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latitude and longitude scale across the
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side
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so that we can determine with some
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accuracy when those tidal flows were
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taken when they were surveyed
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so it could be relatively accurate and
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as i say
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it's high water portsmouth that this
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tidal stream atlas
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is referring to so you need to know
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the time of high water in portsmouth on
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the day that you're sailing
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so that you can work out tidal flow for
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the 12 hours
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the six hours either side of high water
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portsmouth on that day
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and you can see each page has got a
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different flow and you've got slack
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water there on the top page
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and then it starts to flood
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at six hours after high water
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okay so let's first of all look at the
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high water
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portsmouth page and fill that in for the
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appropriate title hour
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for the day that we're sailing so here
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is a
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title atlas uh sorry title hammer
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sorry an almanac and uh you can see the
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title information here for portsmouth
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so the area shown in green shaded
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uh those are uh times as a universal
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time
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or greenwich mean time in other words
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our standard time in the uk
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if you're looking at an area which is
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not shaded so anywhere that's white
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then that is british summer time which
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is one hour ahead of greenwich mean time
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so for an area that is non-shaded
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so for instance april
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we need to add one hour to get to
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british summer time because everything
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is in greenwich mean time
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so let's have a look and let's choose
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saturday the fourth of april
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uh high water is eight fifteen zero
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eight fifteen at three point nine meters
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that's high water
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zero eight fifteen on saturday fourth of
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april that's utc or greenwich mean time
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same thing
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that's our high water in portsmouth so
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um we're going to put a time
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for high water portsmouth onto this
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uh of the tidal flow
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now we have to add one hour because
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we're now in summertime british summer
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time
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so adding one hour to make it bst
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so that gives us 0 9 15 as high water
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portsmouth but
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we need to calculate the tidal hour
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so the tidal hour it's not a difficult
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calculation
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it's basically we're assuming that the
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tide is doing what it says it's doing on
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that page half an hour either side
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of the specific time we've calculated
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from the uh almanac
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so we've got 0 915 bst as high water
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so half an hour before that and half an
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hour after that
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that makes up our entire tidal hour and
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that
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is the time we write in as the beginning
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of the tidal hour
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on this particular page of high water
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portsmouth
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so let's write that in zero eight forty
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five which is half an hour before nine
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fifteen to zero nine forty five that's
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our title hour
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so once we've got our tide layer we can
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then write in
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zero eight forty five onto
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our page for high water
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put in bst so that we know we've done
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the conversion already
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and that tells us that that's what the
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tidal flow is doing at
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that time at high water an hour later
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an hour after 9 45 bst 10 45 bst
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is two hours after and so on and we can
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do that before and after high water
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so that for the entirety of that day we
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know what tide flow is doing
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in that location
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uh now if we just look at
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to clarify what i was saying about the
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numbers we've got 0
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4 comma 0 9 so
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0 4 comma 0 9 represents 0.4 knots of
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tidal flow
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at neaps or 0.9 knots of tidal flow
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at springs at that location
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and it's flowing in the direction of the
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arrow and obviously as the tide ebbs
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and floods the
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direction and the velocity of the water
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will change so the tidal flow will
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change both in direction
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and in speed and if it springs where
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you've got a lot more water moving in
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and out in the same time period then of
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course the tidal flow
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will be stronger that's why we've got
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0.4 or 0.9
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in that location other locations 1.6 and
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3.2
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1.4 and 2.8 so you can get all sorts of
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different fly
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different uh title streams
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okay so that shows you how to calculate
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tidal streams
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both the rate and the direction for any
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given hour
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on any given day as long as you've got
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the title information
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for the standard port being used in the
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arm in the atlas
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and you have an almanac covering those
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tidal high water
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times so as long as you know the high
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water time for the day
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all the times for the day and you have a
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tidal atlas showing
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the day part broken up into
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six hours either side of high water then
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you can always know what your
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tidal flow direction and rate is going
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to be
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depend on whether you're on neeps or
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springs there is also
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um for um people that are getting a
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little bit more advanced
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a computation of rates calculation which
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will give you a more accurate
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interpolation
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between neaps and springs but for for
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the day skipper candidate
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on the roa course if you're halfway
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between neeps and springs
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and you take an average of the two then
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that would be
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good enough i hope that that's of some
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use and
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speak to you again soon i hope you found
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this short video tutorial useful if you
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did please click the like button
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experience you'd like to share
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under this video until next time stay
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