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Back to Basics : How to judge the tides
We need to know as
much as possible about the tide as it is our friend and our enemy. We all
know that the tide rises and falls and what looks like a beautiful bay at
high tide can turn into a foul smelling muddy swamp as the tide recedes. I'm
sure we have all launched from a nice beach only to arrive back to find a
mammoth carry, or drag, over nearly a mile of sand or mud.
Believe it or not,
there is a way to avoid these pitfalls and it is something that can be learned.
There are bits, however, that do appear very close to black magic if
you have little or no knowledge.
You do not have to have been kayaking long to
appreciate that the tide also moves in various directions along or away from
the coast at varying speeds. This latter fact is the more difficult to learn
and the more difficult to allow for.
It is taken as a fact that most paddlers can
maintain 3 knots (mph at sea) so, if the tidal flow is against him at 2
knots, he can only make 1 knot progress. However, if he goes with the flow
he makes 4 knots progress. So it is obvious that we need to paddle with the
flow whenever we can.
On many coasts of Britain, particularly the
NorthWest, the tide can make 6 or even 12 knots! Don’t even think about
paddling against these.
So how do we start to understand? What follows
is not all there is to it, but it may go some way towards helping:
Tide Tables
As the tides work on a Lunar Day (Moons orbit of
earth) which is 24 hours and 50 minutes and there are two high waters and
two low waters each day it is possible to calculate the times of High water
and Low water for each day. The High water will be 50 minutes later each day.
These ‘predictions’ (due to weather and
local conditions times can never be exact) are produced showing all the
times of High and Low water throughout the year. This is for a particular
spot eg. Liverpool or Dover and these places are called ‘standard’ ports.
LIVERPOOL
December 1998
Date
Time
M
Ft
5th Thur
0608
0.4
1.3
LW
1128
10.0
32.8
HW
1830
0.5
1.6
LW
2349
10.1
33.1
HW
Here we can see the times of the tides and also
the height in metres and feet.
The height is very important, as it is from this
we can calculate the speeds of tides. We need to calculate the ‘range’.
This is the difference between High water and Low water. In this case, 9.6m
or 31.5 feet for the morning tide.
However if we now look at the tide table a week
later we will notice a distinct difference.
12th Thur 0523
7.5
24.6 HW
1209
3.3
10.8 LW
Range 4.2 metres or 13.8 feet.
These are called ‘Springs’ for the large
range and ‘Neaps’ for the small range. The Springs occur every 15 days.
If you can imagine a Spring tide in harbour exiting through a harbour gate,
it only has the same time to get out as a Neap tide. In this case, there is
over twice as much water. Which will be faster? Thus you can see that on a
Spring tide we will either have a very quick ride, if we get the direction
right, or else we will be pushed backwards.
So, unless I’m very much mistaken, we don’t
all want to paddle from Liverpool, or even Dover. In the tide tables you
will find a list of ports near to your standard port. These are known as
secondary ports. Next to their entry you will see a + or – sign and a time,
in hours and minutes. You must apply this time to the Standard port and you
will have the time of local High or Low water.
Eg.
Standard Secondary
Liverpool Wick
0523 HW
Difference +0 11
= 0512 hrs.
Of course you may not be exactly at a Secondary
port, so you have to look at the ones either side and make an educated guess.
Well some of it’s witchcraft!
Armed with this information we are able to make
some decisions already. When can we avoid a long carry? And where can we go
to, with the tide, and come back from, with the tide? The latter is
bordering on the art of true Black Magick.
It is also worth looking at some books on
navigation for yachtsmen and learning some more basics about charts and the
symbols found on these. There is lots of information on a chart, other than
a picture of the coast.
In the next issue, we will continue to find out
how to calculate tidal speeds and see how to make the tide work for us.
We are very much obliged to Ken Rawle for stepping
aboard MarineZine and saving our kayaking bacon...he clearly has
more knowledge of the subject on the back of one elbow than we do en masse
and we have thoroughly enjoyed this sensible treatise on the beginnings of
learning to read the tides. We're looking forward to hearing more, next
issue.
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