A Free Tidal Lunch
Canny racing boats battle to eke every tenth of a know of
advantage from the tide. The following guide from Towergate
Insurance, specialist providers of Yacht Insurance, strips the complexities away to
help the more relaxed boaters get there quicker and more easily
without sweating the details. They say that nothing is certain but
death and taxes, but there is something that is an unavoidable
aspect of boating in British waters – the tide. It might flow
strongly, it might flow weakly, but flow it will; a mass movement
of water dragged around the British Isles by the gravitational pull
of the moon. It is an enormous source of free energy, and while
scientists work on efficient ways to harness this energy to power
light bulbs and cities, the British boater can take advantage of a
free tidal lunch just by getting the timing right.
We’ve come up with a handful of things to think about before
departure, to try and ensure that the tide is making life easier,
and not harder.
- Understand the basic concepts: if that sentence about the moon
dragging the water around the British Isles was news, then take a
little time to learn about why we have tides. This video is a great
introduction: http://www.youtube.com/watch?v=CTQ6ciHENgI&feature=youtu.beNgI
- Do the homework on tide heights and times: it may sound
obvious, but it’s hard to take advantage of the tide without
knowing which way it’s flowing and what time it stops and starts
going the other way. There are some great resources available. The
Admiralty (aka; The UK Hydrographic Office) is the oracle when it
comes to British Isles tides, and they have a good online
prediction service that provides seven days of free basic data, not
just for Britain, but
worldwide
- Research the tidal flow: knowing the time and height of the
tide is only the beginning; the really useful information is the
direction and rate of the flow. There are two ways to find out
which way the tide is flowing at any given place, at any given
moment. The first and most traditional method is to look on a
regular Admiralty (other brands are available) chart, and find the
closest tidal diamond. The diamond lists which way and how fast the
tide is flowing at that geographical point at hourly intervals –
there’s an example and a
further explanation right here
- Working out the tidal flow using tidal diamonds can be a bit
time-consuming, but fortunately for many popular sailing areas
(especially in the UK) there are tidal stream atlases. These use
arrows and/or colour to show the direction of flow at different
times – go back to that last link and scroll down a bit to see an
example. We’re very lucky in Britain as the Admiralty have done a
great job of covering British and Northern European waters with a
series of
Tidal Stream Atlases
- The Admiralty aren’t the only people producing them though, and
many variations can be bought from chandlers and bookshops.
Armed with the
relevant time of high water and the right tidal atlas, a few
minutes work will tell us which way the water is travelling and
when that might change. Bearing that in mind, here are some things
to remember when planning a trip:
- The tide runs more strongly at the surface of deep water
compared to shallow water: this is because friction with the seabed
slows up the flow of water close to it, with the effect declining
towards the surface. It follows that in deeper water the
slow-flowing-water at the seabed will have less impact on a boat
floating on the surface. So the simple rule is to head for deep
water when sailing with the tide, and to head for the shallows (but
not too shallow, or you will need that Towergate insurance) when
sailing against the tide.
- The tide runs strongest on the outside of bends: everything
else being equal (like the depth of the water) then the outside of
a bend in a channel is the place to be when sailing with the tide,
and a place to avoid when sailing against the tide.
- The tide changes first on the shoreline and only later does it
turn out in the deep water channels: so when sailing with the tide
stay out in the channel until it has turned, then head inshore to
take advantage of the relief from the tide available in shallower
water.
- Tidal planning: the idea is to spend as much time as possible
with the most advantageous tide. Sometimes this will be obvious
from a glance at the tidal atlas. If the journey is straight down
the coast and the tide will be either flowing with or against the
boat, then if the journey time is expected to be nine hours it
makes sense to leave at a time that provides six hours of sailing
with the tide, and only three hours of sailing against it.
Now, I
know we said we weren’t going to sweat the details, but let’s take
a quick look at a more sophisticated approach for more complicated
journeys. It’s going to require a little calculation. Pick a time
to start the trip, and work out which way the tide is going for the
first hour. Let’s say it’s flowing in the direction the boat is
going at a couple of knots, so write down +2.
Then use the tidal speed and the expected speed of the boat to
calculate roughly where the boat will be at the end of the first
hour. Go back to the tidal atlas to work out what the tide is going
to be for the second hour of the trip. Let’s say that it has
dropped in strength a little to one knot, but is still going with
the boat, so write down +1 under the first number.
Once again use the tidal speed and the expected speed of the
boat to calculate how far the boat will have travelled by the end
of the second hour, and return to the tidal atlas to find out what
the tide is going to be for the third hour of the trip. Perhaps the
tide has now gone slack, and so we need to write 0 under the first
two numbers.
Carry on with this until the tidal planning arrives at the
destination, and then add up the numbers – the sum is the total
amount of tide that the boat will see on the journey. If the answer
is positive then that’s good – just like the first example where we
were sailing for six hours with the tide, and against it for only
three hours. A negative answer is a bad one, so try a different
departure time and see if it helps. A bit of practice and some
experience, and it becomes possible to quickly narrow down the best
time to set off.
