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A tidal gate.

August 30, 2010

Maggie, one of the Stunning Ruins, has requested a brief explanation of my use of the term ‘tidal gate’ in our post about going making passage through the Straits of Messina, so I thought I’d use a bit of down time during the crossing to Corfu to fill write a little bit about the topic.  So make sure you’re sitting comfortably, pay attention and read on – there may yet be questions to follow.

In the waters round UK, the tides have a significant effect on navigation, as the difference between high and low water can be up to 12 metres in places like the Bristol Channel.  On the Menai Straits where we kept ‘Rampage’ before we left UK, the range is about 6 metres.  The effect of this twice daily inrush and emptying of water is to give rise to currents that flow round the coast, changing direction by 180 degrees as the tide turns. 

In the Menai for example (admittedly a fairly extreme one) the tidal currents flow at up to 8 knots in the narrowest parts of the strait.  For a boat like ‘Rampage’, which does 7 knots at best speed under the motor, this means that you have to plan your movements to match the tide.  So, for example, when we wanted to get out of the Menai to sail to Ireland, we had to make sure that we got to the exit from the straits as the tide was starting to ebb (flow out to sea), so that the current was with us.  On our return to port, we had to make sure we arrived as the tide flooding into the straits, about 3 hours after low water so that there was enough water to get us through the shallow entrance water and have the current with us.

With me so far?

The effect I’ve outlined above is known as a tidal gate, because if you don’t get to the right place at the right time, you have to wait until the ‘gate’ opens up again before you can proceed to your destination.  The Menai is an extreme example, where you actually cannot make any progress into the adverse current.  In most places, it’s simply a case of butting into the current and losing time rather than actually being unable to make progress.

Now, the Med is basically non tidal.  There are tides but the range is about 10cm, so for the most part they can be completely ignored.  The only time you might encounter any noticeable effect is when you’re in a harbour and you might notice that the water level goes up and down by a few cm during the day.

There is, however, one little bit of water where the tide actually produces a current of significant proportions and that’s on the Straits of Messina.  The current runs north for 6 hours, then reverses and runs south and so on in a daily cycle.  In fact, the time period is actually about 6 hours and 45 minutes (it’s tied to the lunar day) and the current can be up to 4 knots in places.

When you add to this the fact that the shape of the bottom of the Straits lead to the formation of some pretty large whirlpools, it makes the Straits an entertaining place for a small yacht to be.  Then to add to the fun, just about the narrowest part of the Straits is where the ferries operate – there seems to be about 20 of them, all hurtling back and forth and ignoring anything smaller than they are.

To complete what is already a fun picture, the wind does pretty peculiar things in the Strait as well.  We arrived at the northern entrance in pretty much a flat calm but by the time we were 2 miles in we had about 15 knots of wind pushing us down toward Messina.  The following day, we once again got a terrific wind from the north, which sprung up out of nowhere as we headed down the Strait.

Well, there you are.  Tidal gates explained and a little bit about the Straits of Messina.  Any more questions from the class?

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2 comments

  1. Well thank you Duncan for that very interesting explanation of a tidal gate. Basically you have to go with the flow! Also impressed with your knowledge of the Greek classics. I’m wondering if I should add them to my holiday reading … mmm!


  2. Thank you Duncan. As ever, a very informative lesson! I love your lyrical description of sailing at night too. I, in contrast, went back to school today – far more prosaic, and perhaps less educational. M x



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