Tide gates and Other
costal cruising, tide gates and other tidal effects
can have a considerable effect on a vessels progress.
This is particularly true for sailing boats and the slower motor boats
craft will be affected.
As part to your passage
should take into consideration not just the height of the tide but how
streams are going to affect your progress.
This is most important when coastal sailing.
As well as all the traditional sources of tidal
information there are an
increasing number of electronic tidal planners.
They can even be downloaded as apps on the iPhones.
It should be noted though that some of these apps use
different sets of data
so there may be differences between the app predictions and the
However, just the fact that these are regarded as worth
building apps for,
is testament to the value of tide planning.
Of course this information is still available from traditional
where tide times from your tide tables is used along with tidal
diamonds on the
with tidal atlases.
While it isn’t always possible or desirable to go with the
knowing what it is going to do during your trip can allow you to use it
And one advantage is in making the best use of those tide
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A Tide Gate is the term we use to describe
an area of the coastal
water where the flow of the tide will at certain times make progress in
Usually they are associated with headlands, shallow patches and narrow
These are areas where the tidal steam is
concentrated and the speed of
becomes greater than the open water average.
Simple maths will show that someone traveling at say 4 knots in open
come to a standstill when faced with an extra 4 knots of contrary tide.
But as well as the reduction in traveling speed, that faster tide can
often create choppy seas and dangerous overfalls.
And the slower progress means that you will be in those adverse
longer, causing wear and tear on both the crew and the boat’s gear.
So, event though the Tidal Gate might not be shut, it will be prudent
until it is ‘open’.
Once the tide gate is ‘open’ the tide will be with you, progress
will be faster, safer and more comfortable and any time lost waiting
easily be made up.
Occasionally there might be local anomalies such as inside passages,
alternate routes which do not appear in the general tidal information
these can usually be found in Pilot books for those particular areas.
The prudent navigator will often fare better if he carries the tide so
waits for the turn before venturing further.
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When the tide is forced around any
over shallows or where a channel is constricted, it often produces a
which can cause overfalls.
Most charts mark dangerous overfalls and tide rips with a series of
and a note describing at what state of the tide they are most dangerous.
Often the dangerous area will be in one position on the flood tide and
another on the ebb.
These areas are to be avoided unless you have specific local knowledge
experience of say, an inside passage.
The overfalls often occur as the fast tide flows over an uneven seabed
turbulence and upwelling down tide.
Upwelling of cold water from the bottom can also increase the
fog or mist banks.
And where a fast tide is flowing against the direction of the wind it
cause waves to become steeper and possibly start to break, creating
conditions, which effectively close the tide gate.
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Wind Against Tide.
Very basically waves are caused
be wind blowing across the surface of the water, just as blowing across
surface of your coffee will produce ripples.
The stronger the wind, the longer it has been blowing and the longer
stretch of water (fetch) it has been acting on the larger the waves it
Where the water is also moving, with the tide or current this will
‘apparent’ wind strength across the surface.
So, if the wind and tides are going in the same direction the apparent
thus the waves will be reduced but if they are in opposite directions
the apparent wind and the waves will increase.
There are other factors such as the depth of the water which influence
shape and power of the waves.
And where the water is moving, the effect is further complicated, such
the wave itself is moving in the opposite direction to the current.
The result tends to be short, choppy waves with steep leading edges.
Not only will these be uncomfortable for the small boater but bashing
those steep wave fronts will also slow the vessels progress.
Once the tide turns the effect will be the opposite, the tide gate will
the sea will flatten and progress with the tide will be more
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A bar at the entrance to a harbour or
estuary is another shallow area where the tide or current will be
The bar may only be narrow and the water beyond deep but in the wrong
conditions this can be a dangerous area with steep waves breaking right
These should always be treated with caution, particularly when the tide
running strongly and there is an onshore wind.
Even when there is little or no wind, any swell running into the
be exaggerated as it crosses the bar.
Always allow for plenty of water under the keel.
Your tide tables might predict a sufficient mean level of water for
but where there are waves there will be less than that mean level of
within the troughs.
And bars can and do shift and change shape.
Bars are not normally regarded as tide gates but better to be patient
until there is plenty of spare depth then go in safely and enjoy a
at the other harbor bar.
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