TIDES: PART 1
KNOW THY DEPTH
If we are including any height of tide calculations
in our sailing activities, it is pretty much essential
that we have our depth sounder calibrated so that it
shows us the total depth of water that we are sailing
in. If we do this then everything hangs together.
Depth of water = charted depth + height of tide
If we don’t, then untold mathematical gymnastics
are required.
Fig 2 Tidal curve for Portsmouth using height of tide data for 17 August 2009.
can also affect the height of tide and a actually occurred at 0910 BST.
difference of 20-30cm may be routinely We can then fill in the hour boxes at the
expected. High pressure reduces the height bottom of the curve to cover the times
of tide, because the water is literally we are interested in (Fig 2). We don’t care
pushed sideways towards lower pressure what the tide was doing before 0910, so
areas, and winds blowing for a period of don’t bother writing it in. Each box has 10
time towards the coast will increase the minute graduations.
height of tide as water is pushed towards We then mark the height at low water
the shore. (1.7m) on the lower tide height scale and
All these factors need to be taken into the height of high water (4.0m) on the
Fig 3 Tide table for
Portsmouth,
consideration if we are planning any upper scale and draw a line between them.
17 August 2009.
passage that takes us close to the limits of This is our Tidal Range line.
depth for our boat. It is also important to To find out the height of tide at 1100 we
Fig 1 Approach to the Hamble (Imray 2200.7).
remember at all times that the tide tables draw a line (green) from 10 minutes to the Where can we anchor?
are only predictions. The UKHO has done left of the centre of the 1110 box, going
One time when we really do need to know our
its best, and its best is normally very good, up until we hit the tidal curve. Two curves
tides is when coming to anchor. Unless we are
but the tables come with no guarantees. are shown – the dashed line depicts the
happy to wake up at a jaunty angle, we must
curve for neaps and the solid line shows
ensure that our boat will still be afloat at low
CALCULATING HEIGHT OF TIDE the curve for springs. Because we are just
water. Basically this is just a matter of working out
Tide tables tell us the times at which a day after neaps (neaps occur about 2 how much further the tide will fall and if we add
specific tidal events occur and tidal curves or 3 days after a quarter moon, which is this to our boat’s draught we will find out the least
(Fig 2) show us how the height of tide marked in the tide tables), we draw our
depth in which we should anchor. The equation is:
varies over time. The curves are normally vertical line to just a bit above the dashed
Minimum Depth for Anchoring = Height of tide now
centred around the time of high water. line. We then come across horizontally
- Height at low water + Draught
We will now work through how we until we hit the tidal range line and then
can determine the height of tide at any from this carry on up to the height of tide
Can we go under it?
given time. scale where we find out that the expected
If we want to pass under a bridge or electricity
We are in Portsmouth on Monday, 17 current height of tide is 3.7m.
power line then we need to know the height of
August at 1100. A look at the tide tables
our mast above the waterline. Our charts show the
(Fig 3) tells us that it was high water CAN WE GO OVER IT? ‘Charted Vertical Clearance’, which is the distance
at 0810. This time is Universal Time If we look at Fig 1 and imagine that between the underside of the structure and water
(same as Greenwich Mean Time) and we are coming out of the Hamble
level at Highest Astronomical Tide (HAT). For a
so, because we are now in summer time, and wanting to head northwest for
power cable there may also be a ‘Safe Vertical
adding an hour determines that high water Southampton docks, we could either
Clearance’, which is lower than the charted
travel south between
vertical clearance to allow for a spark gap safety
margin. Always base your calculations on the least
the port and starboard
HAT Highest Astronomical Tide
vertical clearance on offer.
marks until we reached
If our masthead height is comfortably less than
MHWS Mean High Water Springs
the Hamble Point south
the stated vertical clearance then we can proceed.
Current sea level
cardinal and then turn
However, if it is close to or greater than the stated
MHWN Mean High Water Neaps towards Southampton clearance then we need to check how far the
Height of Tide
or we could sharpen current water level is below HAT.
MLWN Mean Low Water Neaps our pencil and see if
Unfortunately, the tide tables do not normally
we have enough height
tell us what the value of HAT is and so the best we
MLWS Mean Low Water Springs
of tide to be able to
can do is take the safe option and assume it is the
Chart Datum / LAT*
cut the corner and save
same as Mean High Water Springs. The equation is:
*LAT is normally the same as Chart Datum, but it can be higher or lower
ourselves about 1M by Minimum vertical clearance = Height of mast
crossing Hamble Spit.
>> above waterline + Height of tide - MHWS
September 09 Sailing Today 103
ST149 Seamanship
TIDE_RTG.indd 79 16/7/09 18:08:54
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