CLIVE LOUGHLIN
SEAMANSHIP
FIG 4 Tide table for
Southampton,
17 August 2009.
Fig 5 Tidal curve for Southampton.
Consulting the chart, we see charted reasonably expect. selected Standard Port.
depths of 16 indicating that if the height of We know we need at least 3.4m For our next exercise we are up the
tide was zero (i.e. the sea level is at Chart height of tide and so, this time, instead Beaulieu River. A study of the chart (Fig 6)
Datum) then this area of the mud bank of working out the height of tide at tells us that the depth of water over the bar
would be 1.6m above sea level. Our boat a specific time, what we are more that guards the river entrance is just 0.9m
draws 1.8m and so we need at least 3.4m interested in are the times when the at Chart Datum. Our boat draws 1.8m and
(1.6 + 1.8) height of tide before we can height of tide is at least 3.4m. So we so we need at least 0.9m of tide or we will
make it across the spit. start by drawing our line (green) at 3.4m not be able to leave.
If we have a quick look at the tide on the height of tide scale, come down The UKHO selects a standard port with
tables for Southampton (which is just 5M until we hit the tidal range line and then a tidal curve closest in shape to that of the
away) (Fig 4) we can see that high water horizontally until we hit the tidal curve. secondary port. We might expect Beaulieu
on 17 August is either 3.9m or 4.1m. If We then draw lines down from each to be linked with Southampton, but in fact
these values had been less than the 3.4m intersection of the tidal curve to give it is instead linked to Portsmouth, which is
that we need to cross the spit then we us a first time of 1220 and a second time a bit further away.
would instantly know that we had to go of 1944. Fig 7 shows the tide table entry for
the long way around the Hamble Point This tells us that in the interval the Beaulieu River. The standard port
cardinal. Fortunately they are higher and between 1220 and 1944 the height of is identified as Portsmouth and the
so we know we have a fighting chance. tide is less than 3.4m. It is now 1100, so corrections are for Stansore Point, which
The tidal curve for Southampton (Fig we need to make haste or be prepared is just outside the river entrance.
5) is centred on the time of low water, either to take the long route around the
because this is more clearly defined in this south cardinal or wait until evening. As SECONDARY TIMINGS
area than the time of high water. The time we start to leave the Hamble, we should The table enables us to correct for the
now is 1100 and, because the next low re-check the time before we head over time of high and low water, relative
water is at 1434 BST (1334 UT), we write the spit. Let’s say it is now 1130. We to Portsmouth, however the correction
this in the LW box and we again draw in can quickly scan our tidal curve (dark depends on the time of day that high or
our tidal range line. blue line) and see that the height of tide low water occurs.
In practice we would probably choose should be about 3.7m. In theory we If high water occurs in Portsmouth
to round times off to the nearest 5 or should have 0.3m (about a foot) of water around 0000 or 1200 then the adjustment
10 minutes and would certainly only under our keel, but we should weigh up for the time of high water at Stansore
do depths to the nearest 0.1m, but for whether we should risk running aground Point is -0030 which means that it will
these exercises we will be precise to aid on a falling tide. If we do decide to occur 30 minutes before high water
identification in the figures. proceed, then we should keep a very firm Portsmouth. Alternatively, if high water
It is quite common for the two low eye on our depth sounder. Portsmouth occurs around 0600 or 1800
tides and two high tides for a given day then the adjustment for Stansore Point
to differ by 0.1m or so, in which case it SECONDARY PORTS is -0010 or 10 minutes earlier. The
is a wise precaution to use the values of It is impractical for the UKHO to provide adjustments for low water are worked out
the lower low water and lower high water tidal curves and full tide tables for all in the same way.
(1.7m and 3.9m), because this gives ports and so some are designated as The times selected for reference are
us the least height of tide that we can ‘Secondary Ports’ and are referenced to a based on the typical timings for high
104 Sailing Today September 09
ST149 Seamanship
TIDE_RTG.indd 80 16/7/09 18:09:05
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