CLIVE LOUGHLIN
SEAMANSHIP
Fig 1 Approach to the Hamble (Imray 2200.7).
Tackling the tides
For some reason, many people get freaked out by tides but, as
Clive Loughlin explains, if you don’t panic and you work through them
logically, there’s really nothing to worry about. This month he shows how to
calculate tidal heights and how they influence our everyday sailing activities.
K
nowing the height of tide at ‘14’ means that the depth of water is 14m Chart Datum and at Portland LAT is 0.2m
a given moment is of vital below Chart Datum. below Chart Datum (source UKHO NP
importance for our sailing 201-09 Table V).
activities if we are to avoid the CHART DATUM The complications that such differences
embarrassment and danger of running Chart Datum is a fixed vertical reference might create for our sailing activities are
aground. In this article we will calculate relative to which all chart depths and thankfully minimised, because all tidal
the height of tide as accurately as possible, heights are based. On the chart itself it data published by the UK Hydrographic
but while we are doing this we must says: “Chart Datum is approximately the Office is based on the Chart Datum of the
always bear in mind that, when on the level of Lowest Astronomical Tide (LAT)”. largest scale Admiralty chart of the area
water, we need to add in a safety margin We can think of Lowest Astronomical concerned, so tide tables can be used, in
that can only be judged by the skipper at Tide as the lowest low water we are conjunction with the depths shown on the
the time. ever likely to encounter under average chart, to tell us the correct amount of water
Fig 1 shows the approach to the Hamble meteorological conditions. The chart we should find under our keels. But it is
River from the Southampton main says ‘approximately’, because it does vary worth remembering that we cannot blindly
shipping channel. In the blue areas we can from one area to another and by a not assume that we can take our 1.8m boat
see the numbers ‘14’, ‘12’ and ‘11’. These inconsiderable amount. For example, in anywhere with at least 1.8m charted depth.
are the charted depths of water and the Chichester Harbour LAT is 0.2m above The wind and atmospheric pressure
102 Sailing Today September 09
ST149 Seamanship
TIDE_RTG.indd 78 16/7/09 18:08:41
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