No two coastal places have exactly the same tidal pattern. There are many factors that influence this, from the shape of the coastline, the funnelling effect of channels and estuaries, the depth of the sea and the proximity of oceans. The times and heights of tides are available from numerous sources in print and online, all over the world. Most sailors who keep their vessels in tidal waters have printed copies aboard, as well as on their electronic devices.
Tide tables for the larger ports and harbours, known as Standard Ports, are published each year and show all the times for high and low water throughout the year. Smaller harbours and havens are referred to as Secondary Ports. My local harbour, Chichester Harbour in southern UK, is a standard port and we will use this as an example to explain some details about understanding tide tables. In this part of the world we have twice daily (semi-diurnal) tides, i.e. two high tides and two low tides every 24 hours.
The information given in the nautical almanac for 20 April 2023 is as follows (see Figure 1): Low Water (LW) is at 0429 Universal Time (UT). We have to add one hour to this time because 20 April is in British Summer Time. So this means LW is at 0529 BST and the height = 0.6m above CD (Chart Datum). High Water (HW) is at 1234 (BST) with a height of 4.9m and so the tidal range is 4.3m. If you look at Figure 1 you will note that 21 April is marked in red which means it is a spring tide on the following day. All this information is useful for a navigator who needs to work out when the water level will be at a specific height or what height of tide we will require to navigate up shallow channels in the harbour.
In order to work these times and levels out, the navigator uses a tidal curve as shown in Figure 2.
A tidal curve is a graphical representation of the predicted tidal heights and times for a specific location. Tidal curves use the same tidal data that is found in tide tables, usually presented as a graph with time on the horizontal axis and tidal height on the vertical axis. The curve is in two parts, the straight linear graph on the left and the curved part on the right (see Figure 2).
The curved part of the drawing shows the pattern of the rise and fall of the tide. Notice that there are two curves, one dotted for neaps and the solid red one for springs. The boxes along the bottom of the graph are where the specific times are entered at one hourly intervals between HW and LW.
Finding a time for a specific height of tide
As an example, let us assume that we wish to drop our anchor at East Head and stop there for a few hours on 20 April where there is a 4.9m height of tide at HW. Our sailing boat has a draft of 1.2m and let us say we will require at least 0.3m below the keel to remain afloat. We can see from the chart that the anchorage inshore from a charted depth contour dries to 2.4m above chart datum at MLWS. At HW we will have plenty of height of tide beneath us, more than 3m (4.9m – 1.5m = 3.4m). We arrive at 1230 and drop our anchor. The depth gauge shows we have 2.8m beneath the keel which tells us we have crossed the 2.4m contour. How long can we stay before we need to anchor in deeper water?
This is where the tidal curves comes in – we can use it to calculate when we have to move further out into deeper water. First, we draw a straight line from MHWS at the top of the graph to MLWS at the bottom (shown in green). Then we draw a line from 3.4m at the top (shown in brown) until it intersects with the green line, then from there to the mean spring curve (in red) , then vertically downwards to the bottom of the graph. We can then read off our answer, which is 20 minutes after 1449, i.e. 1509. To be on the safe side we should raise our anchor at 1500.
Note: Source material for graphics and information taken from Reeds 2023 Nautical Almanac.