The tide tables are used to determine the heights and times of high and low water at all the major ports in Namibia and South Africa. They are also used to determine the height of tide at anytime during the year at these ports. When using the tables certain parameters concerning the safe depth a vessel requires when entering an area of “shallow water”. These are:

• The draught of the vessel. The vertical distance between the keel and the waterline of the vessel.
• The under-keel clearance. The minimum depth below the keel required to safely operate the ship to prevent damage to the rudders/propellers.
• The charted depth of the bottom. This is the depth indicated on a chart of any sandbank/rocks/shoals /channels over which or along which the ship would have to pass.
• The height of tide. This is depth of water above the chart datum predicted for anytime during the year for a particular area.

The following are examples of how the tide tables are used practically.
Example 1.
Your vessel is due to enter Richard’s Bay on the rising tide during the afternoon of 4 May 2003. In order to enter you must cross a shoal with a charted depth of 5.6 metres. The draught of your vessel is 5.2 metres and you require an under-keel clearance of 2.0 metres. When is the earliest you will be able to enter harbour during the afternoon of 4 May?

Prepare the graph by marking the vertical axis “Height of Tide (metres)” and the horizontal axis “Time (hours SAST)”.
Open the SA Tide Tables and in the section giving the hourly heights of tide, look up the page giving the heights of tide for Richard’s Bay during May 2003. On the left hand side of the page the day and date are recorded vertically in sequence. Across the top the hours are given from midnight to 2300.
Look for the line containing the predicted heights of tide for 4 May. Search along the line for the height of tide required (1.6 metres) during the afternoon (after 1200). In this instance the height lies between 1.48 (1500) and 1.82 (1600). In order todraw a graph, take the heights of 1400 (1.07), 1500 (1.48), 1600 (1.82) and 1700 (1.99). Calibrate the graph along the vertical axis by marking the bottom line represent 1.0 m and in ascending order, mark each tenth line with an increase of 0.1 metres, ie 1.1, 1.2, 1.3, etc. Each little block will then represent 0.01 m. Starting at the left hand side, mark the hours along the horizontal axis, allowing twenty blocks to represent one hour.
Plot the heights obtained from the table against the hours and join the plots to produce a graph. Look for the height of tide required on the vertical axis and draw a line parallel to the horizontal axis until it cuts the slope of the graph you have drawn. From this intersection point draw a line parallel to the vertical axis and read off the time on the horizontal axis. In this instance the time is 1521. The answer is therefore: The earliest we may enter is 1521 B.

Example 2.
Your vessel is due to arrive at the entrance to East London harbour at 1200 B on 4 May 2003. A sandbank exists at the mouth of the harbour with a charted depth of 5.2 metres. The draught of your vessel is 4.8 metres and you require an under-keel clearance of 1.5 metres. Will you be able to enter at 1200?

Open the tide tables to the section giving the hourly heights of tide in East London for the month of May. Looking along the left hand side find the row representing the 4 May. Move along the line to the column under 1200. In this instance the height of tide is 0.38 metres. Compare this figure with that obtained through calculation, ie 1.1 metres. It is obvious that you will not be able to enter at 1200 since there is insufficient depth of water at that time.

The following diagram shows the relationship between the depths mentioned in the examples: