1 - Information from the chart

Most often the chart presents succinct tide tables for certain positions.  These positions are marked with the 'square'. The table below shows us an example for two different positions. The first refers to Cowes (UK), the second to a position south of Cowes.

This data only provides us with average high and low waters heights. Moreover, it is merely valid at spring or neap tides. To use it we need to first find out how many hours we are from high water. Secondly, we need to know if it is spring or neap or sometime in between at that particular moment. We shall use this table to solve two types of problems. Finding height of tide at a particular location at a particular time:

• To get over a shoal.

• To pass under a bridge.

Almanacs and many other nautical publications contain predictions of the times of high and low tides at many major standard ports. Also listed are differences in times of tides from these ports for additional secondary ports. To work with this succinct data we need two extra tools:

• To interpolate between high and low water heights we use the Rule of Twelve. We assume the tidal curve to be a perfect sinusoid with a period of 12 hours. The height changes over the full range in the six hours between HW and LW.

  • During first hour after HW the water drops 1/12^th of the full range.

  • During the second hour an additional 2/12^th.

  • During the third hour an additional 3/12^th.

  • During the fourth hour an additional 3/12^th.

  • During the fifth hour an additional 2/12^th.

  • During the sixth hour an additional 1/12^th.

  Hence, two hours after the HW the water has fallen 3/12 of the full range.
   

• To interpolate between spring and neap tides we use the Rule of Seven. Since the change from spring range to neap range can be assumed linear (instead of sinusoid), each day the range changes with 1/7th of difference between the spring and neap ranges.
  Hence, the daily change in range is (spring range - neap range)/7.

Shoal problem:
Our shoal near Cowes has a charted depth of 1 meter and we would like to cross it at about 15:00 hours with our yacht (draft 1,5 m).

From any nautical almanac we find that HW occurs at 03:18 15:53 and LW occurs at 09:45 22:03 at a standard port nearby. We also find that at our location HW occurs one hour later and that spring tide is due in two days. Hence, we have a HW around 17:00.

• Via the rule of seven we find out that today the range is:
  spring range - 2 x ( (spring range - neap range)/7 )
  <=> 4,8 - 2 x ( ( 4,8 - 3,1)/7 ) <=> 4,8 - 2 x 0,25 = 4,3 m.
   

• We also need today's HW height:
  which is Spring HW - 2 days x ( (5,2 -4,3)/7 ) = 5,0 m.
   

• Via the rule of twelve we find out that at two hours before high water the height is:
  5,0 - 3/12 x 4,3 = height at 15:00 hours = 3,9 m.

So, after three interpolations we derive the water height at 1500 hours. Considering the charted depth leads to an observed depth of 4,9 meters, enough for our draft of 1,5 meters.

Bridge problem:
An overhanging rock, power lines or bridges have their clearances charted with respect to another chart datum than LAT. Normally, 'high water' or 'MHW spring' are used as reference planes.

An example:
Above our shoal hangs the 'Cowes bridge'. At 15:00 hours we would like to pass this bridge, which has a charted height of 20 meters to HW. Our mast is 23 meters high. In the example above we found that the water height was 1,1 meters below HW level at that time. Obviously, we will have to wait!
So, at what time will we be able to pass under this bridge?
The water height must be 3 meters lower than HW level (5,0 m). That is almost 9/12 of the range (4,3 m) indicating four hours after HW. Conclusion, we will have to wait at least six hours in total.

2 - Information from tide tables

Instead of mere averages, a tide table provides us each day with the times of high and low water for a particular place. Basically, it is same table like the one we found in the chart, but is extended for every day in a year. By using this method we get more accurate water heights since it involves less interpolation. The example shows us a part of a very detailed tide table, which even includes heights for every hour.

3 - Information from tidal curves

In most tables the tides can also be characterized by a tidal curve. This method substitutes the rule of twelve providing more accurate heights. The left side contains the water height information with the lowest heights to the left where also the chart datum is indicated. The low water height will be marked at the bottom and the high water height will be marked at the top.

The area under the curve will be marked with the time information.

To find the water height at a specific time we need to know first how many hours before or after the HW this is. Then

Often this is done when the curve is not sinusoid and the rule of twelve is rendered useless.