Calculating the Height of Tide
We know by looking in the almanac, app or tide website that we get given the times and heights of High and Low water on a daily basis. The below image is an extract from the RYA Training Almanac showing a spring range day which is indicated by the red colour and a neap range day which is indicated by the blue colour. The image on the right shows the average spring and neap ranges for that port and will help us to work out if the day in question is a Spring, Mid or Neap range tide.
In the example below, we are going to calculate the height of tide for 13:30 on the 25th.
Step 1: Add the HW time to the middle of the hour boxes at the bottom of the curve. We are using the HW time because it is a HW Port. There are a few examples of LW Ports such as Southampton and Poole Harbour. We have chosen the HW at 10:28 UT as it is the nearest to our time of 13:30. Before we add the information into the tidal curve, we need to add the hour on for DST. This is only done in the non shaded days.
Step 2: Add the HW height at 11:28dst and the nearest LW Height which is 5.5
The HW Height gets added to the HW scale at the top of the curve, the LW height to the scale on the bottom of the curve and a line drawn between them. This will act as a reference line when calculating the height of tide at a given time.
Step 3: Once all the information has been added to the tidal curve, it is simply a case of finding the required time on the scale. drawing a line up to the tidal curve, along to the reference line in Step 2 and then up to the HW Height Line.
This means that the Height of tide at 13:30 is 4.2m and it would be added onto to the Chartered Depth on the charts to create Depth of Water.
Finding the time of a specific height:
We can also use the tidal curve in the opposite way once step 2 above is complete. For example, if we needed to find the time before HW that there was 2.5m then we would find 2.5m on the HW scale at the top of the curve, draw a line down to the reference line, across to the tidal curve and finally down to the time scale. Using the example we can see that we reach 2.5m of tide at 07:28 in the morning.
This is a full video of how to calculate a height of tide.