Tidal Calculations
(Standard Ports)
The wizard Yacht Masters amongst you will already be aces at this stuff... but for other mortals and maybe a refresher, this top tip looks at using the tidal information from, in this case, the Reeds Almanac to perform some useful practical calculations. We will look at Portsmouth as an example and will be 'sailing' on Thursday 23rd April 2020.
First of all, if we turn to the tide tables for Portsmouth we can see the following box on the top left hand corner:
This tells us that the tables are based on Standard Time (UT) or, in old money, Greenwich Mean Time. It then tells us that, if we wish to have the tide times in British Summer Time, then we should add one hour in the non-shaded portions of the tables. We can see below that the shaded areas in Reeds are a light green
We can also see that phases of the moon are included in the table and, also useful, we can see that days of Neap and Spring Tides are also annotated (see below):
If we now look at the entry for our day, Thu 23rd April we can get the tidal information for that day. Get into the habit of always checking day and date... 23rd and Thursday... this is a safety measure to stop you picking 23rd March or May for instance (doesn't always work but is a good habit nonetheless). We should write down all the information we need to use the tidal graph - LW Ht, HW Ht, HW Time, Range. We are interested today in the morning of 23rd April so we pick the LW and HW than straddle that period. Notice that, as it is an unshaded area we must apply corrections for BST - we do this as the last thing we do before we enter the tide graph (this will help us later when we tackle Secondary Ports):
We now turn to the Tidal Curve. The first step is to enter our HW Time (corrected). I have rounded it up from 1247 BST to 1250 BST for ease. Again, a good habit to get into is to run the times all the way to Low Water and check that the LW on the graph is fairly close to that on the tide tables. This is a good way of checking if you have missed out an hour while filling in the graph (I kid you not... happens a lot!):
Now enter the LW and HW heights and join them with a line. The graph is now ready for us to use to solve our tidal dilemmas!
Tidal Calculation Example 1.
What is the Height of Tide (HoT) in Portsmouth Harbour at 1100 BST?
The use of the tidal curve to answer this problem is illustrated in pink in the diagram below. Enter the bottom of the graph at 11 and draw a line vertically upwards until it meets the curve. Then draw a line horizontally from this point until you meet the LW - HW line. Use either the HW or LW scale to read off the Height of Tide. In this case the HoT at 1100 is 3.5m. (HoT is of course measured from Chart Datum)
Tidal Calculation Example 2.
What is the earliest we can get to our berth - Charted Depth 0.7m - on the morning of 23rd April? Your vessel has a draught of 1.6m and you want to maintain at least 1.0m below the keel.
This requires a little thinking about before we get to the tidal curve. I always find a little diagram helpful. Put in the numbers you have and that will show you what you need to calculate. In this case we need to calculate the required HoT to give us the required depth at our berth... we can then go to the Tidal Curve to find out at what time the tide reaches this height:
We can see the use of the tidal curve in Blue below. Enter the HW scale with the required HoT. Drop a vertical down to the HW-LW line, then a horizontal across to the curve. In this case there is a separate curve for Springs and Neaps... we have a range of 3.5m... which is nearly Springs... so we shall use the Springs (Red) Curve. We then drop a vertical from this point to the time line. Therefore we can see that the earliest time we will have the required depth at our berth is 0800 BST.
