In our previous article we looked at tidal curves and how we can use them to find the time of a specific height of tide. Following on from this we now take a look at how we calculate a height of tide for a specific time. This is not a complex calculation but it helps to practice how to do this if you haven’t done it for some while. We then explain the Rule of Twelfths.
Find the height of tide for a specific time
Let us suppose that we want to know the height of tide at a specific time and place. In this case it is at East Head in Chichester harbour on 23 April 2023, i.e the same day that we used as an example in the previous article. We want to know what the height of tide will be at 16:00. Once again we can use a tidal curve and input the same figures as before. On 23 April there is a 4.9m height of tide at HW at 1234 BST. Our sailing boat has a draft of 1.2m and we will require at least 0.3m below the keel to remain afloat. Where we want to anchor 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). However our estimated time of arrival is 16:00 and we want to know if there will be enough water to anchor.
Using the same tidal curve as we did in the previous article, we draw a line (shown in brown) from 16:00 at the bottom of graph vertically upwards until it intersects the curve (see Figure 1). Then we continue the line horizontally until it intersects the straight line in green which represents the heights of high and low water, close to MHWS and MLWS. We then draw a line vertically upwards and can read the depth which is marked across the top of the graph. As seen in the graph, the reading at the top is a depth of 1.8m. This is not good news, as although the reading shows 1.8m, the proposed anchorage dries to 2.4m above chart datum, added to which our vessel draws 1.2m. Further inspection of the chart is required if we want to anchor at 16:00 near this location, which shows this is possible in deeper water a little further out from the shore.
Getting in the habit of using tidal curves, therefore, is a very good way to check whether an anchorage is going to allow your boat to stay afloat at low water and how long you may have in a specific spot before you need to move on. It is a good idea to have several blank copies of places you may frequently visit in your chart table.
Rule of Twelfths
For those skippers who need to make a quick calculation or don’t have access to specific tidal curves for their location, it is possible to make an approximation using a system called the Rule of Twelfths. This is a simple method used to estimate the height of tide at any given time during the tidal cycle. It is based on the idea that the rise and fall of the tide is not constant, but follows a predictable pattern than can be divided into twelve parts.
This is how it works:
- First, you need to know the time of high and low tides for the day in question and to ascertain the tidal range.
- Divide the time between high and low tide into twelve equal parts.
- The rule assumes that the rise starts slowly in the first two hours after low water, then speeds up in the third and fourth hours, slowing down again in the fifth and sixth hours.
- In the first hour, the sea level rises at the rate of one twelfth of the total tidal range.
- In the second hour, the sea level rises by two twelfths (one sixth) of the total tidal range.
- In the third and fourth hours the sea level rises by three twelfths of the range per hour.
- In the fifth hour the rate slows down to two twelfths per hour.
- In the sixth hour, the sea level rises more slowly, by one twelfth, until high water is reached.
- After high tide, the sea level falls again, following the same pattern in reverse.
In order to estimate the height of the tide at any given time, you need to work out how many twelfths of the tidal range have occurred since the last low or high tide.
It is important to note that this method is not accurate for all areas and is a general guide only and should only been considered as a rough estimate. In some locations, for example the Solent in the UK, the cycle is affected by local geography, which alters the pattern quite considerably.
Note: Source material for graphics and information taken from Reeds 2023 Nautical Almanac.