Working with tides
Mankind has harnessed tidal energy since pre-history, taking advantage of its ebbs and flows to help explore the globe. The tides are as useful to mariners today as they ever were. All we have to do is understand them.
Tide Tables predict the daily times of high and low water, together with the corresponding heights of each tide at those times, for specific locations around the UK. These locations are known as Standard Ports. Other locations, situated along the coast between the Standard Ports, are known as Secondary Ports. Finding the times and heights of tides at Secondary Ports can be calculated by reference to the Standard Port data.
The difference between high and low water on any particular day – known as its range – is greater on spring tides, when the associated tidal streams are at their strongest. Unsurprisingly, the opposite is true for neap tides.
This is vital information for all boaters, who will find the tidal stream helpful when it's flowing in the direction they intend to travel, and downright obstructive when they have to fight its power.
For navigators in tidal waters, understanding tides is an essential skill. And the basic information necessary to use them to advantage can be found in the Tide Tables which are available in various forms, such as this one on the RYA's site:
- Folding leaflets, often freely available from shore-side pubs and shops. The information is often so small that it's almost impossible to read without a magnifying glass, the producers of this information taking most of the space for their advertising messages.
- Printed booklets available from chandlers and newsagents in coastal towns and villages. They may cost you a couple of pounds but they're much better value than the free leaflets.
- In published Nautical Almanacs. These substantial tomes contain a wealth of other information.
- Local newspapers, in coastal regions.
- Online. There are many freely available sources of tidal and weather information.
- Published Tide Tables (paper or electronic), give reasonably accurate predictions of high and low water times for the Standard Ports. But be careful not to apply the information too precisely for it can sometimes be innaccurate for reasons beyond the compilers' control. For example: strong winds can influence tidal times, while high atmospheric pressure will result in lower tides and lower pressure the reverse.
But What Causes Tides?
Well, they are the up-and-down movements of the oceans caused by the collective gravitational effect of the moon and the sun on the earth. The tidal stream is the flow of water from high to lower areas. The direction and velocity with which they flow is recorded in what are known as Tidal Stream Atlases – a subject too involved to go into here.
When all the Sun, the Moon the Earth are in line, as seen here on the right, the gravitational effect is at its greatest and tides are highest. These are known as spring tides and occur twice each month.
Conversely, when the moon and sun are at right angles to each other as seen from the earth, the effect is at its weakest and tides are correspondingly smaller. These are known as neap tides, and occur midway between spring tides. In most parts of the world there are two high tides and two low tides every day. However, there are some regions – the Meditteranean for example – where they are so weak that for all practical purposes they can effectively be ignored by navigators.
Love them or loathe them, tides are a part of most sailors' lives and we have to learn to live with them.
Estimate tidal rates by using the traditional Rule of Twelfths