Navigation and Chart work - Tides

Tides

What causes the tides?

The main cause of the tides is the Moon. The moon exerts an attraction on the Earth. This gravitational attraction causes a bulge in the water on the Earth's surface at the point nearest to the Moon.

This is because at that point the forces are at the greatest. However, what is not so obvious is why there is a second bulge in the water-on the opposite side of the Earth to the Moon.

This occurs at the point furthest from the Moon, because the Earth and Moon rotate around each other to some degree, the centre of rotation being about 1,000 miles below the Earth's surface. Because of this rotation there is a force similar to that felt if a weight is whirled round on the end of a piece of string, instead of the force holding the string taut, it is acting to pull the sea in to a bulge on the side of the Earth away from the Moon. This results in a second high water.

In addition to this the gravitational attraction of the Moon is weakest at the point of the Earth that is furthest from the Moon.

This is a very simplified explanation of very complex forces but is adequate for practical use. If you want a technical explanation a google search on "cause of tide" will satisfy the most technically minded. Further animations are available at the Nova website.

Earth and Moontidal forces.
These bulges in the oceans depth give us high tides and the points in between, where the water level is depressed gives us the low tides. Because the World rotates in 24 hours, in any 24-hour period most parts or the World will have two high tides and two low tides.

This is not strictly correct because the Moon rotates round the Earth, this movement causes the time of high water to become later by approximately 50 minutes each day.

In addition to the Moon, the Earth is affected by the gravity of the Sun. Because of the great distance to the Sun, the Moon's effect is about 20 times that of the Sun.

Earth, moon and sun spring tide.
The forces from the Sun and Moon can either work together or against each other. They work in concert when they are more or less in line, this occurs if the when the Moon is between the Sun and the Earth or opposite the Sun, in either of these situations the tides will fall lower and rise higher; this is known a spring tide. If the forces cancel out because the Sun, Earth and Moon are at right angles to one another, the tides will not rise so high or fall so low; this is a neap tide.

Earth, Moon and Sun for neap tide.
As it takes the Moon a month to rotate around the Earth, Spring tides occur at two weekly intervals, with a gradual change over a week to a neap tide. So in 1 month there will be two neap and two spring tides.

When the Earth, Moon and Sun are in line, there will be a full or new moon. That is, you will see all the disc of the Moon at full moon, and at new moon the dark side will be towards us and not visible. On clear nights you may be able to see a pale new moon because of the reflection from the Earth shining on the Moon.

This means that spring tides occur at full and new moon, in fact due to friction, there is a delay of 2 days for the tides to reach the maximum.

In the same manner, at neap tides you will be able to see half the Moon (the first and last quarter).

Knowing this you can take a rough guess at whether it is a spring or neap tide without looking in the tide tables. In Reed's Almanac the phases of the moon are marked in the tide tables with little outlines of the moon phase. The approximate dates of spring and neap tides are indicated by date being written in blue or red ink.

Tidal ranges

The range of the tide is the difference in height between low and high tide. On a spring tide the range is about twice the range of a neap tide.

You will come across the following terms frequently.

Mean High Water Springs.

Mean High Water Neaps.

Mean Low Water Neaps.

Mean Low Water Springs

The word mean, means the average. So mean low water spring tides are not the lowest tide. As it is the average, some must be lower!

The diagram below shows you the relationship between the tidal levels. All tide heights are measured above Chart Datum, which is the level below which the tide never normally falls. Tidal Levels.

If the range of a spring tide is about twice that of a neap tide, and it still takes about 6 hours to go from high to low tide, the speed of flow of a spring tide will be about twice that of a neap tide.

Tide tables

There are many types of tide tables. The ones in the Training Almanac are extracts from Reed's Almanac, which is one of the most popular almanacs available in Britain . It is published annually and covers many ports in Europe from Germany to Portugal . A publication of this type is invaluable if you are going to sail any more than a few miles from your homeport. At the other end of the scale, if you only sail locally, you may find a simple tide table available at your local chandlers for 20p is sufficient.

Turn to page 33 in your Training Almanac.

This is the page for Victoria. Always check that you are looking up information for the correct port! Around the coast there are harbour for which the times and height of high and low water have been calculated for each day. These are Standard Ports, they may be major harbours such as Portsmouth or minor ones selected for their importance in the pattern of tidal behaviour like Margate.

Other harbours may not have ready made up tables but the data for these Secondary Ports can be calculated using the tables in the Almanac.

In the top left corner of page 33 there is a box noting the time zone the tables are correct for. The time zone is UT or Universal Time. As far as we are concerned this is the same as GMT.

Under the time zone is written: For Summer Time add one hour to the non-shaded areas.

During the period of the year when Summer Time is in force, we must add one hour to the times given in the table to bring them in to line with the time we are using. From examining the tables you can see that this happens from March 31st to October 26th. In the UK we would refer to British Summer Time (BST) but these practice tables have been created for use all over the World so we will refer to Summer Time.

When you are dealing with tidal information, it is valuable to rewrite the information you will be using, but I suggest that you start by drawing a box in pencil around the day in question. This will ensure that if you look away, you will come back to the correct place! (In the real book, cross out each day when it is finished, you will not need the information again). When you have finished each example remember to erase the box or you could inadvertently use the wrong data in another question.

Many of the problems people encounter during navigation courses are caused by untidy working, get into the habit now of laying your work out in a standard manner and keep it tidy. Remember, when you are skippering a boat, the navigation will be only one of your responsibilities, if you are untidy, and another job intervenes you will lose track of what the numbers mean when you return to the navigation.

We are going to look at September 6th.

September 6th

Victoria Summer Time

LW 0423 0.8

HW 1036 5.4

LW 1637 0.9

HW 2249 5.8

To find the range of the tide we subtract the LW height from the HW height.

Range = 5.8m - 0.9m = 4.9m

To decide if this range is a spring or neap tide. Look on the top of page 36. In the mean range box, the figures for a spring tide range is 4.9m and for a neap is 2.4m (these figures only apply to Victoria! They will be different for each Standard Port).

Therefore this is a spring tide. Provided the calculated range is close to that given in the mean range box we can call it a spring or a neap tide depending upon which is closest.

It may seem a lot of work to lay this information out like this for each question that you do, but if you can establish a thorough method of working from the beginning you will make fewer clerical errors. Most of the errors in navigation are not caused by a lack of understanding, but by untidy working.

Question 1.

Is the range of the tide on March 22nd a spring or neap tide?

Answer.

Additional Resources:


Mailspeed Marine
Contact the British Offshore Sailing School for all your sailing courses and cruises.
Sailtrain Home | Purchase Training Aids and Publications | Contact Us | ©2004 Sailtrain.co.uk | Sports Books