Your RYA course companion – Part 6

A yachtsman in the Med or the Baltic can get away with a cursory knowledge of how tides work, but for those of us in the UK it is an unavoidable part of our cruising life. Robert Avis explains.

A yachtsman in the Med or the Baltic can get away with a cursory knowledge of how tides work, but for those of us in the UK it is an unavoidable part of our cruising life. Robert Avis explains.

Spring tides

The constant movement of water around our planet is caused by the gravitational pull of the sun and moon. When they are on the same side or opposite sides of the earth, they cause larger movements of water known as spring tides.

Helpful hints

Spring tides happen two or three days after a new moon or a full moon.

If high water is in the middle of the day it is a spring tide.

Tidal information

Tidal information is available from various sources, including the Admiralty Tide Tables, almanacs and local harbour authorities and of course, this website.

Wherever the information comes from it follows the same basic format. But it is vital that you check the time zone in which the tides have been set. Universal Time equates to Greenwich Mean Time (GMT). During British summer time (BST) you have to add one hour to tide tables for British ports. Tide times of European ports are set in European standard time, one hour ahead of GMT and shown as “Time Zone – 01.00”. This means that you need to deduct one hour to get to Universal Time (GMT).

Some locally produced tables take BST into account when calculating tide times, so make sure you know which time zone is being used.

Helpful hints

Winds come from a direction but tides go to it.

Check the time zone that your tide tables are set in before making a potentially expensive mistake.

Tidal curves

If you need to calculate the height of tide at a particular time or to find out between what times a certain height is available, there is a more accurate method than the rule of twelfths. A special matrix with tidal curve is found near the tide tables for each Standard Port.

Tidal stream atlas

Another source of tidal stream information is tidal stream atlases. These can be stand-alone booklets, or in some cases they are incorporated into the land area on charts or included in almanacs. They give a simple line drawing of the appropriate area, with the tidal steam shown by arrows and two tidal speeds for each of the six hours before and after high water related to an adjacent port.

Tidal stream atlases are divided into a different page or drawing for each of the six hours before and after high water.

Tidal streams are given in the format “05,10”, which indicates 0.5 of a knot during neap tides and 1.0 knot during spring tides. If between neaps and springs . . . interpolate!

The one point to remember about tidal information is that it is only a prediction and that tidal heights and times can easily be affected by strong winds, higher or lower pressure areas than normal, and other anomalies of the weather.

Position fixing

In last month’s Day Skipper 5, we discussed plotting a position fix on the chart. The problem with a fix is that it’s history by the time you have it plotted on the chart. This problem can be overcome by working up a dead reckoned (DR), or better still an estimated position (EP).

Dead reckoning

A DR is simply a case of taking a past known position and adding to it your course and speed. Let’s say that we took a visual fix at 1715 and plotted it as above. We are on a course of 110° at 15 knots. At 15 knots we know that we will travel 1.5M in six minutes. Therefore we could mark our planned track every 1.5M and mark it at six minute intervals.

Having established a DR, it is possible to look at your watch and have a reasonably accurate idea of you position now rather than at the time of the last fix.

Estimated position

The DR is all very well, but it does not take into account tide or wind. Therefore, if we can fine tune it by adding the anticipated impact of wind and tide, we could be very sure indeed of our position. As in the last example, let’s say that we took a visual fix at 1715 and plotted it as above. LetÕs say we are on a course of 110° at 15 knots. If we calculated that there was a southerly tidal stream of two knots and a 20-knot beam wind which our experience told us would blow us an additional 5° to starboard, then we could plot it like this.

Having generated an EP we may discover that the continuation of our ground track will pass over an area of danger, or worse still we may already have passed over an area of danger.

Neap Tides

When the gravitational pull of the sun and moon are not either together or far apart, there is less effect, causing a smaller movement of water, known as neap tides.

The rule of twelfths

There are various ways of establishing the height of tide at any time between high and low water. The rough-and-ready method is the rule of twelfths.

This presumes that the height of tide rises or falls by 1/12th of the range in the first hour, by 2/12ths during the second hour, 3/12ths during the third and fourth hours and then 2/12ths in the fifth hour and 1/12th in the sixth hour. Although this is not the most accurate way of establishing a tidal height, it is satisfactory if you are not working to the nearest centimetre.

Tidal diamonds

If you have ever tried entering the Needles Channel three hours after high water you will have encountered a tremendous cross-tide that will have made getting between the Bridge Buoy and the South West Shingles buoy an interesting manoeuvre.

There are many ways of pre-determining tidal streams. Probably the most economic method of working out the likely tidal stream is to use the tidal diamonds or tidal circles printed on charts themselves.

Magenta-coloured tidal diamonds are found in strategic positions on Admiralty charts to indicate the position of tidal information, which is shown in a table elsewhere on the chart. Other cartographers use different shapes to show similar information.

It is important to remember that tides go in the direction of the bearing. (Winds come from the direction of the bearing.)

Powerboaters and wind

Leeway is the effect the track of the wind has on your boat. It has always been an accepted problem for the sailing man: he relies on the wind to push him along, but unless it is directly astern, it has a tendency to push him sideways as well as forward.

Many powerboaters believe that leeway is not a problem for them. However, it can be a major difficulty, particularly for those with lightweight or high-sided craft and especially when travelling slowly.

Someone once told me that when manoeuvring a powerboat the effect of 20 knots of wind equates to that of one knot of tide, and I think they weren’t far wrong.

There is no easy formula for calculating the amount to allow for leeway in a powerboat because there are too many variables. However, be aware that it is a potential problem, and only personal experience will help you determine how much you need to take it into consideration.

Helpful hints

Leeway can affect powerboats as much as sailing boats in certain conditions.

Course to steer

Probably the one topic that gives Day Skipper students the most grief is calculating a course to steer. If only it were possible to anticipate the effect of wind and tide, we could calculate a course that would take us over safe ground.

We need to decide our start and finish positions and the approximate distance between them – let’s say 16 miles. We know that our cruising speed is 15 knots and we have calculated that the tidal stream is setting 200° at 1.5 knots. It is a calm day so there is no leeway involved.

You should start by laying off the tide direction from the start point. We know that the passage will take about one hour, so the length of the tide line will be 1.5 miles. Once the tide is marked on the chart, add three arrows to the line to indicate tide. Then, using a pair of compasses, open them to 15M (the distance we will travel in one hour). Place the pointed end on the outer edge of the tide line and draw an arc on the planned track. Join the end of the tide to the point where the arc cuts the planned track and this is the course to steer to compensate for the tide. It is extremely unlikely that your compass arc will pass through the finish point. If it does, check that you have done it correctly.

When calculating speeds and distances you don’t always need to use one hour. So long as the tidal speed and vessel’s speed is calculated for the same time, you will always get the same course to steer.

Helpful hints

Tide tables are only a prediction. Many meteorological factors can affect their accuracy.


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