- a little to ponder for those new to tidal waters

I like being at sea. Being near the land means that there are more things to look at but also more things to worry about and be worried by. When one is a mile or two offshore, those bêtes noires of the dinghy sailor, water scooters and water skiers are comparatively rare. Other craft are scarcer too so concern about rights of way is less harrying. The wind is steadier being undisturbed by land and buildings and the seas are more regular. I find it is the time when sailing is at its most mentally relaxing. One should be aware however that it can become physically strenuous! My idea of heaven is when the wind is blowing force 2 - 3, the sun is shining and the land is an indistinct shape on the horizon. Nevertheless it is an established fact that recreational sailors who are based on good sheltered sailing water often find it difficult to venture out into more open seas. There are 35 foot yachts in Chichester Yacht Basin that have never been outside Chichester harbour. There are similar yachts at Portsmouth that never leave the Solent in many years of so called cruising. This is true at many places round the world from Vancouver to Sydney; but those who have to emerge from their base straight into open water seem much less inhibited. If they want to sail they can't afford to be! However, if we are going to venture out to sea we should be able to find our way about.

As dinghy sailors we have to do very little true navigation, even on more extended trips. However there are two topics that we should try to be knowledgeable about, tides and transits. It would be beyond the scope of this article to explain everything about tides but to simplify the subject we can tabulate the essential information.

Basic Facts About Tides

1. When we talk about 'tides' we are normally referring, to either tidal streams - the horizontal movement, or tidal heights - the vertical movement, of water.

2. The tides are caused by the pull of the moon and to a lesser extent, the sun. When they are working together we have faster streams and greater rise and fall (range) - and vice versa.

3. When the tidal streams are fastest and the range of height is largest the tides are known as 'spring' tides (from the Nordic meaning 'a swell'). When the opposite applies they are known as 'neap' tides. Spring tides and neap tides each occur about once a fortnight. On any one part of the coast they will occur when high water occurs at the same time of the day. For example springs will be found to occur on the east coast when high tide occurs at about 1300.

4. Tide Tables tell us somewhat more than just the times of high and low water. If we take the height of the high tide in our tables and take away the height of the low tide it will give us the range. The height of tides are measured relative to a certain level, usually mean low water springs, regardless of being springs or neaps. Spring tides not only rise higher but they also fall further than neaps. The tide tables will show lower heights for low water and greater heights for high water during the spring tide phase. A greater range in other words. 5. The tide rises and falls twice in each 25 hours approximately taking just over 6 hours to rise or fall between high and low water.

6. The amount that the tide rises or falls in each of the six hour periods can be measured approximately in twelfths. From low water, in the first hour it rises one twelfth of the range of that day's tide, two twelfths in the second hour, three twelfths in each of the third and fourth hours, two twelfths in the fifth hour and one twelfth in the final hour before high water. As the water goes down a similar situation arises. When the tidal range was given in fathoms or feet the calculation was much easier than now it is given in metres.

7. In an estuary the times of high tide and slack water (or low tide and slack water) both occur pretty much at the same time at any one point. Slack water occurs when the water is stationary in relation to the sea bed. As the water takes time to fill the estuary the time of high water near the mouth of the estuary occurs earlier than high water further from the sea.

8. Along the coast this convenient situation does not normally occur. Although tidal information is given on most charts you are earnestly advised to buy one of the Pocket Tidal Stream Atlases for your area, published by the Admiralty. This will show the strength and direction of the tidal streams based on the tide tables for the nearest Standard Port. For the east and south coasts this would be Dover. These Atlases do not change like tide tables do, so one of them lasts for years. It is in any case quite cheap.

9. Tidal streams usually run faster in and out of an estuary, than they do along the coast.

Using the Tides

In order to experience the open sea it is not necessary to make a long coastal passage. If you have become competent at handling your dinghy in reasonably strong breezes and have a properly equipped boat you can learn much by making an out and return trip. I would suggest however that you should not do this with an offshore wind if this is your first venture; for obvious reasons. No doubt you will check the weather forecast before you go but in order to get the most out of your trip you will also require good visibility. You should also buy a chart of the vicinity and a tidal streams atlas for the area although most charts have adequate tidal stream information on them. Tide tables should also be consulted. You will not need to know the fine details of the tides of the day but it will be as well to know the direction and strength of the currents at the time you will be out sailing. As you emerge from your usual harbour or estuary you just set off straight towards the horizon. If you are on a broad reach, check by lining up two fixed conspicuous objects immediately astern (or ahead if there are any) such as a buoy, a point on the land, gasometer and alter course until they remain in line. That way you compensate for the tidal stream without having to do any calculations.

Alternatively take a compass bearing on the buoy and try to keep it constant. Of course the tidal stream will change during your trip. The longer your outward trip the more it will have changed by the time you turn round. I am assuming however that if it is your first venture you will not go far. If the wind is onshore you will have to beat out with the advantage that you will have a fair wind for the return and then the tidal stream will have less chance to affect your course. Keep the windward legs fairly short as then you will be able to appreciate how the tidal stream along the coast is affecting you and compensate accordingly. A long leg to windward with a strong tide against your windward bow can set you a long way down the coast which may need hard work to get back. The opposite side of this effect is that often you will be able to hold one leg much longer before tacking as the tide flowing against your lee bow will keep you heading up towards the wind. This is known as 'lee bowing' the tide. You should however read a bit about the local tides before you go as the first little trip will then teach you much more in a practical way about the way tides affect you. A half hour will take you between two and three miles offshore unless you are beating.

As you move away from the land you should be constantly looking along the coast in both directions as well as behind you. That way you will appreciate how the appearance of the coast changes with distance off. The headland that was so conspicuous to the east may gradually merge with the coast as you move out to sea, but another one will probably appear beyond it. With the possession of a chart you can see how far out you would be when the second headland appears by imagining a line drawn between the two headlands and extended. This alignment of charted objects is known as a transit, as was the buoy and land previously mentioned, and most pilotage as opposed to offshore navigation depends on them. The use of transits can ensure you avoid dangers and enable you to set courses without too much use of the compass. If you can practise this awareness of where you are in relation to objects on the chart, without having to consult the compass continually you will find it a great advantage in dinghy cruising. If you have emerged from an estuary you may find that what was a great inlet of the sea when you were in it, gradually disappears. This will teach you that when you are looking for a harbour entrance from offshore, how valuable are landmarks either side of the entrance and inland as well as any buoys in the offing.

Wave Formations

When the wind is blowing against the direction of the tidal stream, the apparent wind, as far as the water is concerned, is blowing harder. The waves therefore will be steeper and higher. It often happens that when you are beating with the tide in your favour the water is rougher than you expect - well that's better than having the rougher conditions when you have the tide against you! This of course occurs when you are running against the tide but you don't mind so much then. Keep your weight out of the ends of the dinghy, particularly the stern if you can. It is easy to get pooped, even in an estuary as the waves will inevitably be steeper.

Wave shapes can help your pilotage. If the wind is against the tidal stream, the centre of the channel where the stream runs fastest will be the bumpiest. Likewise there will be smooth water in the same place if the wind is blowing in the same direction as the tidal stream. This can be very useful when seeking a channel in shallow muddy water. A whole estuary can be affected in this way. After slack water a previously choppy estuary will suddenly become calm. Be aware however that the opposite can happen. You might need waterproofs even in force 2.

Friends and Enemies

Beginners often think that the only natural force that is important when sailing is the wind. This is far from the truth. As far as getting where you want to go is concerned the tide is the master, as well as in the way it affects the waves. Ignore this truth at your peril. A typical sailing dinghy speed is 4 knots. A moderate tide might run at 2 knots. If you are both going the same way you are travelling at 6 knots. If the tide is against you, will make only 2 knots. If you are beating you will not make the 4 knots through the water directly to windward but at some 45°, so it is doubtful if you will make any progress at all. There again we can see from the above that the tide can affect the roughness of the sea. The good thing about all this is that the tide can be counted on. The weather is fickle but the tides are sure. The magic of their secrets can be yours for the price of tables and an atlas.