Tide and low tide - periodic fluctuations in the level of the ocean or sea resulting from the tidal forces of the moon and the sun. Tides cause changes in sea level, as well as periodic currents, known as tidal currents, which make tidal prediction important for coastal navigation.
The intensity of these phenomena depends on many factors, however, the most important of them is the degree of connection of water bodies with the oceans. The more closed the pond, the lower the degree of manifestation of tidal phenomena.
So, for example, in the Baltic, Black and Caspian seas, these phenomena are practically not noticeable.
On the other hand, if there is a narrowing bay or river mouth at the place of tidal formation of a sufficiently large amplitude, this can lead to the formation of a powerful tidal wave (tidal boron), which rises upstream, sometimes hundreds of kilometers. Places where tidal boron is observed:
- Amazon River - altitude up to 4 meters, speed up to 25 km / h
- Fuchunjiang River (Hangzhou, China) - the highest tidal boron in the world, up to 9 meters high, speed up to 40 km / h
- Ptikodiak River (Fundy Bay, Canada) - the height reached 2 meters, now it is heavily weakened by a dam
- Cook Bay, one of the arms (Alaska) - height up to 2 meters, speed 20 km / h
The lunar interval is the length of time from the moment the moon passes through the point of the highest position above the horizon or the lowest position below the horizon (that is, the moment the moon passes through the celestial meridian) in your area on that day until the highest water level is reached at high tide.
Although the magnitude of the Sun's gravitational force for the globe is almost 200 times greater than the gravitational forces of the moon, the tidal forces generated by the moon are almost twice that of the sun. This is due to the fact that tidal forces depend not on the magnitude of the gravitational field, but on the degree of its heterogeneity. With increasing distance from the source of the field, the inhomogeneity decreases faster than the magnitude of the field itself. Since the Sun is almost 400 times farther from the Earth than the Moon, the tidal forces caused by solar attraction are weaker.
Also, one of the causes of the ebb and flow of the ebb is the daily (proper) rotation of the Earth. Masses of water of the oceans, having the shape of an ellipsoid, the major axis of which does not coincide with the axis of rotation of the Earth, participate in its rotation around this axis. This leads to the fact that in the reference system connected with the earth's surface, two waves run across the ocean along the opposite sides of the globe, leading at each point of the ocean coast to periodic, twice-daily repeating ebb phenomena, alternating with the tides.
Thus, the key points in explaining tidal phenomena are:
- daily rotation of the globe
- the deformation of the water surface covering the earth's surface, turning the latter into an ellipsoid.
The absence of one of these factors precludes the occurrence of tides.
In explaining the causes of the tides, attention is usually drawn only to the second of these factors. But the conventional explanation of the phenomenon under consideration only by the action of tidal forces is incomplete.
The tidal wave, having the shape of the ellipsoid mentioned above, is a superposition of two "two-humped" waves formed as a result of the gravitational interaction of the planetary pair Earth - Moon and the gravitational interaction of this pair with the central luminary - the Sun on one side. In addition, the factor determining the formation of this wave is the inertia forces that occur when the celestial bodies revolve around their centers of mass.
The annually repeating tidal cycle remains unchanged due to the exact compensation of the forces of attraction between the Sun and the center of mass of the planetary pair and the inertia forces applied to this center.
Since the position of the Moon and the Sun relative to the Earth periodically changes, the intensity of the resulting tidal phenomena also changes.
The mechanism of the phenomenon of "ebb and flow"
The nature of the formation of ebbs and flows is already well understood. For many years, scientists have investigated the causes and results of this phenomenon.
- Such fluctuations in the level of ground water can be shown in the following system
- Gradually, the water level rises, reaching its highest point. This phenomenon is called full water.
- After a certain period of time, the water begins to subside. Scientists have given the definition of ebb to this process.
- For about six hours, the water continues to go to its minimum point. This change is called the term “low water”.
Thus, the whole process takes about 12.5 hours. A similar natural phenomenon occurs twice a day, so it can be called cyclical. The vertical interval between the points of alternating waves of complete and small formation is called the amplitude of the tide.
You can notice some regularity if you observe the tide process in the same place for a month. The results of the analysis are interesting: daily small and full water changes its location. With such a natural factor as the formation of a new moon and the full moon, the levels of the studied objects are moving away from each other.
Therefore, this makes the tidal amplitude the maximum twice a month. Also, the occurrence of the smallest amplitude periodically occurs, when, after the characteristic influence of the moon, the levels of small and full waters gradually approach each other.
The effect of lunar energy on the ebbs and flows
Although the influence of the Sun on the cause of the ebb and flow is undeniable, the influence of lunar activity is of the greatest importance in this matter. In order to feel the significant impact of satellite gravity on our planet, it is necessary to follow the difference in the attraction of the moon in different regions of the Earth.
The results of the experiment will show that the difference in their parameters is rather small. The thing is that the point on the Earth’s surface closest to the Moon is subject to external influences literally 6% more than the most distant. It is safe to say that this separation of forces pushes the Earth in the direction of the Moon-Earth trajectory.
Taking into account the fact that our planet constantly turns around its axis during the day, a double tidal wave travels twice around the perimeter of the created extension. This is accompanied by the creation of the so-called double “valleys”, the height of which, in principle, does not exceed 2 meters in the oceans.
On the terrestrial land such fluctuations reach a maximum of 40-43 centimeters, which in most cases goes unnoticed by the inhabitants of our planet.
All this leads to the fact that we do not feel the power of the ebb and flow either on land or in the water element. You can observe a similar phenomenon on a narrow strip of the coastline, because the waters of the ocean or sea by inertia sometimes gain impressive height.
From the foregoing, we can conclude that the ebbs and flows are most associated with the moon. This makes research in this area the most interesting and relevant.
The influence of the sun on the ebbs and flows
Significant remoteness of the main star of the solar system from our planet affects the fact that its gravitational effect is less noticeable. As a source of energy, the Sun is certainly much more massive than the Moon, but nevertheless the impressive distance between two celestial objects makes itself felt. The amplitude of solar tides is almost half that of the tidal processes of the Earth’s satellite.
A well-known fact is that during the full moon and the growth of the moon, all three celestial bodies - the Earth, the Moon and the Sun - are located on one straight line. This leads to the folding of lunar and solar tides.
In the period of the direction from our planet to its satellite and the main star of the solar system, which differs from each other by 90 degrees, there is some influence of the Sun on the process under study. There is an increase in the level of low tide and a decrease in the level of the tide of earth waters.
All evidence indicates that solar activity also affects the energy of the tides on the surface of our planet.
The main varieties of ebbs and flows
You can classify this concept by the duration of the tidal cycle. The delimitation will be recorded using the following items:
- Semi-daily changes in the surface of the body of water. Such transformations consist in two full and the same amount of incomplete waters. The parameters of alternating amplitudes are almost equal to each other and look like a sinusoidal type curve. They are most localized in the waters of the Barents Sea, on the vast line of the coastal strip of the White Sea and on the territory of almost the entire Atlantic Ocean.
- Daily fluctuations in water level. Their process consists in one full and incomplete water for a period calculated within a day. A similar phenomenon is observed in the Pacific Ocean, and its formation is extremely rare. During the passage of the Earth's satellite through the equatorial zone, the effect of standing water is possible. If the Moon is declining with the lowest rate, small tides of an equatorial nature arise. At the highest numbers, the process of formation of tropical tides occurs, accompanied by the greatest power of water intake.
- Mixed tides. This concept includes the presence of semidiurnal and diurnal tides of the wrong configuration. The semidiurnal changes in the level of the earth's water shell, which have an irregular configuration, are in many ways similar to semidiurnal tides. In the altered diurnal tides, one can observe a tendency to diurnal fluctuations, depending on the degree of declination of the moon. Most prone to mixed tides of the Pacific Ocean.
- Abnormal tides. These rises and falls of water do not fit the description of some of the signs from the above items. This anomaly is associated with the concept of “shallow water”, which changes the cycle of rise and fall of the water level. The influence of this process is especially pronounced in river mouths, where the tides are shorter in time than the tides. Such a cataclysm can be observed in some sections of the English Channel and in the currents of the White Sea.
There are also types of ebbs and flows that do not fall under these characteristics, but they are extremely rare. Research in this area is ongoing, because many questions arise that require decoding of specialists.
Earth Tide Chart
There is a so-called tide table. It is necessary for people who depend on the nature of their activities on changes in the earth's water level. To have accurate information on this phenomenon, you need to pay attention to:
- Designation of the area where it is important to know the data on the tides. It is worth remembering that even closely located objects will have different characteristics of the phenomenon of interest.
- Finding the necessary information using Internet resources. For more accurate information, you can visit the port of the studied region.
- Concretization of the time needed for accurate data. This aspect depends on whether information is needed for a specific day or whether the study schedule is more flexible.
- Work with the table in the mode of arising needs. It will display all the information about the tides.
A novice who needs to decipher such a phenomenon will greatly benefit from the tidal chart. To work with a similar table, the following recommendations will help:
- The columns at the top of the table indicate the days and dates of the alleged occurrence. This item will allow you to find out the point in determining the time frame of the studied.
- Under the temporary accounting line are numbers in two rows. In the format of the day, the decoding of the phases of sunrise of the Moon and the Sun is placed here.
- Below is a waveform chart. These indicators record peaks (tides) and troughs (low tides) of the waters of the study area.
- After calculating the amplitude of the waves, the data of the entry of celestial bodies are located, which affect the changes in the Earth's water shell. This aspect will allow you to observe the activity of the moon and the sun.
- On both sides of the table you can see numbers with plus and minus indicators. This analysis is important for determining the level of rise or fall of water, measured in meters.
All these indicators cannot guarantee one hundred percent information, because nature itself dictates to us the parameters by which its structural changes occur.
Killer waves: hypotheses and consequences of the phenomenon
This phenomenon causes a lot of controversy among people who trust only unconditional facts. The fact is that wandering waves do not fit into any system of this phenomenon.
The study of this object became possible with the help of satellites of the radar format. These designs made it possible to fix a dozen waves of super-large amplitude over a period of a couple of weeks. The size of such a rise in a water block is about 25 meters, which indicates the grandeur of the phenomenon under study.
Killer waves directly affect human activity, because over the past decades, such anomalies have carried huge vessels such as supertankers and container ships into the oceanic depths. The nature of the formation of this stunning paradox is unknown: giant waves form instantly and also quickly disappear.
There are many hypotheses regarding the reason for the formation of such a whim of nature, but the appearance of whirlpools (single waves due to the collision of two solitons) is possible when the activity of the Sun and Moon intervenes. This issue is still becoming the reason for discussions among scientists specializing in this topic.
The influence of the tides on the organisms that inhabit the Earth
The ebbs and flows in the ocean and sea especially affect marine life. This phenomenon exerts the greatest pressure on the inhabitants of coastal waters. Thanks to this change in the level of the earth's water, organisms that lead a sedentary lifestyle develop.
These include mollusks that are perfectly adapted to the vibrations of the Earth’s liquid shell. Oysters at the highest tides begin to multiply actively, which indicates that they respond favorably to such changes in the structure of the water element.
But not all organisms respond so favorably to external changes. Many species of living things suffer from periodic fluctuations in water level.
Although nature takes its own and coordinates changes in the overall balance of the planet, biological substances adapt to the conditions presented by the activity of the Moon and the Sun.
The impact of tides on human life
This phenomenon affects the general condition of a person more than the phases of the moon, to which the human body can be immune. However, the most tides affect the production activities of the inhabitants of our planet. It is unrealistic to influence the structure and energy of the tides of the sea, as well as the oceanic sphere, because their nature depends on the gravity of the Sun and Moon.
Basically, this cyclical phenomenon brings only destruction and trouble. Modern technologies allow us to direct this negative factor in a positive direction.
An example of such innovative solutions is the pools of the type of traps for such fluctuations in the water balance. They should be built in order to make the project cost-effective and practical.
For this, it is necessary to create similar pools of a rather significant size and volume. Power plants to maintain the effect of the tidal power of the Earth’s water resources are new, but rather promising.
The study of the concept of ebbs and flows of the Earth, their influence on the life cycle of the planet, the mystery of the occurrence of killer waves - all these remain the main questions for scientists specializing in this field. The solution to these aspects is also interesting to ordinary people who are interested in the problems of the influence of foreign factors on planet Earth.
First, some general information about tidal phenomena. The ebbs and flows - the phenomena of periodic changes in sea levels are subject to strict laws arising from changes in the relative position of the three celestial bodies - the Moon, Earth and the Sun. The attraction of the moon and the attraction of the sun cause "bulges" on the surface of the oceans, as celestial bodies tend to attract objects on the surface of the earth.
The main force forming tides is the moon. From its attraction, the water undergoes a horizontal displacement relative to the solid earth’s crust and a flow of water arises towards a place located directly under the moon. The accumulation of masses of water in one place and forms the tide, and at points equidistant from the tide point to a quarter of the earth's circumference - the tide. Симметричная волна образуется и на противостоящей стороне земной окружности: её появление вызвано тем, что вода на обратной стороне, в силу большей удалённости от Луны, притягивается меньше чем сама Земля.
Размер приливной волны в открытом океане – ничтожный, всего лишь около 30-50 сантиметров. Однако при приближении к береговой линии приливы выходят на мелкие глубины и могут достигать 14-15 метров.
Между двумя последовательными приливами или двумя отливами в каждом отдельно взятом месте проходит примерно 12 ч 25 мин, период в два полных цикла между последовательными приливами - 24 ч 50 мин - называется приливными (или лунными) сутками. This fact can be used in the absence of an almanac or tide tables, assuming that the tides will come about 50-60 minutes later the next day.
Full water is the highest level of water observed per day or half a day at high tide. The lowest level at low tide is called low tide, and the moment of reaching these marks is called tidal or low tide.
Syzygy and quadrature
Since gravitational forces act on the Earth from the luminaries of the solar system moving relative to each other, the real picture of tides is caused by the superposition of several processes of different periodicities and in reality is much more complicated than can be described in a short article.
The most noticeable (and interesting for the navigator) is the so-called half-month inequality, which forms syzygy and quadrature tides. Approximately with a frequency of 15 days, there come moments when the Moon and the Sun are on the same line relative to the Earth.
Then the attraction of the Sun and the Moon add up - the tides become higher and the low tides are lower than if they were caused only by the attraction of the Moon. Such tides are called syzygy (spring tides, or springs). At moments when the attractive forces of the luminaries are mutually perpendicular - the tides become lower and the tides are higher than at other times. These are quadrature tides (neap tides, neaps). Syzygy and quadrature are the most noticeable and periodic changes in tidal heights - other changes (monthly, semi-annual and annual) are not so pronounced, although they are reflected in the table of tidal heights.
A regular change in sea level is accompanied by no less regularly changing currents. In syzygy, since the difference in water between high and low water is greater, the currents are stronger, in quadrature the difference is smaller and the currents become weaker.
In different places of the globe, the nature of the tides may differ. There are semidiurnal, diurnal and mixed tides. The semidiurnal tides are directly related to the quadrature and syzygy fluctuations of the tide - in such places there are two tidal cycles per day, slightly differing in height. Daily tides, with one ebb and flow, are typical for tropical waters. In mixed tides, two successive cycles also occur, but the heights of full and shallow waters can differ significantly (such tides can be observed in the Pacific Ocean, for example, in Vladivostok or Petropavlovsk-Kamchatsky).
Since the waters of Europe and the White Sea that are most accessible to yachtsmen in Russia are characterized by semidiurnal tides, with two tides per day, in the future, in this article, we will consider only them.
As we can see, the picture of the tides is subject to complex processes, the independent calculation of the height and time of the onset of the tides for navigational needs is almost impossible.
To obtain the height and time of the onset of full and low water at the right points, tidal tables are used. These tables are published both by national hydrographic agencies of different countries, and by private companies. The following sources are most known and accessible to the yachtsman:
McMillan & Reeds Almanac
The most famous European marine almanac for small vessels is published by a private company and contains data on tides and tidal currents for the coasts of the British Isles, the Atlantic coast of Europe and the North Sea. The almanac is published annually, and in addition to tidal data, it contains a lot of valuable flight information, first-aid instructions and large-scale maps of several thousand European ports. Data in the almanac are for the calendar year covered, from January to December. The Almanac is available for purchase in September of the previous year. The almanac can be ordered by mail or purchased at almost any port in Europe. I recommend it as the main tool for navigation in the tidal waters of Europe - all major sailing schools are taught how to use the Reeds almanac, it is easy to purchase or borrow from a charter company.
Programs and the Internet
At the end of the 20th century, many software products appeared on the market, providing a convenient and cheap alternative to paper atlases and tables. Programs such as Tides, Tide Plotter, tidal modules of MaxSea and Seapro contain almost everything you need to navigate in tidal conditions. The site [Easy Tide] (http://www.ukho.gov.uk/easytide/EasyTide/index.aspx) is very convenient. Of particular interest are programs for handheld computers and smartphones - an ordinary tablet in a waterproof case can replace a rather thick volume of tidal tables and display information of a navigator in a few seconds - for example, TidesPlanner
However, in this barrel of honey there is a drop of tar. The situation with free and shareware software is complicated by the fact that the International Hydrographic Union transferred in 1999 the management of the rights to hydrographic data on tides to individual countries, abolishing the de facto non-commercial use of tidal data.
Thus, nonprofit programs can only use data that nonprofit access is permitted by hydrographic organizations of copyright holder countries. Some countries (such as the United Kingdom) have stated that their rights to non-commercial use have been revoked, while others, such as the United States, provide broad access to their data. Because of this, the coverage of non-commercial tidal almanacs such as WxTide is incomplete and may contain significant errors in some places.
If the program uses data distributed on a commercial basis - usually in the form of a subscription to an annual update - then most likely the data in it will be completely identical to those published in paper editions. Among such programs are navigation systems MaxSea and SeaPro, software atlas Tide Plotter and many other programs and electronic systems.
Tidal tables for some regions can also be found on the Internet. When using them, you should also pay attention to the data source: free table generators created by amateurs can contain serious errors.
For primary ports, the almanac provides data on the height and time of the onset of full and low water for each day of the year. (pic) It is obligatory to indicate in which time zone the countdown should be taken - taking into account winter and summer time. The dates of the onset of syzygy are marked in red and quadratures in blue, so that the navigator can instantly determine what phase of the tide it is in. Each table is accompanied by a graph of changes in sea level, which is used to quickly determine the current tide height. Typically, two curves are indicated on the graph: red for syzygy and blue for quadrature tides.
Since it is technically impossible to provide data for all ports of Europe without exception, in one book, most of the small harbors are given in the almanac in the format of secondary ports (Fig.). The height of the water in these ports is found from the data for the nearest main port using special corrections to the time of onset and the height of the full water in the port. (pic)
Without going into details about the use of data on secondary ports, we note that, for example, Langstone port, located near Portsmouth, is no more than 10 minutes behind it, and the tide height does not differ by more than 10 cm. But Port Bembridge, lying 20 miles (rice), it can differ by an entire hour and have a water height of 1.5 meters less than Portsmouth. As a rule, the farther the secondary port is from the main one, the more corrections there are and the more carefully you need to be when calculating the time you call them.
A mini-atlas of 12 maps is shown for each navigation area in the almanac, on which the direction and speed of the current at selected points are indicated by arrows. Maps usually cover a 12-hour period between 5 hours before the onset of full water to a point of +6 hours to the time of full water. In the almanac, they are designated, respectively, as HW-x or HW + y, in the range of HW-5 to HW + 6 (fig.) The time used by this mini-atlas is determined relative to the reference port indicated on the maps, in the form “5 hours before full water in Dover. " That is, if the high water in Dover comes at 1200, time in the interval 1630-1730 falls into the interval HW + 5.
Each of the arrows on the map shows two current velocities: one for the syzygy tide, the second for the quadrature. These values are used when plotting for currents.
In order to calculate which course you need to follow in order to move to some point in the flow, we have to solve a simple graphic problem well known to all navigators. Let me briefly recall the way to solve it.
Suppose we are at point A and want to come to point B, which is 5 miles away from us. Our yacht is sailing at a speed of about 7 knots. To simplify the problem, suppose that the flow in the next hour is constant in speed and direction and equal to 2 nodes in a direction strictly south (180). The solution to this problem is carried out in three stages.
From point A we set aside the vector AB of the flow beyond 1 hour - 2 miles heading 180.
Set the meter solution to 7 miles using the scale on the vertical edge of the map. Putting one leg of the compass at point B, place the other on a line connecting points A and B. Note at this point point G
The angle between the BG line and the north direction is the course we need to go to arrive at point B, taking into account the current.
If you can do these three simple steps, then I congratulate you, you have the basic method of plotting in current conditions. However, if you carefully followed all the explanations and manipulations with the ruler and pencil, one thing remains unclear: how did we get the value of the flow at the point we need?
Tidal diamonds are the easiest and fastest way to determine the magnitude of the flow at a point. They look like lilac rhombuses with a letter in the center, mapped:
In the corner of the map with tidal rhombs there is a table whose columns correspond to the data for each rhombus, and the rows correspond to the hours counted in minus and plus the time of full water at the reference port (HW-6 to HW + 6).
The use of English terminology is not accidental here, since I have never come across a GUNIO card equipped with this extremely convenient tool. Having learned from the almanac or the tide table the time of full water onset at the reference port, we, knowing the current time, can obtain for the point located near the rhombus the value of the direction and speed of the current.
Atlases are a slightly less accurate method for determining the direction of the current. In tidal atlases, map maps are published that indicate the direction of flow at selected points with arrows. Near each arrow, current velocities are indicated. Depending on the atlas near the arrow, the direction of the current can also be indicated, but more often the direction of the arrow has to be determined independently, using a protractor. Atlas is a less convenient way than rhombuses, and if rhombuses are marked on the map, I usually use them.
The calculation of the flow value by each of the two methods consists of three stages:
- Reference Port Definition
- Determination of the tidal phase at the reference port
- Determination of flow parameters
The determination of the reference port is made on the basis of additional data given in the rhombus or in the atlas. For the diamond table from the example, the reference port is Plymouth. The reference port for the atlas is necessarily indicated on each map.
To determine the phase of the tide, let us turn to the almanac tabulating the data for Plymouth for today.
Suppose the high tide comes at 1408 GMT, and we need to determine what phase of the tide falls on July 1642. The full water time (PW, in the English almanac - HW) according to the almanac is considered to continue from 1338 to 1438, adjusted for summer time - from 1238 to 1338. Then point 1642 falls in the interval from 1638 to 1738 - PW + 4.
If you use the almanac, then to quickly determine the phase of the tide, it is convenient to plot the times of the onset of each hour of the tide phase directly on the tidal curve before going out to sea (Fig.):
If desired, the same table can be plotted on the margins of the notebook used for calculations, or on the margins of the map:
Having determined the phase of the tide in the reference port, we can proceed to the calculation of the flow velocity. Slight difficulty: in the table of rhombs and in the atlas for each point there are two values of the current velocity: one for quadrature, and the second for syzygy tides. What value should we take for calculations if we are somewhere in the middle between the phases of the tide?
If the difference between the values is significant, then to determine the value of the current velocity at the moment, you will have to solve a simple arithmetic proportion. In practice, if the error in determining the flow does not exceed 0.1-0.3 knots, you can safely use an approximate estimate. Say, if we are somewhere in the middle between quadrature and syzygy, we can take the average value between the two indicated in the table or atlas. If the tide is very close to quadrature or syzygy (for example, syzygy occurs the next day), you can safely take the corresponding boundary value from the table. To determine whether the tide is now - quadrature or syzygy - use the indicated in the table or almanac
The “correct”, long method for calculating the flow velocity is also uncomplicated, but requires some arithmetic operations.
Suppose that in the table for rhombus A at 1642 GMT (as we calculated - HW + 4), the current is shown in the direction 090 at a speed of 1.0 knots in quadrature and 4.0 knots in syzygy. The difference between high and low water on this day is 2.2 meters, high water in syzygy is 2.7 m, low water in syzygy is 0.1 m
The formula itself is extremely simple:
In the formula PV-MV - this is the difference between the ebb and flow of the tide today, PVsizigii-MVsizigii - the difference between high and low water in syzygy. Vsprings is the speed of the current in syzygy. For our example, we get:
If simple arithmetic scares you, keep in mind that in practice, approximate calculations in the mind are usually enough to "quickly" determine the speed of the tide. For example, if the difference in syzygy is 2.6m, and today it is 2.5m - we can take the value of the current equal to 3.9 knots without calculations, which will be very close to the truth. Errors in 0.1-0.3 knots will give a maximum discrepancy of 1-3 cable for 1 hour of transition, which can be considered to be within the acceptable range for a small yacht.
The above methods require the use of an atlas of currents. But what if it does not exist, and there are only tide tables and approximate information about the maximum value of the current, which are published on some maps? In this case, you can use the "principle 50/90" or, as it is also called the "principle of thirds."
According to the 50/90 rule, the time from standing to the maximum or minimum of the current velocity is divided into three parts, one hour at a time. The speed at the end of the first third is taken as 50% of the maximum, at the end of the second third - as 90% of the maximum, and at the end of the third third - as 100% of the maximum. Then the flow rate begins to fall, the same 3 hours: 90% -50% -0%. Thus, the flow reaches its maximum value and again drops to zero in 6 hours. For example, if we expect a maximum current of 12 knots, then at the first and last hour of the tide, the tide speed is assumed to be 6 knots, at the second hour and fourth, at 10 knots and in the third at 12 knots:
|0 knots||6 knots||10 knots||12 knots||10 knots||6 knots||0|
When using this rule, it is important to remember that the curves of speed and tide / ebb heights in the nearest port of reference may not coincide, and it is necessary to know when exactly at a point of interest to us the water stands. To do this, you can use the calculation of the time of full water in the nearest additional (secondary) port, or practical knowledge of the local water area.
To speed up the course laying on the course (for example, in a race, when you need to quickly check whether the boat goes to the mark), a method is used in which instead of taking the exact distance in miles from the map edge, the solution uses a similar triangle. On the edge of the "Breton" plotter or parallel ruler, scales in inches and centimeters are plotted. They can be used instead of the meter, so as not to waste time moving from the meter to the plotter and vice versa.
For example, for our laying, we put it on maps: for a current of 2 knots - 2 cm, for a speed of a yacht of 7 knots - 7 cm. Полученный в результате курс будет эквивалентен предыдущему, но делается значительно быстрее, особенно в условиях небольшого штурманского стола.
Инструменты и приборы для навигации в приливной зоне
Необходимыми инструментами для использования счисления в условиях меняющейся скорости течения и высоты воды являются хорошо откалиброванный электронный лаг, с автоматическим отсчетом пройденного пути и эхолот. Обращу ваше внимание на слова «хорошо» and «откалиброванный», поскольку в условиях повсеместного господства систем электронной навигации лагу придается вспомогательное значение – как системе измерения скорости. Very often, they forget to clean the lag and periodically check the adequacy of its testimony, and, as you know, a lag showing a lower speed is much more dangerous than its complete absence. Take care of the lag on your yacht and periodically check it on the “measured mile” or other means (for example, GPS)
An echo sounder is no less important for determining a place by means of "traditional" navigation. It requires significantly less maintenance than the lag, but it is nice to periodically check it using data from the sea chart and tide tables.
Another small addition to an important addition: if your echo sounder shows the depth under the keel, then do not forget to add the value of draft of your boat to its readings.
The problems of reckoning on the course are solved in a similar way. In the log every hour the heading, lag count and wind direction are noted (for drift assessment). The numeral point is found by the usual method described in all textbooks, just as in the case without currents. After this, the current vectors for a countable period are postponed from the obtained point.
Let's face it - reckoning in the face of changing currents is not the easiest thing, and not the most accurate. The values of the currents in the rhombuses and atlas are given with values for each hour, that is, pretty roughly, the location of the data on the current map may not completely cover all the anomalies. We add here all sorts of calculation errors, lag errors and other devices ... and it becomes clear that when moving in the tidal zone, it is necessary to control the progress of the yacht through observations as often as possible. This can be done in classic ways to determine the location in the view of the coast, or using GPS.
Discrepancy and currents
What to do if you found a yacht deviating from your route line at the passage? There is no ready-made recipe and it all depends on the specific situation in which you find yourself. The most important thing is to notice the departure from the course in time and take the necessary corrective actions. If the discrepancy is small (for example, it is within 2-3 cables per hour) and does not lead to danger, then you can continue to move on by pre-laying. Do not forget that if the tidal currents cancel out, then the line of movement will be indirect. Even in the case of a current that does not change direction, the line of movement may not coincide with the planned direction line due to the approximate values of the current velocity in the table.
If, after all, the discrepancy is too large, then the navigator has no choice but to adjust the gasket, continuing it from the observable point. In this case, you need to try to find out the cause of the big discrepancy and eliminate it. This can be an unadjusted deviation, a strong wind drift, a systematic steering error, or simply incorrect laying. Do not panic: meticulously and calmly check your calculations and the correct choice of data from the almanac. If you find that your course leads to danger, do not hesitate to go to a drift, or turn back to find out and fix the problem. Make sure that no one threw a jacket with a mobile phone on the binnacle, that the helmsman did not use “those constant red and white lights on the horizon” as a guide - anything can happen! In my experience, when calculating on a yacht, the most common mistakes are using the wrong time zone in the almanac and underestimating the drift.
To eliminate the risks associated with the discrepancy and errors of the laying, it is recommended to break the laying in such a way as to know exactly its place on the approach and at the exit from the dangerous section of the track. For example (Fig), when approaching narrowness, you can plan your way in such a way that you first go to the point near the noticeable buoy A and end the passage between dangerous shallows at easily identifiable point B. If possible, the position in the narrowness can be controlled with the help of guards and dangerous bearings.
Tides and safety
In addition to the obvious and expected effects - changes in sea level and currents - the tides carry some unexpected and very unpleasant consequences for an unprepared yachtsman. Every year, tens of thousands of yachts without special adventures travel in the tidal zone. Nevertheless, one should not neglect safety precautions and forget about the surprises that the tides are preparing for us.
Bustling and swift
Suloi - a sharp, erratic wave crowd - is observed near cans, sharp uplifts of the bottom and prominent capes in the sea. (rice) It appears in those places where the underwater current meets a sharp change in depth. On the surface of the sea in these places there is a random swell, breakers and whirlpools (swirls), the most severe excitement will be located behind the obstacle, if you look along the stream. Such places are especially dangerous in fresh weather, because in addition to the growth of excitement on the shallows, it is superimposed on it, increasing the overall height of the waves, a random crush. For example, on the Wicklow Bank bank in the Irish Sea, with a sea swell of 1-1.5 meters, on the edge of the bank itself, a yacht can be expected to rip up to 3 meters high. During the peak of the current, the entire strait between Ireland and Scotland - the Mull of Kintyre - turns into one huge rip, with 100-200 meter whirlpools and breakers up to 3-4 meters high. The author happened to pass through Mull with a fair 6-point wind during an offshore race - he has no desire to repeat this experience. Permanent rip can also be observed in estuaries of large rivers. Bustles, swifts and associated breakers and whirlpools are indicated on the maps as follows:
If you have no other choice, then such places are best to go through the times of high or low water, when the current is practically absent. (For example, from the picture it is very convenient to use a tidal rhombus to determine the time the water stood at the cape).
Bystrins arise wherever where the speed of the tidal current accelerates when entering a narrow or approaching an outstanding promontory. Almost any cape can be expected swiftness of one degree or another and they should be passed with great care, especially if the wind blows against the current. The effect of quickness on excitement is very strong and can be extremely unexpected for an inexperienced yachtsman or navigator who made a mistake in laying. In European waters, there are many notorious hurricanes, for example the Raz Blanchard or the Alderney flight, in which the syzygy current can reach 12 knots (!), And the breakers can reach 6-8 meters.
It doesn’t matter with a tailwind or headwind, you go if you fall into a swift situation in the “wind
How does this happen?
Every evening in the sky we see the largest satellite of our planet - the moon, and in the afternoon - the sun. Their tandem is the main generator of the tides on Earth. Since the moon is much closer than the sun, its influence on the tides is much stronger. The gravitational force of the Moon and the Sun forms 2 large tidal waves on our planet, both from the side of the Moon, and from the opposite. These waves follow the moon and make a complete revolution in 24 hours and 48 minutes. And since there are only 2 waves, in one day 2 tides and 2 low tides occur. Twice a month during the full moon and new moon, the water level in the ocean changes dramatically. This is due to the fact that the moon and the sun are on the same line to the earth, and their effect is amplified. This time is not the best for surfing due to the rapid change in water level and sharp differences between the ebb and flow. This, in turn, greatly limits the riding time for certain spots.
How do the tides relate to surfing and how to “read” the schedule?
Tides directly affect surfing. The conditions for a good gurney on each spot are different and this factor is one of the most basic. Each surf spot is unique in its own way, and its best conditions are associated with a certain water level in the ocean. So, before you go surfing, be sure to check these details and, based on this, choose a place. To do this, the tide schedule will help you. Moreover, people who love surfing have taken care of us for a long time and every year they specially compile the tides on different spots around the planet, including Bali. The schedule looks like this:
Attention! Do not get confused!
- Many beginner surfers after completing courses in surf schools say with one voice that you need to ride on the tide. Believe me, this is not entirely true. Each spot is unique and has its own specific set of conditions for perfect skiing, so before you go to any spot, you should carefully study its nuances.
- The water level in the ocean does not depend on the swell (wave height), however, everywhere there are exceptions. At some spots in the tide, the waves become more massive, respectively larger. But the most important thing: do not confuse tide chart with swell chart - these are two different things. In order to find out what wavelengths (swell) will be in the near future, you will need to go to specialized sites that will give you complete information. Most Popular:
In the beginning, this may seem difficult and perhaps it’s all like a math lesson, but our experienced gurus will always help you figure it out and tell you when and where to go for a ride!