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Lesson 7 Tide

The origin of tide

The first of the two "tidal waves" is the result of the gravitation between the earth, sun and moon. The moon pulls the oceans to the side of the earth where the moon is. But there is also another "tidal wave" on the other side as well (the side where is no moon). This "tidal wave" is the result of the fact that the moon and earth together have 1 center of gravity, that is not exactly in he center of the earth, but about 1500 kilometers below the earth's crust, while the center of the earth is at a depth of approximately 6400 kilometers. The centrifugal force of the earth-moon together is greatest on the sides of both celestial bodies facing away from each other. This is the reason of the second "tidal wave". Consider the moon and the earth as one system that rotates in the universe, and the water flows to the other side of the earth where there is no moon.

Because the earth rotates in 24 hours, it will be high tide twice a day and low tide twice a day. So two times flood and two times ebb tide.

getijden drempel

Springs and neaps

If the sun, the moon and the earth are in one line with earth other, the height tide will be relatively high and the low tide will be relatively low. The range is relatively large and as a result the tidal currents will run relatively strong. This is a situation of springs.
In case the sun, the moon and the earth are nog in one line with each other, the high tide is less high and the low tide is less low. The range is relatively smaller and as a result the tidal currents will be less strong. This situation is called neaps.

 

Looking at the moon we can determine if it is neaps or springs. In Holland, two days after full moon and two days after new moon (you will see no moon in that case) it will be springs. Two days after first quarter (you will see the right side) and two days after last quarter / third quarter (you will see the left side) it will be neaps. Because the moon circles around the earth in one month, it will be springs two times every month, and two times neaps every month. The two days differ per location on earth. In Holland it is two days because it takes two days before the "tidal wave", that starts on the open oceans, reaches the Dutch coast.

To be more precise, it is springs on days with the largest average range. It is not always exactly 2 days after full moon or new moon. Also consider that it can be full moon at 23.55 or at 00.05.

Before starting to calculate, we recommend to make a drawing of the situation. Draw a timeline with the dates, the phases of the moon, and the days it is neaps or springs.

Definitions

Horizontal movements of the tides are tidal currents and vertical movements determine the height of the water and the range.
Chart datum is the level that is used as a reference to put the depths in the nautical chart, for example mean lowest low water springs, lowest astronomical tide, etc.
The height of tide is the distance from the chart datum to the observed water level. To calculate the total depth, you need to add the height of tide to the depth in the nautical chart.
Drying heights are for example sandbanks that are above chart datum. These "negative depths" are underlined on the nautical chart.
The flood is the tidal current that causes high water. Please don't confuse the definition flood with high water, as it is not the same.
Ebb tide is the period between high tide and low tide.
On shallow waters like in the north of Holland there are shallow areas where the tidal currents meet and result in high water on that location. On those places there will never be a strong current so it becomes even more shallow. In most cases we can only pass these areas when it's high tide, but that is exactly what's the interesting part of it. We can sail with the flood to the shallow area, the tide turns and we sail again with the ebb. This means we will have the tidal current in our direction all the time and we can pass the shallow areas when it is high tide.

Tidal diamonds

There are tidal diamonds in the nautical chart with a letter in it that corresponds with the letter in the table with tidal currents. In order to determine the tidal current's direction and rate, we need a tide table to determine the time of HW for the standard port of the table on the relevant day (in the image below it is Hoek van Holland). Suppose we are sailing near diamond A at noon and it is high-tide Hoek Van Holland at 10.15 a.m. and it is springs, then we look in the table column at diamond A, at 2 hours after high tide, in the left column (springs). The current at diamond A at 12.00 is 26 degrees and 2.7 knots.

wiebertjes

That tidal stream in the direction of 26 degrees and with a rate of 2,7 knots will run from 11.45 until 12.45. See the table below in which we also placed an extra column for the calculated rate, between springs and neaps.

Time +/- from untill direction Springs

Neaps

calculated
rate

04.15 -6 03.45 04.45 203º 3,8 2,5  
05.15 -5 04.45 05.45 205º 4,0 2,5  
06.15 -4 05.45 06.45 204º 3,1 1,8  
07.15 -3 06.45 07.45 195º 1,3 0,7  
08.15 -2 07.45 08.45 043º 1,1 0,5  
09.15 -1 08.45 09.45 030º 3,1 1,6  
10.15 HW 09.45 10.45 027º 4,0 2,3  
11.15 +1 10.45 11.45 026º 3,8 2,2  
12.15 +2 11.45 12.45 026º 2,7 1,6  
13.15 +3 12.45 13.45 024º 1,3 0,9  
14.15 +4 13.45 14.45 197º 0,9 0,2  
15.15 +5 14.45 15.45 201º 2,9 1,3  
16.15 +6 15.45 16.45 203º 4,0 2,2  

Often the exam states that you will be at Waypoint x at around 11:45 and a trip to Waypoint y will sail and the trip will then take about an hour. In that case we can use the current with the direction of 26 degrees and with the speed of 2.7 knots, because it runs from 11.45 to 12.45. So if we leave at 11.45, then that is exactly right.

Tidal level squares

The nautical chart also contains squares with a letter in it. These squares correspond to the table with vertical tide data. Here you can see the height of tide at for example MHWS (mean high water springs), MHWN (mean high water death neaps), MLWS (mean low water springs) and MLWN (mean low water neaps). If you want to calculate the height of tide between springs and neaps you need to use the 1 / 7th rule to calculate the height of tide at HW or LW for a day between springs and neaps.

vierkantjes

HP33

In nautical books like the HP33 it is possible to find the height of tide per day and even per hour. In the nautical chart you are only able to find the maximum and minimum heights of tide on days that are exactly springs or neaps.

HP33

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