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Calculating tides is an interesting exercise that?s important to understand before setting out to sail. While there are many websites that list tide times and heights (starting with SHOM and Marée.Info), we?ll explain how to calculate them using a tide table (in this case, the Bloc Marine), which contains all the necessary information.
A tide calculation is performed in 6 steps, which we detail here, with a numerical example for each step.
1 - Tidal coefficients
The tidal coefficient indicates the tidal range. It is a calculated numerical value that ranges from 20 for the lowest coefficients to 120. A tide is said to be at "dead water" (ME) when the coefficient is below 70 and at "live water" (VE) when it is above 70.
2 - The Tide Table
Find the reference port associated with your navigation area.
There are reference ports and affiliated ports. There may be a slight difference in time and water level between the two. This difference varies depending on whether the tide is rising or falling.
Example :
Morgat (a port in Finistère that is part of Brest)
There is a time difference ranging from 5 minutes (high tide at VE) to 20 minutes (low tide at VE) and a difference in water level ranging from -40 cm (high tide at VE) to -10 cm (low tide at VE).
Be sure to note these values; they will be used to adjust the final calculation.
3 - Tidal range
In the tide table, you can find the day you plan to sail. It lists the tide times, but more importantly, the water levels.
Tidal range is the difference between the water level at high tide and that at low tide.
Example :
Sunday, October 25, tidal coefficient of 89, meaning high tide
- Low tide in Brest: 1.50 m
- High tide in Brest: 7.10 m
- Tidal range in Brest = 7.10 %E2euros%93 1.50 = 5.60 m
After correction for Morgat, the values are:
- Low tide: 1.50 - 0.10 = 1.40 m
- High tide: 7.10 - 0.40 = 6.70 m
- Tidal range = 6.70 %E2euros%93 1.40 = 5.30 m
4 - Tide Time
Tidal time is close to standard time. However, the calculation is refined by taking the time of low tide %E2euros%93 the time of high tide and dividing it by 6.
Example :
Sunday, October 25
- Low tide in Brest: 9:10 a.m.
- High tide in Brest: 3:06 p.m.
- Low tide in Morgat: 9:10 a.m. ? 12:20 a.m. = 8 hours and 50 minutes
- High tide in Morgat: 3:06 p.m. ? 12:05 a.m. = 3:01 p.m.
- Tide times in Morgat: 3:01 p.m.?8:50 a.m. = 6 hours, 11 minutes
- Tide time: 6:11/6 = 1:02
5 - The Rule of Twelfths
Over the course of a tide (about 6 hours), the incoming and outgoing currents do not flow at a constant rate. At the beginning of the tide, the current rises (or falls) gently. At the peak of the tide, it is at its strongest. And at the end of the tide, it slows down.
It is commonly said that water levels change according to the rule of twelfths.
- Hour 1 1/12th
- Hour 2 2/12th
- Hour 3 3/12th
- Hour 4 3/12th
- Hour 5 2/12th
- Hour 6 1/12th
To find the "twelfth" of a tide, take the tidal range and divide it by 12.
Example :
Sunday, October 25
Tidal range at Morgat: 5.30 m
Twelfth: 5.30/12 = 0.44 m
Hour 1 the sea level rises by 1/12th or 0.44 m
Hour 2 the sea level rises by 2/12th or 0.88 m
Hour 3 the tide rises by 3/12th or 1.32 m
Hour 4 the sea level rises by 3/12 or 1.32 m
Hour 5 the sea level rises by 2/12th or 0.88 m
Hour 6 the sea level rises by 1/12th or 0.44 m
Thus, by the 4th tidal hour, the sea will have risen by 0.44 + 0.88 + 1.32 + 1.32 = 3.96 m
6 - Find the water depth at a specific location on the map
On the charts, zero corresponds to a coefficient of 120 (the highest coefficient that is never actually reached). This is therefore the lowest of the lowest tides. According to a tide gauge reading
Example :
The 0.50-meter-deep sounder at the mouth of Morgat Harbor will already be covered by 2.00 meters of water at low tide.
Sunday, October 25, in Morgat
Low tide: 8:50 a.m.
Water depth: 1.40 m
That is, 0.50 + 1.40 m: 1.90 m
At the 4th tidal hour, it will be:
8:50 a.m. + (4 × 1:02) = 12:58 p.m.
The water level will be
1.90 + 3.96 = 5.86 m
There will therefore be 5.86 m above the 0.50 m-high probe at the harbor entrance.
