Tides

This post is inspired by Tide Simplified by Philip Clegg and the book Sea kayak navigation by Franco Ferrero.

Everything on this planet is affected by the gravitational pull of the moon, and to a lesser extent the sun. The moon orbits this planet and we experience high water when the moon is nearest to  (full moon) or furthest away from wherever we are (new moon). Low water is experienced when there is quarter moon.

There is about 6 hours difference between high water and low water. This means that during a bit more than a 24 hours period, you will experience two high water and two low water.

The tide can form currents near land and it will therefore affect sea kayakers. Some currents are strong while others are weak. This is dependent upon local topography and how the tide is pulled through narrow sounds or overfalls.

There are some rules of thumb which is handy to be aware of as a sea kayaker.

50/90 rule:
As mentioned above it takes 6 hours from high water to low water. This means that at the beginning of the first hour the high water is at slack tide. The tide is standing still a brief moment, before it slowly begins to sink towards low water. After 6 hours low water is reached and the tide is again slack for a short moment before it starts to increase towards high water.

So how fast is the tidal current moving between the 1st and the 6th hour after high or low water? First you need to know the maximum speed of the tidal current at the area you are interested in. Let´s say the max speed of the tidal current is 6 knots. The 50/90 rule says that the speed of the tidal current will be 50% of the max speed in the first hour after slack and 90% in the second hour, as illustrated below.

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Rule of twelfth:
This rule is about the water level of the tide. The tide increase or decrease according to the following rule:
1st hour after slack: 1/12 of total tidal difference (1%)
2nd hour: 2/12 (25%)
3rd hour: 3/12 (50%)
4th hour: 3/12 (75%)
5th hour: 2/12 (80%)
6th hour: 1/12 (100%)

So if the tidal difference between high water and low water is 5 meters. We can estimate that in the 4th hour after low water the tide will be 9/12 (1+2+3+3=9) of 5 meters. 9/12=0,75 or 75%.
75% of 5 meters is a bit less than 4 meters, 3,75 to be exact.

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The most important tool you have however is your eye. Look at the water, buoys and anchored boats to decide what is happening to the tide. Find out if the tide is flooding or ebbing and make a paddle plan so you can work with the tide and not against it.

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