Tides are important to Sea Kayakers

As a Sea kayaker, the influence of the tides and currents can make a big difference to our paddling day, so it is very important to take them into account even if we are only going out for an hour or so. That hour could turn into a couple hours or not reaching your destination at all, if you don’t take the tides into account. The tide is explained as the rise and fall of the sea level in height in relation to the coast. This rise and fall creates currents with huge volumes of sea water which moves in and back out, away from the coast. These tidal currents are measured as the horizontal speed of the water in knots or kilometers per hour compared to the sea bed. In this post I am going to explain tides and what the tides are affected by. In a future post I will explore how the tide affect currents and how we can use them to help us in our paddles.

The tides are caused by the gravitational effects of the Earth, the Moon and the Sun. Firstly the ocean water is kept at equal levels around the earth with the inward gravitational pull of the earth and the centrifugal force of the earth spinning pushing outwards. Then the moon disrupts the balance by pulling the ocean water towards itself with the moon’s gravitational pull. As the moon orbits around the earth and the earth rotates, this bulge of ocean moves around the earth causing a high & low tides.
An interesting fact is the water on the opposite side of Earth, facing away from the Moon also bulges outward (high tide). The Moon and the Earth revolve together around a common gravitational center between them, or center of mass. To explain this, remember those hammer throwers in the Olympics, as they spin around before they let the hammer go, they have to lean further and further back as they spin around with the hammer. This puts a center of mass between the hammer thrower and the hammer. The link between the thrower and the hammer, represents the gravitational pull of the moon and the earth. The centrifugal force of the two objects spinning balances out the gravitational pull and keeps them apart. The ocean on the opposite side of the earth has less gravitational pull on it. The ocean bulges outwards because the centRifugal force is stronger than the gravitational pull of the moon.

The same forces are at play between the earth and the sun, but even though the sun is much bigger than the moon, it is over 380 times further away, so it’s effect is smaller. You will have noticed that the sun and moon don’t appear in the same position in the sky in relation to each other each night. It is easy to see at different times through a monthly and yearly cycle, they are going to effect the ocean tides differently. When the sun, earth and moon line up, (at full moon or new moon),that is going to create increased tides which are called spring tides. They are not called spring tides because of the season, but because they spring higher.

At the first quarter & third quarter moon, when the sun and moon are at 90degrees to each other, the high tides are much smaller in height. These lower high tides are called neap tides. The height of the tides can vary during the month due to the distance the moon is in relation to the earth. When the moon is closest to the earth during the moon cycle it is called the perigee. When the moon is at it’s most distance, it is called the apogee.

Tides most commonly happen around the world twice a day called diurnal. Tides can also occur as two highs and two lows per day as they do here in North Queensland. This called semi-diural. The moon takes about 24 hours and 50 minutes to pass over the same point on the earth. So the high tide is staggered 50 minutes later each day.

There are also other factors that affect the tides, the moon’s declination (angular height above the equator), local geography of the coastline, topography of the local ocean floor and depth of the water and other considerations.

One full tidal circle:

SLACK TIDE – the tide has reached its highest point. The water is not moving anymore. It’s the TURN.
LOW TIDE – the tide now starts to GO OUT. It’s low tide from the moment the water starts to go out until it is at its minimum point. When it reaches its minimum, the water stops moving, and it’s the turn again.
SLACK TIDE – the tide has reached its lowest point. The water is not moving anymore. It’s the TURN.
HIGH TIDE – the tide now starts to COME IN. It’s high tide from the moment the water starts to come in until it is at its maximum point. When it reaches its maximum, the water stops moving, it’s the TURN.

As you can see from this chart for Wednesday 25th December 2013 for Mackay Harbour from the Australian Government Bureau of Meteorology Web site I can find out the tides. The Slack High Tide is at 4:04 am with a height of 3.87 meters – 10:01 am a slack low tide of 2.24 meters. Therefore the low tide is running out for almost 6 hours. The following high tide is running in for 6 hours and 16 minutes until 16:17 or 4:17pm.

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