How To: Understand Tides for Swimmers, Part 1 – Theory

One occasional bee I have in my swimming bonnet is the lack of understanding of tides amongst many OW swimmers. It is however quite understandable, if the location where you swim isn’t particularly affected by tides.

But what about expedition swim planning or trips to spots you don’t know? Or from a safety point of view planning a new participation swim? Or if like me you are prone to going solo. It’s good for swimmers to understand these factors.

In Dover, I spent an hour explaining tides generally, and how they affect the Channel, to two Italian relay swimmers, Martino and Michale (The first ever successful Italian relay team). In their case they swim in the Mediterranean, which is not very tidal, as any of you have been there will know. And they had no clue about tides. And it reminded me I was going to do some stuff here on tides months ago.

So we better start with the theory (don’t worry, it’s easy). Because some people who do know something about tides repeat the theory only, to the exclusion of local effects, or just don’t believe there are local effects. I learned this stuff when surfing and it was certainly my case when surfing, never realising that I was incorrect until I started swimming.

Ok. Whether you know it or not, tides are caused by the Moon AND the Sun. (A lot of people think it’s just the Moon), with the Moon being the main influence.

When the Sun, Earth and Moon are in a Direct Line, Spring tides are caused. (Pic 1. Spring Tides Diagram).

(These tides are called sub-lunar and antipodal).

(One of the biggest misunderstandings of non-coastal people is thinking Spring Tides happen in the Spring, instead of actually every two weeks).

In the case where you can’t see the Moon, that’s called a New Moon (another common misunderstanding) and the Moon is between the Sun and the Earth. Both gravitational forces, of the Sun and Moon, combine to pull the sea water away from the earth’s surface, the Spring Tides.
A Full Moon is when the Moon is on the opposite side of the Earth from the Sun, but the gravitational forces are still directly in-line.
Spring tides therefore happens approximately twice a month.

It’s “approximately” because the tide cycle follows the Lunar cycle and is around six and a quarter hours, meaning a Tide Period, one High and one Low tide, every twelve and a half hours. So you don’t have exactly four tides per day (24 hours), which is why you can’t go swimming on the same high tide every Saturday morning (or whenever) as the tides shift time gradually every day. Occasionally you get a second Full Moon within one calendar month, which is called a Blue Moon. (This is where the well known phrase originates.)

At the New Moon, the tides are called sub-lunar, while those on the other side of the world, away from the Moon’s closest influence are called antipodal, as the Moon’s gravitational effect is lessened and the Earth is pulled more than the ocean.

With Spring Tides, since the water is pulled MORE, that means the tides are both HIGHER and LOWER. The difference can be up to almost double the height. This is really important and I’ll come back to it at the end.

So the New Moon is the first Spring Tide. The second then occurs at the Full Moon. Full Moon is actually a Half Moon in lunar phase terms, that is, Second Quarter.

A Full Moon in NOT a New Moon!

So what about Neap tides?

Neaps also obviously then occur every other two weeks from the Springs. They happen when the Sun and Moon and Earth are at Quadrature, that is not directly aligned. This is around the First and Third Quarter periods of the Moon,.

The Sun and moon’s gravitational influence are acting in partial opposition (transverse) on the ocean, so the gravity pulling the water away from the surface is lessened. The Sun is pulling in one line and the Moon is pulling perpendicular to that direction.
(Pic 2 -Neap tides).

Ok. Got that?

Briefly:

Spring tides are higher and lower.
Neap’s and Spring’s cycle around each other with each occurring about twice a month.

Moving on… and back to the issue to tide height.

The single biggest misunderstanding of tides is just assuming that tides only go in and out, and therefore up and down. I think it’s a misunderstanding because again (non-coastal) people don’t pause to ask themselves what happens to all that water.

With tidal movement there is something called “The Rule of Twelfths”.

For this we average a tide into 6 hours.

In the first hour before AND after high (or low) tide, the tide will move one-twelfths of the total distance that it will move over the full 6 hour tide. So that’s 1 twelfth PLUS 1 twelfth equals 2 twelfths moved in two hours.

In the second tide before AND after high (or low) tide, the tide will move two-twelfths of the total distance that it will move over the full 6 hour tide. And that’s 2 twelfths PLUS 2 twelfth equals 4 twelfths moved in these two hours.

So for four of the six hours the die moves 2 twelfths plus 4 twelfths equals 6 twelfths which is only half the distance, in two-thirds of the time.

For the remaining 2 hours, which is the third and fourth hour of the tide, the tide moves three-twelfths in each hour, so it move the other half of the total distance in only one third of the time.

So…this means, that the tide is moving fastest in the central 2 hours of the tide.

Hour Distance (Speed)
1 1/12th
2 2/12ths
3 3/12ths
4 3/12ths
5 2/12ths
6 1/12th

So theory says: the tide is at it slackest, lowest or no movement, around high and low tides and the tide is moving at it’s quickest at mid tide. That speed is then directly related to whether it is Spring or Neap.

A Spring tide will have a greater range from higher to lower, so more water will move, so the speed of the running tide will be greater. A Spring High will be higher, a Spring Low will be lower.

For example a Spring Low will expose kelp at Tramore pier but a Neap Low Tide won’t.

In Ireland we have quite a big Tidal Range. Lowest Spring Tides are only 0.1 metres (above the Mean Datum of 0), while highest Spring Tides can be well over 6 metres. In the English Channel the highest is above 7 metres. The biggest tidal range in the world is 15 metres but the Mediterranean is only a metre or so!

So, almost there…with Spring Tides the two issues which you have to consider are:

  • Tidal Range. (Will it affect your ability to swim at a particular location, for example when it’s low tide, like at Sandycove?
  • Will the speed of the tide affect your ability to swim a particular distance in a particular time? (For example timing a swim across T-Bay or across the Waterford Estuary, or across the English Channel).

BUT…

Recall: this is Theory. It will apply directly in some places, but not in other because of local effects. This is very important. I cannot tell you your local topography and geography.

Part 2.

About these ads

One thought on “How To: Understand Tides for Swimmers, Part 1 – Theory

  1. Pingback: Beginish 2011 – open water racing on the edge of the world – Part 2 | LoneSwimmer

What do you think?

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s