Tag Archives: Tides

Ballymacaw – Swimming a new location 2

I love swimming at my favourite places such as Kilfarassey, Sandycove and the Guillamenes. but I also love swimming at new places and there aren’t that many left to me on the Waterford Coast. It’s been some time since I did Project Copper Coast, swimming from Powerstown Head as far as Stradbally. There’s a gap of about two kilometres still unswum at Ballyvoyle Head, then all of Dungarvan Bay is swum (I hope to close that gap this year). There’s a long inaccessible stretch of coast with high cliffs from Helvick Head to Ardmore Bay, which stretch of coast is home to Ireland’s highest lighthouse at Mine Head and also still untackled apart from a couple of swims off Clare’s boat back in 2010. In 2011, I wrote a post on swimming a new location (Whiting Bay) and how I went about it, and this covers a similar theme of swimming a new location, but with different considerations.

Last year I travelled away from the Copper Coast closer to the Waterford Estuary, on the far (east) side of Tramore Bay and before Dunmore East, a less-travelled stretch of coast, and did an exploratory swim out of Portally Cove, where I discovered strong westerly currents running toward Dunmore East. 

The May Holiday weekend brought some very rare sun and a little bit of warmth, and a belief that I was finally recovering from a protracted chest infection. The water temperature seemed stable at around 10 degrees in Kilfarassey, so I decided I’d spent the day on the coast at the far side of Tramore Bay again.

Saleens warning sign
Saleens warning sign

We started the morning at the Saleens, the beach and channel at the east side of Tramore Bay. The channel separates the Back Bay, a tidal lagoon from the main bay and as such has a very strong current running through it.

From there we moved onward to Ballymacaw on the far side of Powerstown Head, which I’d only ever visited twice previously on a bad day and low tide. This occasion was a nice day, close to high tide. Like Portally, Ballymacaw is another tiny narrow and short high-sided cove, on the west side of the estuary but away from any  main road. If you remember, tidal range here in Ireland is about 5 metres average so at low tide both Portally and Ballymacaw Coves are almost dry and at high tide the coves are completely flooded. Prior to swimming Dee and I walked the path through the dense gorse bushes out to the old slipway, and then out beyond the cove entry for a good look outside the cove. Eastwards the next headland is Swines Head, to where I had swum from out of Portally. West from Ballymacaw is toward Powerstown Head and inaccessible from land, though the coast and cliffs are typically only about five to ten metres high, there are no roads.

Ballymacaw Cove
Ballymacaw Cove & the old slipway – (the new lens Polarizer is working out!)

The wind was fresh, about Force Three and there was plenty of movement in the water. With still cool water, it was earlier than usually to be doing an exploratory swim so it would need to be short. Not least because with my weight loss and less exposure training than usually, I’ve lost some of my hardening and feeling 45 minutes is about enough currently without wanting to push hard into a colder state. For this short exploratory swim at a new location, I had a number of things to evaluate and weigh beforehand

  • Swimming time
  • Currents
  • Rocks
  • Water state (roughness)
  • Wind direction
East from Ballymacaw to Swines Head
Looking east from Ballymacaw to Swines Head

Our walk out to the cliff outside the cove entrance gave a good view of the coast on either side. Also the water state of the sea and a good look at the rough water around the cove entrance. The cove itself was completely flat but right at the ten to fifteen metre-wide entrance there was a lot of movement in the water and reefs just visibly breaking the surface on the west side. The sea outside the cove had plenty of onshore wind, blowing south-westerly onto shore at a slight angle and the water was very choppy though with no big swell. Chop waves were one to two metres high.

Ballymacaw Cove entrance
Ballymacaw Cove entrance and the old slipway

Back at the car, I changed and explained my plan to Dee. The cove is about 300 metres long at high tide, it might take me four to five minutes to reach the entrance and the rough water at which point I would disappear from her view. With the wind blowing onshore but with a slight westerly element, I would swim into the chop. It was high tide, and though most people don’t believe me, high on the Waterford coast is NOT slack tide and I knew the tidal current would still be running east, though I couldn’t estimate any local eddy current effects which would run anti-clockwise. I also knew that there had been strong westerly currents from the west moving in this direction previously when I’d swum out of Portally and I would always choose to swim into an unknown current when heading out. The obvious rationale is that I don’t want to get carried too far away from a starting place by a strong current, and possibly have too difficult a swim back while getting cold.

So I would swim west for 15 minutes after leaving the cove, evaluating travelled distance as I went. If there was no current I would be then have 15 minutes back, plus another few minutes getting back to the beach, 40 minutes total. I wear a watch always when swimming open water so I’d be able to judge. Dee asked at what point should she start worrying, so I said 45 minutes, at which point she could walk up on the path to give her a better chance to see me.

As I was about to get ready a couple of guys were also getting changed into scuba gear. They were somewhat familiar with the cove, and indicated no items of concern, except a steep drop-off to 10 metres at the eastern exit of the cove and a consequent sharp drop in temperature. Just before I was ready to get in however, the worst of all possible arrivals, appeared in the bay: Three jetskis. Even in the flat water of the cove I didn’t want to risk getting in so I got back in the car. The jetskis tied up to the outside old slipway, and the guys came inland along the winding gorse path. they could only have come out of Dunmore East, the only possible water entrance for many miles. They came along the path, obviously heading for the pub near the cove. I had a chat and let them know I was heading out and there were already divers out there. They were nicer chaps but while I can’t be certain they were going for a drink, there was no-where else to go on that road and drinking and being on jetski isn’t illegal here, as far as I know. Another reason to add to my nervousness about jetskis.

Ballymacaw angler
Ballymacaw angler

It’s a very long lead-in for a short swim. As expected I reached the cove entrance after four and half minutes and immediately hit a line of choppy water. Just under the surface was a long reef reaching out from the west side of the entrance. I passed an angler who was positioned on rocks at the est side of the entrance and headed westward. The chop was coming south-westerly with the wind, about a metre and a half high. The jetskiers had warned me it was “big out there”. One a half metres of chop isn’t big, just messy and slow. After fifteen of grinding through it, I had travelled the glorious distance of maybe 400 metres! The westerly tidal current I’d expected was running strong. I released Duck #4 and turned back to the Cove entrance, impossible to see from seaward unless you are directly in front and close. The swim that had taken 15 minutes out took 5 minutes back!

Ballymacaw Cove entrance
Ballymacaw Cove entrance from the sea

Getting into the cove was quick over the reefs with the waves at the reef entrance providing a quick surf into calm water. I’d had been 30 minutes, so I swam to the beach in the warmer water at the high tide mark, and turned back for a couple of laps. I’d forgotten how tough it was to swim out of water that had helped you recover from much colder water. Warm water  feels nice…if you are not leaving it for cold water. Swimming back out the cove was brutal. The warmer water had restarted my circulation so I had inadvertently initiated Afterdrop, cooling faster, and now I was hit by colder water again. I lasted another 10 minutes  before I I was out of the water.

But the purpose of the swim, an initial scouting swim at a new relatively unknown location, though short, was successful. I’d like to stress that when swimming a new location, having a plan, an understanding of the constraints and possible problems and an idea of how to approach it, are all important.

I repeat that tides are a vital consideration for many locations and a solid understanding is essential for safety and swimming new locations in tidal areas. 

Sea pinks against the sky. yes, it's time for me to start taking lots of photos of sea pinks again.
Sea pinks against the sky. Yes, it’s the time of year for me to start taking lots of photos of sea pinks again.

How To: Using Tide Tables

Because I live and swim in Ireland, I am constantly made aware of the large tidal range here.

I’ve written extensively about tides previously because I feel they are an aspect of open water swimming not appreciated by enough swimmers and because global variations can mean that many people never see nor even realise the apparent extremities of a higher tidal range in other locations. I therefore think a better understanding of tides is important for open waters for safety reasons.

To understand tides better is to increase your knowledge, your range of options and responses and locations and therefore your safety around the coast. Combined with this is that tidal knowledge is sometimes incorrect, that people make very basic incorrect assumptions, that the tide goes directly in and out from the shore regardless of the coastal position, is amongst the most common misconception (which is only true in some locations).

Because of this North East Atlantic tidal variation, most experienced Irish and United Kingdom sea-goers are used to checking tide times when the sea is not immediately visible to them daily.

You can revisit some of the more detailed tide articles I’ve written but for a brief recap let’s remember that each tide is about six hours and fifteen minutes, which means that high and low tide times change each day. A practical consequence of this is that Sandycove, which is usually swum above half-tide, usually only swim times designated for group swims every second weekend. (I am luckier at the Guillamenes as it is deeper water and can be swum on any tide).

Let’s a look at some graphs and data of a daily tide cycle, for the week I’m writing this. This data comes from MagicSeaweed’s Tramore tide report.  The undulating sine wave indicates the rising and falling tide. You can see that there are four tides in each 24 hour period and that each tide on this current cycle varies from just under six hours to about six and a half, with rising tides being longer than falling tides. On each chart you can also see the tide heights of high and low tide. As the four days pass the range between high and low decreases, and the high tide gets lower as the low tide gets higher, all indicators that the tide is moving from a Spring tide to a Neap tide, this pattern of changes from springs to neaps and visa versa repeating every two weeks.

tide graph 2

Tide programs and applications are usually similar in this presentation and a good understanding can mean a quick glance at a tide table can tell you a lot. Since I know that spring tides here are over 5 metres, I can tell immediately from this where in the lunar tide cycle we are. Lower tidal range means lower tidal currents, (not usually a concern for me anyway), important information for some locations.

The other usual tide tool, which I prefer myself, is an annual national tide table. These are currently about €3.00 for the pocket-sized book and I keep one in the car. There are two types of tide table books. Those often issued by the local port or regional publishing company, and a national one. Ireland is small enough that a localised tide table is too specific and of little utility if one is visiting the far side of the country.

OLYMPUS DIGITAL CAMERAI find physical Tide Tables more utilitarian. Always to hand when needed, and useful for longer term planning many months in advance. Free and online tide apps usually don’t provide future tide times.

More importantly in Ireland, the island nature of the country makes the tidal situation far more complicated than many people realise, with the tides washing around the coast in diverging or even opposing directions. Therefore the Tide Table is sub-divided into five regions with further tide time offsets (delays) to even more localised ports. This provides a level of forecasting that gives a far greater level of accuracy.

OLYMPUS DIGITAL CAMERAThe detail is of the same type, date and day, high and low water (tide), with tide heights and in this case, moon phase to indicate more easily the spring and neap tides. On the page above can also be seen the variation of other locations from the Cobh location, Cobh being the Standard (reference) Port, i.e. the main tidal location, for the south-west to south-east Irish coast. A fuller list of Secondary Ports for each region is also included.

What’s equally important about these tide tables, and hidden in a note inside the back, is that the data is compiled from the United Kingdom Hydrographic Office, which, bizarrely and which I haven’t mentioned in a couple of years, owns the tide data for all of the United Kingdom and Ireland, and from which it must be licensed. Not that I am in favour of this arrangement, but it does mean you can be sure of the table accuracy, unlike with many free tide applications where license fees haven’t been paid.

OLYMPUS DIGITAL CAMERA

A local sources of tidal information if you are unsure or without any better information is to check any RNLI  or local inshore rescue stations across Ireland and the United Kingdom which at least usually display the month’s local tide times on the outside of the stations. These always use the accurate UKHO (below) tide data. Some broadsheet newspapers can carry the information also in the weather section.

The important points therefore are:

* Tide tables are essential for coastal safety in area with high tidal ranges, such as across the continental European coast.

* When using tide tables note the tidal height as well and high and low tide times.

* If you are using tide prediction tools, safety is important and to this end, the origin of the data is vital.

Limiting Factors in Marathon Swimming – Part 2 – Environmental Factors

In Part One I covered the physiological limiting factors in marathon swimming.

The various environmental aspects of a swim are not insignificant. They are especially important in that they all lay outside the swimmer’s control and often even outside the control of the support crew.

Water Temperature

Thermometer
Thermometer

This is generally a known factor prior to a swim. Swims are either cool or cold water like the English and North Channels or warm water swims like Maui, Rottnest, Manhattan or Chloe Maccardels’ upcoming Cuba to Florida attempt. A few fall into an intermediate category defined more by the swimmer’s experience, such as the Catalina and Gibraltar Channels. Sudden changes in temperature are rare in marathon swimming and where they are possible they are also understood; such as South Africa’s west coast which is prone to sudden wide water temperature changes, and the California coast where the sudden transition from very deep water to a shallower continental shelf very close to the  mainland can cause cold water upwelling at the end of a marathon swim. Air temperature is obviously much more variable and a condition of the weather but extremes of air temperature are not usual during a swim. A five degree Celsius differential can be significant for a swimmer if such a drop is also accompanied with a breeze or wind which can sap the swimmer of body heat.

Jellyfish

Lion's Mane jellyfish
Lion’s Mane jellyfish

The recent and future attempts at a distance and time records by necessity are held in warmer waters such as Cuba to Florida.  These water are home to jellyfish with debilitating stings such as Box Jellyfish. While the cold waters  of the North and English Channels are home to Lion’s Mane and Portuguese Man O’War’s endurance records are less likely and jellyfish stings in the English Channel are rarely more than intermittent, though the North Channel (the Mouth of Hell) can have miles of Lion’s Mane blooms, part of what makes it the ultimate channel swim. Attempts to swim in these waters divide swimmers in two ways: whether attempts should be made in locations not considered possible without additional protection or exceptions to the usual rules, and if so are jellyfish protection suits acceptable or the thin edge of a wedge that will inevitably lead to more overt (or hidden) performance enhancing suits? (See Evan’s analysis of his survey of marathon swimmers for an excellent overview of the contradictions of divisions and unity in the community).

Sharks

The Man In The Grey Suit is a subject of great concern (and discussion) for distance swimmers. Not of any real concern here in the north-eastern Atlantic, they are a greater hazard in the warmer waters elsewhere, particularly California, the Caribbean, Hawaii, South Africa and Australia. The Cook Strait Channel swim in New Zealand is unique in having a shark evacuation rule. Shark cages have been used for marathon swims in the Caribbean and South Africa at least. Shark cages are however considered swim assistance as they increase the swimmer’s speed through eddy current drag. Other possible control methods include electronic shark repellents (whose effectiveness is not entirely assured or quantified), armed boat crew or armed or otherwise scuba diver outriders.

Tides

These are amongst  the most variable of environmental factors and therefore potentially also the most limiting. Because swimmers move slowly relative to even a sailing boat, we are vulnerable to slight deviations, miscalculations or just insufficient data, the most likely cause. Even in such a well-travelled and mapped location as the English Channel, especially for swimming, pilots will occasionally speak of tides arriving early or late or with a difference force than expected. Tidal currents are understood at a larger scale, hundred of years of navigation have mapped the seas for craft, not for swimmers. Tides act in a similar chaotic way to a weather system, which means that small deviations will always creep in. The only way to improve accuracy of prediction is to improve the data, and this is not practically possible or even desired for small tidal variations. As swims occur in less well-known or new locations, the likelihood of discovering unknown local variations outside marine charts increases. Half a knot current, barely detectable to a boat, is enough to deviate a swim over hours from a projected or necessary course.

Global tides
Global tides

Crew and boat

Any English Channel pilot will confirm that one of the most likely causes of unsuccessful Channel swims is poor selection of support crew. The most likely cause is mal-du-mer, seasickness. For some people seasickness is a completely debilitating ailment that can sap all willpower and strength and there is no way to know whom it will strike. The solution of course is to have experienced crew. Even this can fail because people experienced on powered craft will be at the mercy of the choppy water amplified on an almost stationary craft. Other crew issues can also arise, whether accidents or other illness. Anyone who hasn’t been on a rocking boat looking down on a swimmer is unlikely to understand! And not unknown are mechanical problems on the pilot-boat. Most pilots are by necessity practical mechanics able to address problems as they arise, but not all problems can be fixed with a wrench and hammer while rocking about on the sea.

Channel boat The Viking Princess
Channel boat The Viking Princess out of the water

Weather

Weather changes are the bane of English and North Channel swimmers particularly. Other Channels like Tsugaru and Gibraltar and Cook are also subject to constantly variable and unpredictable weather patterns. If you are used to the predictable weather of the west US coast, with morning offshore and afternoon onshore breezes, knowing your swim will almost certainly take place with a 48 window, the difficulty of allocating two weeks or even long (like the North Channel) and still being completely unsure of getting in the water is shocking. Weather constraints obviously ran the full gamut. In the North, English and Gibraltar channels the main concern is wind (and its effect on the seas). Fog can also be a problem with 2012′s Channel season infamously seeing three solos on one day abandoned within a kilometre of France for the first time in 137 years. I’ve warned previously that fog may be the most dangerous weather condition for swimmers. In warmer humid climes like Round Manhattan, and the Caribbean, lightning storms are a serious cause for worry, a swimmer or boat caught exposed out on the water is in real danger. Having to wait for or even postpone a swim is something many marathon swimmers have undergone and the mental pressure this brings is often not inconsiderable, which I will discuss further in the next and final part.

Coming in part three, Psychological Factors.

 

H2Open_tides

My first published article is in this month’s H2Open magazine

I was pretty excited a few month’s ago to be asked by Simon Griffiths, editor of the world’s first and only open water swimming magazine, to write a post on a subject I feel pretty strongly about; tides. You feel strongly about tides, Donal? You are even weirder than we thought. The magazine has previously featured loneswimmer.com and our other shared project with Evan Morrison, marathonswimmers.org, whose own blog freshwaterswimmer.com has also been featured.

Open water swimming in Ireland and the UK are primarily governed by two aspects, cold and tides. Therefore I’ve previously written a lot about both, and will continue to so do.

Incorrect (or no) understanding of tides (and waves) lead to deaths around the shores of Britain and Ireland every year, and the media don’t help the situation by regularly attributing drownings to freak waves. Tides are critical for any swimmer visiting the coast or estuaries.

Simon asked me for a comprehensive article covering theory, safety, hazards, and useful resources. I even took the photos for the article, using Newtown & Guillamenes for my images of tidal range and hazards.

The magazine is available for iPad (search H2Open) through Apple’s newstand and there are Apps for both Android and iPhone, and it’s available as subscription or individual issues (for a mere £2.99!), all from here. The App is free and comes with a free 16 page sample magazine.

Western end of Kilfarassey beach

Explaining a critical open water swimming factor: Tidal Range

Recently I wrote about how I consider safe entry and exit points and possibility of swimming at all tide times to be a critical requirement of a good open water swim location.

Kids growing tend to think the whole world is the same as their local experience. Though I didn’t live by the coast growing up, it never occurred to me that the seashore was different in other places. In Ireland, if you were visiting a new beach, you knew you had to be careful of incoming tides and of not being cut off. The first time I visited the Mediterranean I was really surprised by how little the tide seemed to move. So even now I tend to forget that tidal access and depth is not an issue in many places in the world. But it’s far safer for someone who comes from a high tidal range location to travel to a low range area than the opposite.

Many people now know the Bay of Fundy in Canada has the world’s highest tide, due to estuarine forcing (pushing more water into a smaller space), with a height of up to 15 metres and the lowest tidal regions are called amphidromes, with no height change.

Tidal range is the height difference between low tide and high tide. The tidal range in Ireland averages six metres. This is called a Macromareal tide, a tidal range above four metres. The average open ocean tidal range is only just over half a metre. And the Mediterranean is micromareal, less than two metres range. In between, from two to four metres range is mesomareal.

Why this is the case I explained a long time ago in Tides for Swimmers, Part One and Part Two.

In the English Channel the range about seven metres. Even average is misleading. During a low neap tide, the range in Ireland can be as little as four metres. During a high it can be as much as seven metres. In Ireland on a neap tide the low will not drop to as low as the open water mean of 0 metres, but might only drop to 1.3 metres, and will only go up to over just over 4 metres, whereas during a spring tide, the range may be from 0.0 metres to over during the spring spring and autumnal spring tides. Spring spring is not a mistake, it the spring tides that occur during spring.

Today in fact is a neap tide, and in Tramore the neap tide is 0.5 metres and the high tide is 4.2 metres.

There are a few serious safety implications of this.

  • Will you have a planned known safe exit if the water is going to be a different height to when you start swimming?
  • What will be the effect of the tidal current where you are swimming? The greater the tidal range, the greater the tidal current.
  • What different challenges will come into play on your planned route at different tide times? Will dangerous reefs appear? Will swim landmarks disappear?

What happens if you show up in a new country and have no idea of the tidal range and want to swim? Well as always, first check with locals before you swim.

But how do you recognise a high tidal range? The simple answer is to look for the high tide line.

On a beach that will be a line of debris, twigs, leaves, kelp or rubbish or even a change in the sand quality.

Don’t assume that a high tide line won’t happen the day you are swimming. Was the moon full or dark the previous night? If either, it’s a spring tide. A half-moon is a neap tide.

If there’s no beach, rocks are even better indicator. The difference between the low and high tide point is called the intertidal zone.

Français : Verrucaria maura, Kergulan, Goulien...
Around Ireland and elsewhere, rocks that close to the high tide mark will get covered in a salt resistant lichen, Verrucaria maura, making the rocks black.
Verrucaria maura doesn’t start at the low tide mark by the way, it generally starts at about the mean high tide point, HWA.

The rocks beneath the low tide point will retain their original colour, the rocks above the high tide line will often be yellow or orange with less salt resistant lichens such as Xanthoria parietina or Caloplaca marina, all of which are visible in my Copper Coast Swims. Of course sometimes the rocks are dark anyway, but high tide lines are easy to see. Here are the rocks on the far side of the Guillamene Cove at about mid tide.

It is still difficult to appreciate just what that range can mean. So … some more photos I’ve taken.

Here’s the Guillamene from the cliff top road. At low NEAP tide, all the steps are exposed. At low SPRING tide, there’s a ladder below the steps which is exposed to about 4 or 5 steps. Look at the colour range of the rocks. At the lowest point and up they are a sandy limestone colour (and covered in barnacles) and get blacker as they get higher. On a high spring tide, without any wind, the water reaches to just under the front triangular platform. Just above that line the rocks are completely black from the lichen.

Newtown Cove at high neap tide.

Considering hazards, this photo below of the west end of Kilfarassey gives an indication. With the tide only about one metre below high in this image, various reefs are starting to appear. Almost all are covered at high tide, some of them only centimetres below the mean high water surface on a calm day.

A 6 metre tidal range (almost 20 feet) is the height of a house, three times the height of a tall person. It’s very very significant.

 

So be safe, and take note of the conditions, tide and tide range and plan accordingly.

world tidal constituent

HOW TO: Tides For Swimmers, Part 2 – Local Effects

Part 1 – Theory

So, why a different post for what we can call the observed local effects?

Well, depending where you are in the world, this may not be needed. There are some places where the observed local effects match those predicted by theory.

However, why isn’t that the case for everywhere?

Simply, the oceans aren’t uniformly shaped basins. They have obviously irregularly shaped coasts, and differing sizes and depths, all of which interfere with the tidal wave. Gravitational effects (lunar and solar) cause waves. But inertia and drag also interfere with the tides.

This is a map of global tides. We need to look at the Global situation to understand the local situation.

world tidal constituent

Whoa there.

Okay, how about this simpler-to-read one?

global tides

All right, that second one’s a bit easier to read. Forget everything else and have a look at the Atlantic. Notice there is a node, a black point, in the centre of the North Atlantic, south of Greenland, east of Newfoundland, from which lines radiate out?

That’s the Atlantic Amphidromic Point.

This means it’s the point in the Atlantic where the tidal range is zero, i.e. it has no tide, it’s all at the same tide height. The tides radiate out and away from that.

The further you go the greater the tidal range.

Each of those lines reaching indicate different tide heights. The further away from the amphidromic point the greater the tide height. Each of those lines also has a number associated which is the (very) approximate tide range.

Now go back to the coloured chart. See how one you get up to Europe the tidal range (colour) gets higher (a deeper colour)? Compare the difference between California (and the Catalina Channel) and the Atlantic Isles and the English Channel.

Now lets back up again. Imagine the Atlantic is a rectangular basin. You pick it up sloosh the water from on side to another. Assuming you are not spilling it out, and keep it moving, there’s a point somewhere toward the middle where the water is hardly moving up and down, just backwards and forwards. The water at the ends though will pile up against the basin for the “high tide”.

Now also think of the moving water. All the water doesn’t move at the same time and most of the water from either side will never get to the other side. So this affects the locality of the tide even within the basin.

Now imagine swishing the water and trying to add a circular motion. It gets more complicated.

This also then applies on a global scale. The tidal wave doesn’t all go from A to Z and back to Z every 12 hours. A to Z is a long way with lots of intermediate points. So lets the water from A goes to C, the water from B goes to D etc. Or better yet. The water from Kerry makes it to West Cork, the water from West Cork makes it to Cork, the water from Cork makes it to Youghal etc. So that means each area’s tide is at a different time, as you already know.

So that’s the whole Atlantic situation: Here’s an animation of the simple and complex (real) models.

Imagine this applying to Ireland. Think of that tide coming towards Ireland. It’s coming from west. So imagine it arriving as a single wavefront. Some flowing into the west coast, some flowing up around Donegal, across the north coast, some flowing across the south coast, around Carnsore Point and up the east coast.

Ok, the tide wave doesn’t cover all the area. As said previously it’s actually water moving back and away from the originating point. And then what? See it?

Ireland is an island, with water flowing around it. Logically that flow must reach around the coast and each flow start to interfere with other. Which is does. There’s actually an amphidromic point north of Ireland near Skye, and in the Irish Sea just south of the Isle of Man and yet Lancashire and Cumbria, on the NE coast of England, not that far away, have the highest tides at eight metres and Carnsore in Wexford, is just 1.75 metres. According to the Irish Cruising Club, there is an amphidromic point close to the east coast 25 miles north of Carnsore.

But we’re not finished. So you’ve heard of Capes, Cape Horn etc. Ok you channel swimmers, your favourite, Cap de Griz. Well in geographical terms, a Cape is a headland that significantly affects the ocean currents around it.

It does this, and also other headlands to a lesser extent, by essentially forcing the tide to flow through a smaller opening like putting you finger over the end of a tap. (Hello Channel tides!) Or maybe the coast “falls away” froma tide flow, having the opposite effect.

And then there’s the estuarine effect where a tide flowing into a estuary gets higher as it is forced further in. The tide here outside my door is about 7 metres, 20 miles from the sea, where is about 5m normally.

The Irish Sea, because of the narrow North Channel, also acts as estuary, forcing higher tides further north along it. In fact there are really varying effects in the Irish Sea.

Courtown for example sometimes has 5 tides instead of 4, with a tiny variation between them. The tides around Kilmore Quay also can be 8 knots. Some other places have similar very strong tides, while a few miles away there are negligible tides.

So lots of effects. In the case of the south coast of Ireland, the only bit I feel qualified and experienced enough to talk about, all this combines to produce a “lag” effect, where the slack portion of the tide is NOT necessarily at the high and low water marks, as we saw is the theoretical case in the first post. Instead, depending where along the coast you can be, slack water may be the middle of the tide or other times.

The charts to determine this are published by the South West Cruising club and printed every year in Reed’s Almanac, the yearly sailing almanac, so I can’t really post them here. PM me if you want them and i’ll send them on, though they are a bit complicated apparently first time you see them.

All of which serves to demonstrate that adhering, as so many swimmers do, to a theoretical tide idea should be tempered with acknowledging that it’s necessarily the truth.

For good tide knowledge, sailors and kayakers are the best. Like us they are at the mercy of tides, but are further out so their knowledge is often more extensive and their literature is far more developed.

HOW TO: 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.