From Steeple Point – a question of scale

Talking of tides and waves (here and here):

The ebb tide is running fast leaving wakes trailing from both the buoy and the fixed mark.

The buoy is floating, attached only by its anchor line. The water is passing more or less unimpeded below it, leaving a clean wake; whereas the fixed mark totally disrupts the flow, resulting in very confused water downtide.

Between the two, you can see the wake from another buoy – dying down but still confusing both of the above wakes.

A short while later, the tide has built up enough to submerge the buoy.

In the foreground are eddies from the uneven bottom, causing smooth upwellings of water.

Should we be interested in this?

Aren’t the two images merely pictures of a spring tide ebbing?

Well, it’s a matter of scale. If we want to know more about the sea, this is a good place to be.

Now we move on.

Below is a an image of Ham Stone, between Bolt Head and Bolt Tail on the South Devon Coast.

The effect of the rock on the tide can clearly be seen. Although the tide is not running as fast as in the images above, the water will be confused here especially at the border with the main flow. However, for a small boat, there will be temporary shelter from the main flow of the tide.

In fact, this boat is fishing downtide of a wreck on the sea floor, the disturbing currents attracting food for the fish, the fish attracting the fishermen.

Ham Stone, South Devon

Compare this with another phenomenon – this time rocks interrupting the swell.

These are waves in motion over the surface of the sea rather than the sea itself being in motion.

Instead of causing the waves to spread outwards, the drag effect of the rocks causes them to slow down and swing inwards, so that the sea is confused on what might have been the sheltered side.

Even if the water was deep enough for a small boat, there would be no shelter from the swell here.

Rocks between St Ives and Zennor, Cornwall

In practice, what happens around the coast depends on the swell, the tide and the size of the various obstructions, whether above the surface of the sea or on the seabed – (not to mention the weather).

So, let’s up the scale again.

The fishing boat on the right has chosen to go between the headland and Godrevy Light, avoiding the long haul out round the off-lying reefs.

It’s about one hour after low water and the tide is running against him – the direction of the tidal stream can be seen to the right of the island

Godrevy Light, Cornwall

It’s running faster between the island and the mainland than further out to sea – but nowhere near fast enough to hold him up.

Although, as you can see, he is having to work at it.

At the western end, the swell is swinging round the end of the island against him, just as in the image of the rocks near Zennor.

He is keeping well over to the right to minimise the effects.

And when he leaves the local effect of the island he makes appreciably faster progress towards his home port.

It’s a matter of scale.

The sea is doing its thing on a vast scale – slopping around the planet under the firm but distant control of the moon and the sun and the vagaries of the weather.

For the most part, we see it locally – we watch it, we study it, often we eulogise it (as you will see in the next two posts), but in the end we have no control over it.

The fisherman chose his time according to the tide and the weather.

He could not choose the tide or the weather to suit his time.

5 thoughts on “From Steeple Point – a question of scale

  1. Hello

    I really like the pictures, they do a great job at describing the phenomenon.

    I always found the current caused by the tide here impressive! I noticed it for the first time at the channel markers on the west side of Drake’s island where the flow creates a vibrant company of vortices around the poles as it passes through (Especially when going towards low water).

    The fisherman of course knows better (perhaps by experience) to choose a path that passes closer to one side of the “channel” he is trying to go through. Someone might say that this is because the velocity of the water (that he will have to work against anyway) is lower there due to the effects of friction… In fact it’s a typical textbook flow profile (

    So the fisherman is taking multiple shortcuts…through space…and scores of hydrodynamics notes 😀

  2. Pingback: From Steeple Point – a hatful of wind « bill's boatblog

  3. Pingback: From Steeple Point – a hatful of wind « bill's boatblog

  4. Bill Nice observations Wave action never fails to astound me. I remember quartering seas off bolt head went past us, but “bounced” back off the cliffs to create very confused water, even in settled weather the tides around there demand huge respect!

  5. When I originally commented I clicked the “Notify me when new comments are added” checkbox and
    now each time a comment is added I get four emails with
    the same comment. Is there any way you can remove me from that service?
    Many thanks!

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