The Sea

From Steeple Point – a question of scale

/ Bill / 5 Comments

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.

From Steeple Point – waves (cont.)

/ Bill / 3 Comments

The previous afternoon the weather was similar. . .

a light swell is coming off the Atlantic.

Waves are in motion, just visible, moving towards us across the surface of the water – at speed.

The actual water they pass through is barely changing position, describing small, slow, circular, vertical orbits.

Finally, almost at the last moment, a small section of a wave is cut off  by the rocks-  guided into a gully.

Forced into the tight space, it loses speed as it drags against the sides. But it retains its energy.

Thus gaining height, it trips over itself, breaking on some underwater obstruction.

And now it is the water itself  – (at a ton per cubic metre), that is surfing down the face of the wave, accelerating towards us.

In speeding up, it quickly reaches its end, dashing itself with abandon on the Breakwater – while, a few seconds later, the rest of the wave, still in the freedom of comparatively open water only a few feet away, rolls sedately up the stones.

And here, it is lifted by the swell above the surrounding sea, only to sluice downhill across the rocks, seeking balance with the main body of water.

From Steeple Point – waves

/ Bill / 3 Comments

Last Saturday, browsing through my favourite second-hand bookshop – Books by the Sea in Bude, I found a book on seamanship by John Russell – The Shell Book of Seamanship, published in 1974.

I already have another book of his – Yachtmaster Offshore, published in 1977 for the RYA  Seamanship Foundation and bought around that time.

Note the publishing dates – before a lot of things that have happened since.

I like his attitude.

From the blurb inside the dust cover: ” . . . ‘safety equipment’ is a misnomer. It is emergency or survival equipment. True safety comes from good seamanship which minimises the incidence of accidents and that is what this book is all about. Examples of true safety equipment . . . are the humble electric fuse, the lifeline and the harness . . . and the pound or two of slush that every one of us carries around for life in his skull.”

It is the slush, of course, that is the problem.

~~~

Chapter 3 is entitled ‘The Sea’ and deals with waves.

Coincidentally, my photos of the weekend included waves and I have put some together with John Russell’s text to see how they fit.

The weather was governed by high pressure.

The sky was blue, with occasional light cloud.

Such wind that there was was north westerly as the Cornish flag on Chapel Rock shows.

The sea was flat – you would not expect any waves other than the residual swell from weather far out in the Atlantic.

And this is what we had – the chance to look at individual waves washing ashore.

~~~

“When the wind stops blowing or changes direction, the sea it caused continues to travel on as a swell . . . Without the energy of the wind to sustain them the waves of a swell gradually decrease in height, but their period and length continue to increase, although at a diminishing rate: thus they become less obvious but move faster as they travel away from the original area.”

“. . . When the wave enters water less deep than half the wave length it begins to feel the interference of the sea bed. Its length decreases without alteration in its period, so it goes more slowly, while its height, after an initial slight decrease, begins to increase rapidly with decreasing depth. This causes the swell to become shorter and steeper . . .”

“When the depth of water falls to one-tenth of the deep-water wave length the increase in height becomes very marked, the progressive deceleration causes crowding with steepening and narrowing of the crests, retardation of the troughs steepens the wave fronts more than their backs and the wave is ready to break at the least provocation. At a depth equal to one-twenty-fifth of the deep-water wave length the relationship between length and period disappears, the wave speed becomes dependent on depth alone and it breaks.”

“For a given speed the energy of a breaking wave depends on how much solid water, as opposed to air and water, it contains, but with sea water at a ton a cubic metre even a modest, well-aerated crest produces  a clout equivalent to collision with a small car.”

“It seldom happens that the fronts of advancing waves are parallel to the bottom contours, so one side reaches the critical depth and begins to slow down before the rest with a result similar to optical refraction. Refraction causes the swells to swing round and align themselves with the bottom contours.”

“Waves frequently cross and even when they travel in the same direction their different characteristics blend to give results that do not appear in either system alone. When the difference in length is pronounced, as commonly occurs when  a sea is meeting or being overtaken by an old swell, the two component waves retain their identities. But when waves of only slightly different period and length combine they produce groups of noticeably higher waves interspersed at intervals with groups of remarkably lower ones as the component waves move in and out of phase,”

~~~

It is worth concentrating on the extracts above. Even if not written in customary blog language, they are a very good description of an often ill-described phenomenon.

You might ask why a book on seamanship should include a section on waves. Well . . . that’s where the ‘slush’ comes in.

“

For love of a boat – reflection

/ Bill / 1 Comment

Following on from yesterday’s post . . .

. . . . . . . . . . .  For the origins and full set of images in this series, here

“There’s not much sail in that collection” – Ed.

No, there’s not. There’s something else I’m  . . .

“A revival of working boats, locally-built, local materials, ‘close-to-the-sea’, preferably under sail? – Sounds pretty specialist, limitist, elitist to me.” – Ed.

No, listen, I’ve been trying to . . .

“And some of those boats look pretty badly kept. If their owners don’t look after them, why should we care?” – Ed

Listen, will you? Just listen!

I have been recording the boats for lots of reasons – (not the least being that I enjoy doing it).

For me, they reflect two things – the people who built them and the places where they were built.

When I was young, there was a song we used to sing along to. It had a verse:

“And they were all built out of ticky-tacky and they were all built just the same.”

Well, there’s plenty of ticky-tacky still around, and not only are things being built the same we are now being ‘encouraged’ to think the same.

It’s not so much about tradition, or being tied to certain materials, or blessed with certain skills (although that all comes into it).

It’s about people who set out to build boats that achieve beauty through a combination of their function, their structure and the knowledge, attitudes and skills that went into their construction.

No, they are not necessarily classical, nor tidy, nor showy, they merely reflect the lives of those immediately around them – about as far from ticky-tackiness and sameness as you can get.

That’s my take on it, anyway.

“Oh, really. What’s for tea?” – Ed.


For love of a boat – The St Ives Jumbo Association

/ Bill / 2 Comments

I have added a link to the St Ives Jumbo Association.

How I missed the Boats in the Bay events I don’t know – (head down – working I should think).

Anyway, this must satisfy just about every aspect of boats that the “Love of a boat” column represents:

  • A revival of working boats,
  • ‘Locally-built’, local materials,
  • ‘Close-to-the-sea’,
  • Preferably under sail,
  • And Cornish to round it off.

Fair winds to them.

(Sit back and enjoy Alban Roinard’s video)

We shall be in St Ives next month for a weekend. I hope to follow this up with a photo at least.

For the origins and full set of images in this series, here

South Devon sunrise

/ Bill / 1 Comment

There are a few occasions in the year when my journey to work coincides with sunrise.

from Labrador Bay, 1st December 2009

These are the ships I mentioned in a post from Southwold last September – still there, still waiting for trade.

I have learnt more about them since, in particular the concern they have created in some quarters – here.

~~~

Fifteen minutes later, five miles further on, the sun higher, the perspective lower, the same ships . . .

from Meadfoot Beach, 1st December 2009

On sailing a Folksong – of dolphins and speed

/ Bill / 2 Comments

AA has come up with an answer to my ‘the speed of dolphins’ question via this link.

Under the title “Dolphins swim so fast it hurts” the author reports:

“What is the fastest a dolphin can swim? Near the surface, no more than 54 kilometres per hour. Why? Because it hurts it to swim faster.Those are the findings of a pair of researchers from the Israel Institute of Technology in Haifa.  But tuna, they say, do not suffer the same problem. Gil Iosilevskii and Danny Weihs carried out a series of calculations to model the tail and fins of fish such as tuna and mackerel, and cetaceans such as dolphins. The aim was to determine what limits the maximum speed at which these creatures can swim. The researchers found that although muscle power is the limiting factor for small fish, this is not the case for larger and more powerful swimmers such as tuna and dolphins. . . .”

Citing cavitation – (the same problem that causes erosion in propellers), as the painful limiting factor, they give 10-15 metres per second (36-54 kilometres per hour) as a maximum.

~~~

So how does this tie in with man’s maximum speed on water without an engine?

For that, you have to look at Hydroptere achieving 51.3 knots over 500 metres

It seems they built an aeroplane and then found a way of gluing it to the surface of the water.

By the way, if you are a wooden-boat person, don’t for a moment think that boat-builders haven’t for ever been constantly developing their skills and technology to improve the speed and/or capacity of their craft, especially where commerce or glory were involved.

It’s not for nothing that the organisers of class-racing have had to place limits on boat specifications to make racing fairer – and don’t for a moment think that individual racers aren’t for ever looking for ways to quietly (very, very quietly) improve the performance of their own boats.

Hopefully, technology will come out of Hydroptere that will filter down to the rest of us.

(And let’s hope they continue to sail where there’s no traffic).

~~~

Which brings me to Blue Mistrss and a more prosaic rate of travel!

When the Folksong were built, one of the accepted methods of calculating maximum boat speed was as follows:

“The speed that a yacht’s hull can be made to travel through water is related to waterline length.

The formula for an average sea-going yacht of conventional shape is:

Speed in knots = 1.4 x Square root of the L.W.L. in feet

The multiplier is altered according to the type of hull. It may range from 1.25 for a tubby hull to 1.5  for a large racing yacht.”

Therefore Blue Mistress’ theoretical maximum speed at L.W.L 19’ 8”: (I have made no allowance for hull shape)

= 1.4 x square root of 19.66 ft = 1.4 x 4.434 = 6.2 knots

I guess there are several other calculations now, but that was then.

The maximum speed (recorded on my handheld gps) on last Sunday’s sail was 6.8 knots.

The best ever is 10.4 knots, remembering that this is speed-over-the-ground rather than speed-through-the-water, i.e. there was an element of tide in the speed recorded – and in the case of 10.4 knots it was a spring tide plus surfing that helped, which makes it even slower than Hydroptere where, presumably, for their record to stand, the water was slack.

Oh, and also not forgetting that my numbers would have to be achieved for a mere nano-second to satisfy the gps, not a timed distance over 500 metres!

~~~

But there’s one distinct advantage for Blue Mistress here  – I bet Hydroptere’s crew didn’t have time for the dolphins.