From Steeple Point – waves

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.

3 thoughts on “From Steeple Point – waves

  1. Hello

    Due to the weather conditions we mainly stayed into the Plym River today at times even going up to the entrance to Hooey lake where i could catch a glimpse of the Blue Mistress. But mostly, it was White (thick) Mist out there…since Wednesday 🙂

    Speaking of waves…Perhaps you would be interested in Tony Butt’s Surf Science book. An excellent book which caught my interest and also gives an overview of the intricate ways that systems in nature are coupled and operate.

    (The latest addition to the For Love Of A Boat series was excellent 🙂 )

  2. Pingback: From Steeple Point – a question of scale « bill’s boatblog

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

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