Staying the right way up: wind and knots

I have just downloaded the US Sailing Directions to Antarctica, a weighty tome full of dark warnings. The British would call it the Pilot Book, the detailed description of the coastline and sailing conditions of the area. (The Admiralty Pilot is not available electronically and costs at least £60.)  In addition to general comments on cold, wind, icebergs, currents and magnetic variations, The Pilot says of the Antarctic Peninsula: 

Storms, high pressure systems, and local topography produce variable, and often strong, winds throughout the region. …From November through May, wind speed averages range from 5 to 15 knots while gales blow on about 1 to 8 days per month. January and February are usually the quietest months.

I will be there in March, when 5-15 knots sounds ok (though Europa isn't sailing anywhere is a 5 knot zephyr). Except for all those gale days. Ho hum. A gale is usually defined as Force 8 on the Beaufort Scale, when the wind is blowing at 35-44 knots. In open water this creates a ‘very rough to high’ sea-state, with waves not expected to be more than 7.5 meters high- a wee bit bigger than the height of average two-storey home. (I’ve put an explanation of knots and related topics at the end of this post, as several nearly-nerds have asked me what the measurement means.)

The detailed descriptions of sea-state are often useful to sailors. They can help to estimate wind-speed, but you have to take into account factors like the tide, the fetch (distance over which wind and wave trains have built up) and where land has constricted or blocked the sea. Subject to all that, at Force 8, those waves are expected to be beginning to break at the crests. That white water (properly called spindrift) is blown in streaks along the direction of the wind.

I know some of you who are professional sailors and lifeboat crew will not be too worried about those conditions. I have once or twice been at sea in that sort of weather (though in much smaller boats than Europa) and I’m still here. Of course, this is far from the top of the scale. The wind can get a lot stronger than 44 knots. Also, in general, it is breaking waves which is why sailors obsess about positioning their boat in relation to each individual wave.

Plus of course, there’s Drakes Passage, that narrow bit between the Horn and the Antarctic Peninsula. We will sail south-south-east from the bottom corner of Tierra del Fuego, wander around a bit and then head back out north-east into the Southern Ocean. That is, travelling with the prevailing wind, but in that bit of the sea between the Atlantic, Antarctica and Drake’s Passage. This picture is taken from a great blog about sailing Europa in these waters.  

The thing about the Southern Ocean is that it doesn’t have land to stop the wind and waves piling up. The current swirls, and the tide, and the prevailing westerlies blow and blow, skittering in happy, wild freedom across tens of miles of latitude. Then they get to the narrow bit and of course they spurt and splutter. A bit like the garden hose when you put a kink in it to increase the pressure.

Well, not very like. The Sailing Directions say:

The routes [to Drake’s Passage] that approach Antarctica from the Atlantic Ocean usually encounter the roughest conditions. Winds from SW through NW are responsible for the gales that blow 10 to 20 per cent of the time even in the middle of summer.

Europa is very good at staying the right way up. I’m sure it will all be good.  But when you get blogs which even electronically reek of misery, you will know why.

For the nerds: knots, Beaufort and breaking waves.  A knot is a nautical mile per hour. A nautical mile is 1.15 of a statute mile, at 1852 metres (about 6076 feet for the Americans). So a knot is a bit faster than a mile-per-hour.

So, lands-people say in confusion, why not use the ‘normal’ mile, or even the more common kilometre? For the very beautiful reason that a nautical mile is a function of the size of the planet. At the equator one minute of latitude is 1 nautical mile. (It is 0.06 longer at the Poles, or 111.2m but that need not worry us even at 62° south.) So our unit of distance, rather than being the result of shadowy centurions marching across empire or the arbitrary distinctions of the revolutionary French, is a measure of the circumference of the Earth.

I love that fact. (When writing nautical science fiction of course, it means re-calculating everything so you can measure knots for the planet you have created. That’s a whole other problem.)

Admiral Beaufort devised his scale of wind forces in 1805. He took some earlier work, and it has been revised since, but the basic principle remains as he wrote it down in his notebooks. The UK Meteorological Office have a good history of the scheme. Sailors still use it today.

The turbulence and kinetic energy of breaking waves takes me rapidly into the world of physics, far beyond my comprehension or capacity to explain. An abstract of a 2014 paper in the Journal of Physical Oceanography aims to model the impact of breaking seas in hurricane conditions, explaining the roughness of the sea surface:

dominant breaking waves in the ocean under hurricane-force winds affect the drag and near-surface airflow turbulence [including] wake turbulence produced by dominant-scale breakers. Effects of unresolved roughness [include] short breakers, nonbreaking waves, and sea foam …. Nevertheless, dominant-scale breakers are more fully exposed to high winds and produce more intense wakes individually. As a result, they support a large portion of the total drag and significantly influence the turbulence for many ocean conditions that are likely to occur. The intense wake turbulence is characterized by flow separation, upward bursts of wind, and upward flux of the turbulent kinetic energy (TKE), all of which may influence sea spray dispersion.

I think this means that it’s a lot rougher at the surface than lower down, while waves fall over each other and individually interact with the wind. Lots of kinetic energy is throwing spindrift across the sea, giving it that menacing look of white teeth snapping at your decks and fighting to drag you down.

More practical for sailors  is that bible of cruisers, the book Heavy Weather Sailing. I have the fifth edition published somewhere around 2000. In it Andrew Claughton of the Wolfson Institute at Southampton University states the issue clearly:

‘it is breaking waves that cause capsize; if the yacht is caught beam-on to breaking waves of sufficient size, then the exaggerated steepness of the breaking wave front, couple with the impact of the jet-like torrent of the breaking crest, will knock the yacht down … when the breaking wave is 6m high, this appears to be a capsize certainty in any boat. … big waves in themselves are not the problem.’

That’s the point of seamanship in rough seas: staying the right way up. I am looking forward to learning some of the tricks of managing a square-rigger with enough sea-room to run.

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2 Comments

  1. Oh Sarah, the Drake Passage! Do take your sea sickness pills – I can still feel the mega roll and having to tie the doors of the wardrobe together even 20 years on…..

    Jo

    • I don’t think Europa has wardrobes, as such! I have a pharmacopeia being built up … And it’s only 4 days, right. (Don’t be fooled – I’m dreading the seasickness!)

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