Friday 12 March 2010

Katabatic Wind and Catenary Pull Knowledge for Sailboat Voyage Planners

The English language is littered with nautical expressions and words. Most of them originate from the wood and sail days of the British Navy, two and three hundred years ago. Expressions such as 'toe the line', 'tarred with the same brush', 'three sheets to the wind', and the most famous of all, 'splice the mainbrace', along with many many more are very much alive and well and part of everyday language today. This is a whole separate subject in itself and many sites can be found on the internet devoted to this phenomenon.

Two phrases which are more contemporary, roll wonderfully of the tongue and are relevent today as part of a cruising sailors store of knowledge are 'katabatic wind' and 'catenary pull'. Say these several times and you will see what I mean.

The following is a short extract from my ebook 'Voyage of the Little Ship 'Tere Moana':

'A thunderous thump, preceded by a low howling whistle abruptly wakes the crew from their slumbers. She realises it is the first of many wind gusts attacking her out of the long valley, but is surprised at the ferocity of it. She reels under the assault, heels over and carves quite an arc around her anchor point.

These night katabatic gusts are created by the cooler air higher up the mountain rolling down the valley, gathering speed and strength as it gushes downward, flowing over the lower slopes and out to sea, where it is finally spent. Anything in its path prior to that point receives a good hammering. Hence the noise on impact and subsequent discomfort. She is a little affronted and feels that it doesn’t need to be quite that violent.

The captain comes topside to check the anchor is still holding. Slipping back into his bunk he adjusts his hatch to control the airflow into the aft cabin. Subsequent gusts arrive intermittently throughout the night, but none seem to have the intensity of the first one. Maybe they become used to them?

Morning comes up all dewy with a fine gauze like haze. Bathing in the early morning sun, the errant valley beams innocently back at them, looking blameless and gentle and quite incapable of handing out the buffeting they had received during the night.

Heading for shore to explore, our crew run the dinghy up onto the gently sloping beach..........'

Katabatic wind:
While the land breeze is rather gentle, not so the katabatic wind.
It doesn't believe in sweet dreams.

After a warm, windless and cloud-free summer day, the katabatic wind will blow in strong gusts. The cooler air from above will flow down the deep valleys and rush out to sea.

This local wind, which tends to spoil the most pleasant nights, can easily reach 25 or even 30 knots. It blows from late evening until sunrise. Kata means "downward

Extract and images courtesy 'Expedition Sail'

More from 'Voyage of the Little Ship 'Tere Moana':

'Quietly cruising along the shoreline, this time using the inside channel, her intention is to anchor for the night off the famous Hotel Taharaa, nestled in a small bay midway back to Papeete Port.
Slipping around the headland in the now darkness, the bay opens up revealing a stream of lights tumbling down the cliff face. This is the hotel and it is
interestingly built using the slope of the land.

Fifteen fathoms of chain rode rattling into the black water announces her arrival. Reversing hard into the draw of the anchor digging in, she pulls up short and knows the anchor is set. Motor off, she lets the catenary pull of the rode drift her forward again, with the chain making a satisfying bubbling hiss as it sinks back into the inky water. She rocks gently back and forth, eventually coming to rest, her bow rounding into what little breeze remains from the day.

Dinner that night is served in the cockpit.........'

Catenary curve/pull:
Catenary is a word which describes the mathematical nature of a certain curve, comparable with other terms such as parabola and hyperbola. While the path of a football kicked into the air is a parabola, a catenary is seen in the real world when a flexible line hangs between two points. It is the effect of gravity, a uniformly directional force, acting along its length. A typical example is the chain and rope connecting your anchor to your boat.

This catenary has the convenient effect of lowering the effective angle of pull on the anchor, which is the positive result we are striving for. Clearly, the heavier the rode, the better this effect, and the greater the pull will need to be to negate it (i.e. to pull the rode straight). Hence, the lore is to use heavy chain behind the anchor.

Extract and images courtesy Rocna Knowledge Base

You can read more about these and other weather phenomena in my ebook 'Voyage of the Little Ship 'Tere Moana' downloadable from my website

Monday 8 March 2010

Fibreglass Repair Work for Sailboats

Several years ago I bumped into some coral in the dark whilst attempting to park one New Years' Eve in the very crowded anchorage at Mustique in the 'Grenadines' of the Caribbean Windward Islands. My yacht drew 2.05 metres and in the process of navigating our way throught the one hundred other sailboats at anchor for the New Year celebrations, the forward bottom edge of the keel hit some coral.

Fortunately we were feeling our way in at not much more than one knot, but the impact and resulting grinding sound sent the heart rate skyrocketing. I knew that the keel was solidly constructed and we had nothing to worry about, but also knew that on return to Rodney Bay Marina, she would have to be lifted and the damage repaired.

Out of the water we inspected the damage and having time on my hands I elected to tackle this task myself. In the event it was relatively easy with the main difficulty being fairing down the cured glass to exactly the right shape. With an electric grinder and fine sander and a good dollop of patience this was achieved so that the finished job, once antifouled again, was indistinguishable from the original. Gazing at the finished job with great satisfaction I came to the conclusion that a lot of these jobs that we automatically call in the experts for, could on many occasions, be achieved by ourselves. From that moment on I resolved to follow this path in the future with, in the main, very satisfactory results.

A p.s to that Mustique trip was that some weeks later visiting there again after all the New Year revellers had departed we happened into the Cotton House bar which was virtually empty except for two figures at the dim far end of the room. Who were they? without dropping any names, they were two very famous band leaders who own property on the island.

Getting back to the subject of fibreglassing, I have come across a very thoughtful book on the subject and written by Roger Marshall.

The following is an extract from 'yachting' section:

'Fibreglass repairing - not as difficult as we thought' .
Doing your own fibreglassing has in the past an exercise that many an otherwise handy yacht owner has shied away from.

However, a new publication just might contain all you need to know to carry out a repair to that latest fibreglass damage on your boat, instead of calling in the professionals.

The cost of the book may be recoverable hundreds of times over if you have the time, the inclination and the bravery to have a go.

Fiberglass Boat Repairs Illustrated, available at some chandleries and bookstores but also online this month, is a comprehensive guide to making repairs to a fibreglass boat and how to finish and paint those repairs.

Written by Roger Marshall, winner of numerous awards for marine technical and magazine writing and author of 14 nautical books, the nine chapters of Fiberglass Boat Repairs Illustrated include:

Fiberglass Boat Repairs Illustrated - all you need to know:
1. How a fibreglass boat is built, identifying hull damage
2. materials, tools and basic techniques; gelcoat restoration;
3. Making minor repairs
4. Making major repairs
5. Hull, keel and rudder fairing
6. Identifying and making osmosis repairs and
7. Finishing and painting a repair job.

Then there is an appendix on building a temporary Shrink WrapT shed in which to do boat repair work year-round.

With more than 200 pictures and drawings, the book shows repair projects as done by the author and other professional and amateur boat builders, from simply polishing the gelcoat or repairing a ding in the paint work, to much larger projects such as making a transom well guard to keep water from flooding over the transom.

The most ambitious project is a complete hull and keel reconstruction on a boat that went aground and was seriously damaged.

Fiberglass Boat Repairs Illustrated is published by International Marine/McGraw-Hill. It's a paperback of some 192 pages, and it sells for just US $24.95 - not bad if it will save you the cost of those expensive professionals.

Extract courtesy

You can read more about hull repair and glassing whilst cruising in my ebook 'Voyage of the Little Ship 'Tere Moana' downloadable from my website

Tuesday 2 March 2010

Concordia Sinking by Microburst Sad Loss for Trainee Sailors

The recent sinking off the Brazil coast of the Canadian training vessel 'Concordia' is a sad and shocking loss, not only to the sail training world and all sailors everywhere, but also to the 'Tall Ships' organisations around the world.

She was a beautiful example of a sailing ship and under full sail evoked the high romance of the 'age of sail'.

Fortunately no lives were lost which is a testament to the excellent ' at sea' drills in which the crew and students had been trained.

Obviously in time a full investigation and report into the incident will be forthcoming, but in the meantime here is an extract from the first interview of Captain William Curry:

"The ship's captain, William Curry, has said although the Concordia's crew had prepared the day before for what they anticipated would be rough weather, the ship suddenly keeled.

When it keeled again the ship's sails were exposed to the powerful wind and within 15 seconds the boat was lying on its side and began to sink. The captain said it slipped beneath the waves 30 minutes later.

Curry called it a miracle that everyone on board made it into rafts and survived after the Concordia apparently experienced a weather phenomenon known as a "microburst" - a sudden, violent downdraft of wind - that instantly crippled the vessel Wednesday.

Sawyer said he hadn't heard of the phenomenon before, but that investigators would get expert opinion on it."

Extract courtesy Winnipeg Free Press

This term of 'microburst' in relation to weather was a new one on me as well, so I thought we would take a look at this weather phenomenon.
Anyone who has an interest in flying would know what 'wind shear' is and it would appear that a 'microburst' is the event that would cause it.
The following item comes from wikipedia:
The term was defined by senior weather expert Tetsuya Theodore Fujita as affecting an area 4 km (2.5 mi) in diameter or less, distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas. Fujita also coined the term macroburst for downbursts larger than 4 km (2.5 mi), a scale of size known as the mesoscale.

A distinction can be made between a wet microburst which consists of precipitation and a dry microburst which consists of 'virga'. They generally are formed by precipitation-cooled air rushing to the surface, but they perhaps also could be powered from the high speed winds of the jet stream deflected to the surface in a thunderstorm (see downburst).

Microbursts are recognized as capable of generating wind speeds higher than 75 m/s (168 mph; 270 km/h).

Dry microburst:
When rain falls below cloud base or is mixed with dry air, it begins to evaporate and this evaporation process cools the air. The cool air descends and accelerates as it approaches the ground. When the cool air approaches the ground, it spreads out in all directions and this divergence of the wind is the signature of the microburst. High winds spread out in this type of pattern showing little or no curvature are known as straight-line winds.

Dry microbursts, produced by high based thunderstorms that generate little surface rainfall, occur in environments characterized by a thermodynamic profile exhibiting an inverted-V at thermal and moisture profile, as viewed on a Skew-T log-P thermodynamic diagram. (Wakimoto, 1985) developed a conceptual model (over the High Plains of the United States) of a dry microburst environment that comprised three important variables: mid-level moisture, a deep and dry adiabatic lapse rate in the sub-cloud layer, and low surface relative humidity.

Wet microburst:
Wet microbursts are downbursts accompanied by significant precipitation at the surface which are warmer than their environment (Wakimoto, 1998). These downbursts rely more on the drag of precipitation for downward acceleration of parcels than negative buoyancy which tend to drive "dry" microbursts. As a result, higher mixing ratios are necessary for these downbursts to form (hence the name "wet" microbursts). Melting of ice, particularly hail, appears to play an important role in downburst formation (Wakimoto and Bringi, 1988), especially in the lowest one kilometer above ground level (Proctor, 1989). These factors, among others, make forecasting wet microbursts a difficult task.

Extract courtesy Wikipedia

So, you can imagine then the Concordia sailing along under reduced sail - she was sailing under topsails only - in moderate conditions and suddenly being hit by this microburst, heeling over, the downdraft filling her topsails and pushing the vessel further over until she was in a full knockdown situation. With the mast and topsails in the water and being held down, the sails would immediately fill with water, adding to the overall inertia. From there, the stricken ship could not right herself, and sank very quickly.

It must have been a sad and shocking sight for the crew and trainees watching their home of a few minutes before slipping from sight so rapidlly.

There will be many lessons learnt from this incident and the final report will be anticipated with great interest - I only trust it does not develop into a witch hunt into the captain and crew, who, in the event were responsible for a miraculous saving of the lives of all souls on board.

Finally, the good news is that a sister ship will be built to continue the training tradition of the Class Afloat sail training school in Lunenberg, Nova Scotia.

You can read more about unusual weather phenomena and heavy weather sailing in my ebook 'Voyage of the Little Ship 'Tere Moana' downloadable from my website