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The blue illustration is derived directly from the 9 metre Shadow variable geometry boat CAD program, shown in the unfolded configuration for fast and silent sailing. Her control computer is designed to ensure that whatever the strength or direction of a gusty wind the Shadow and her crew are continuously protected against overpowering and capsize. Pulling her control lever back into the astern quadrant will put the air brakes on. More details and another illustration are provided in the Technology section.
The benefits of modern automatic wingsail installations for leisure craft fall into two clearly defined classes. The first is for sailing yachts, which are already relatively ecologically acceptable craft in that their crews try to use the free, clean ocean winds for propulsion for at least 80% to 90% of the time. The traditional cloth and rope rig of such craft, unchanged in principle since the 17th century, brings in its wake certain inherent disadvantages, which yachtsmen have learnt to live with if not necessarily to enjoy, especially when the weather turns heavy.
The cloth sail cannot produce thrust safely and controllably below a certain minimum level, because after the sheets have been eased beyond a certain point the sail will flog, and the next stage must be reefing:- the reduction in area of the sail. This occasionally challenging procedure (especially in adverse weather conditions) is unnecessary in wingsailing, since a modern wingsail can be “throttled down” from 100% to 0% of thrust at the touch of its thrust lever.
Secondly, the boom and mainsheet arrangements of the traditionally rigged yacht mean that gybing, the passage of the wind across the stern from one side to the other, must be accomplished with great care on the part of the crew. This is because it is perfectly possible for the boom swinging across to knock an unsuspecting crewman overboard, and in extreme circumstances an accidental gybe can even bring the whole rig crashing down to the deck.
In modern, fully automatic wingsails the tailboom, when gybing, passes quietly and safely across the centreline of the vessel, so that in many cases passengers aboard a wingsail yacht have not even noticed that a gybe has taken place.
The control computer of a modern wingsail, in addition to arranging for tacking and gybing to proceed uneventfully, keeps a watching brief on windspeed and direction and will not permit excessive windforce (which could overpower or even capsize the yacht) to be applied to the vessel. Thus an inexperienced pilot can sit at the controls in perfect safety with the comforting knowledge that if the thrust control lever is set at 100% and a powerful gust arrives the wingsail will automatically cut that thrust level back to 60%, 40% or 20% or whatever percentage of thrust is safe, and the vessel will continue steadily on its way.
The special characteristics of a Walker-type wingsail are particularly valuable in sailing yachts. The diagram shows the difference between a conventionally rigged trimaran yacht and the same craft re-rigged with a wingsail. It can be seen that whereas the cloth rigged craft is heavily pressed to leeward, and is in danger of capsizing over her lee bow, the wingsail yacht is actually heeling safely to windward. Many experienced sailors have insisted that heeling to windward is impossible. The diagram clearly illustrates the truth. Importantly, at a touch of her thrust lever the wing control tail can be centralised and the thrust reduced to any level preferred by the pilot.
By contrast, the conventional yacht's boom may be jammed hard against her shrouds. Bearing away and turning towards the wind are both relatively challenging alternatives.
Powerboats can be replaced by modern wingsail yachts, retaining their ease of control but eliminating noise, vibration and pollution of both air and sea. Costing less to buy and dramatically less to run, the wingsail yacht can offer speeds of around 20 knots in the right wind conditions.
John Walker wrote an interesting account of the first transatlantic crossing by a Walker wingsail yacht, called “Wings across the water”. To read it, click the link.
Other links:
Commercial Shipping
Environment