The term ‘airfoil sail’ has only come into use in recent years. As is well known, it refers to a sail that retains its shape, much like an aeroplane wing, in contrast to the flexible canvas sail.
Just as self-supporting aircraft wings no longer require bracing, so too can a profiled sail be designed in such a way that shrouds, stays and the like are no longer necessary. By using a wooden frame covered with canvas for small designs, and by choosing light metal as the construction material for large surfaces, it is possible to ensure that the weight of the sail itself differs only marginally from that of the corresponding canvas sail. The deck-mounted supports, of course, do add to the weight.
The cross-section must be a symmetrical profile in order to allow the sail to be used on both the starboard and port bows. If a streamlined cross-section is chosen, the sail has exceptionally low drag whilst providing good force perpendicular to the axis of symmetry; furthermore, the strength requirements can be easily met with such a shape. The profiled sail utilises the wind considerably better than a square-rigged sail. This is particularly true when sailing close-hauled. It is possible to sail significantly closer to the wind with a profiled sail than with a square-rigged sail, as the latter still heels over at a small angle of attack.
As far as I am aware, the public first heard of profiled sails through Anton Flettner’s lecture to the Society of Naval Architects in November 1924. However, the idea of using profiled sails instead of flat sails must surely have occurred to many a sailor or seaman before that, provided they had taken a keen interest in aerodynamics. For example, the author of these lines mentioned the profiled sail in a paper submitted in May 1921 to the Shipbuilding Department of the Technical University of Berlin on ‘The effect of the wind on sails and the possibility of improving the rigging of large sailing ships on the basis of aerodynamic considerations’. The problem was not addressed in detail in that paper, however, because the author felt that there were nautical and technical reasons against it.
The concerns expressed at the time remain valid today; they must also have occurred to Flettner and prompted him to refrain from using profiled sails on board large ships and to switch to the rotating cylinder. For one thing, the question arises as to whether the rotating mountings for the profiled sails on deck would not be too heavy if they were to withstand all the stresses caused by the ship’s rolling motion and wind pressure. Secondly, the behaviour of the profiled sails in a storm must be examined. There can be no question of dismantling the sails. Making the sails telescopically retractable would be too expensive for a merchant ship. The only option, therefore, is to disengage the sails, that is, to release the mechanism used to set the sails at the required angle to the wind. The sails are then free to move on their pivot and can yield to any change in wind direction. Due to the streamlined cross-section, the drag of the profile – which automatically adjusts itself to the direction of least resistance – is significantly lower than the drag of the old reefed rigging fitted to the same ship — just as the Flettner auxiliary rudder steered the main rudder. Whilst this may theoretically allay fears regarding the behaviour of the profiled sails in a storm, its implementation in practice will nevertheless meet with the greatest resistance. Every sailor will be reluctant to have a rigid sail on board over which he has no control in heavy weather, and which he must instead leave to its own devices so as not to endanger his ship.
In 1922, Flettner had experiments carried out on profiled sails at the Aerodynamic Research Institute in Göttingen. It is reported that an improvement of 50 to 60 per cent was achieved compared with a canvas sail. To increase the efficiency of the symmetrical profile, Flettner designed the rear end of the profile to be rotatable, so that it could be set at a certain angle to the front section. This resulted in an asymmetrical wing similar to the curved wings of aeroplanes, the adjustable rear section of which could be rotated to one side or the other depending on the direction of the wind. The angle of attack relative to the wind was achieved by an auxiliary surface mounted behind the main sail, which controlled the main sail’s angle.
The Germania shipyard in Kiel, which was collaborating with Flettner in the field of new ship rigging, fitted one of its workboats in late summer 1922 with a profiled sail that was designed without an adjustable rear end, but which did feature the aforementioned auxiliary sail for trimming to the wind. The auxiliary sail is adjusted by means of a handwheel so that the freely movable main sail assumes the desired angle to the wind direction. To my knowledge, this profiled sail remains the only one ever built in Germany. Incidentally, on board the vessel is Dipl.-Ing. Corseck, known to readers of ‘Yacht’ for his articles on aerodynamics, who was commissioned at the time by the Germania shipyard to carry out the tests.
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