True and apparent wind
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True wind and apparent wind are terms from seafaring .
True wind
In seafaring, real wind is the direction and wind speed of the meteorological wind , as measured by a coastal measuring station, for example. In shipping, the wind direction is indicated with cardinal points of the compass. The wind speed is given either in Beaufort or in meters per second. Modern wind measurement systems on sailing ships can approximately calculate the true wind direction and speed if they are connected to a GPS source that indicates the course and speed of the ship over the ground.
Airstream
The head wind is the "head wind" caused by the movement of a vehicle (driving). It is therefore as fast as the speed of movement of the vehicle and the direction of movement of the vehicle (by 180 °) in the opposite direction. This case can be observed when there is no wind. The two vectors of airflow and apparent wind are the same in direction and size when there is no wind.
Apparent wind
The apparent wind (also called relative wind or on- board wind ) is important in shipping and especially when sailing . It is the wind perceived on the moving ship, which results from the interaction (the vectorial addition) of the true wind and the airstream . The apparent wind always comes in earlier than the true wind, unless you are driving exactly in front of the wind (which means that both come from behind).
Sail-powered vehicles always use the apparent wind. On a sailing ship , the position of the sails also depends on the apparent wind.
The faster a sailing boat goes, the more forward the apparent wind comes in, the closer the pods have to be fetched. The maximum height of the wind which can sail a boat depends on the smallest angle from the apparent wind generated on the still advancing. The faster the boat sails, the greater the angle to the true wind. Very fast boats (e.g. catamarans or dinghies) therefore often have a worse turning angle than otherwise rather clumsy heavy yachts.
But the apparent wind can also be experienced on a bike . If you drive in the direction in which the wind is blowing and are just as fast as the wind is blowing, then the air "appears to be", although you can see the wind blowing through the trees (airflow and real wind are vectorially exactly against each other and neutralize each other).
From a physical point of view, apparent wind is the movement of air that you notice when you choose the rest system of the respective vehicle as the reference system .
literature
- Seamanship. Yachting manual. 30th edition, Delius Klasing Verlag , Bielefeld 2013, ISBN 978-3-7688-3248-9 .
- Harbord, Davis J .: Seafaring AZ. Munich: F. Schneider, 1987, ISBN 3-505-09664-4
Web links
- Hannes Preiss: The true and the apparent wind. 2001, archived from the original on January 9, 2015 ; accessed on February 5, 2017 . (from: Hannes Preiss: . The physics of sailing in 2001, archived from the original on 21 December 2009 ; accessed on 5 February 2017 . )