Windward speed

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As Luvgeschwindigkeit is known in sailing , the speed at which a boat a goal that lies exactly in wind direction can achieve. The attainable windward speed (with a given wind) is a good parameter for the cruising characteristics of a ship.

Instead of the German expression, the English abbreviation VMG for Velocity Made Good can be found in many publications .

Since a sailing ship cannot sail directly against the wind, it is necessary to cross in order to reach a destination in the direction of the wind. For a given wind, the windward speed depends on two factors: the course of the ship relative to the wind and the speed on this course. As a rule, only the speed through the water is considered, although in reality the speed over the ground, which can be different due to currents, is actually more interesting if you want to reach a certain destination.

Diagram 1: Comparison of the windward speed of different boat types. The distance covered in constant time to windward is greatest with the catamaran, although it has to cover the longest distance.

The speed of a sailing ship increases the further astern the ( apparent ) wind falls. This is a direct result of the physical properties of a sailing ship. The achievable speed can be recorded for a ship type in a so-called polar diagram . The close wind angle for optimal altitude can be seen directly from the polar diagram. This can vary from boat to boat. Diagram 1 shows three different ships that are sailing on their optimal close-hauled course. Boat A is a conventional keel yacht, its optimal angle is typically around 45 °. Boat B is a dinghy, it does not reach such a high angle, as the drift is much greater due to the much smaller lateral plan and the lack of keel weight. In return, the dinghy sails much faster and can therefore achieve an overall larger VMG. Even more extreme is the catamaran C, which has to cover a much longer distance, but still reaches the destination first due to its higher speed through the water. The catamaran is at a disadvantage if there is not enough sea space for the generous cross strokes and it therefore has to turn often. Particularly poor cross properties have the old square rigger : They reached about 67.5 ° (6 line on the true wind) on account of their "box-shaped" hull coupled with large drift .

The current windward speed can be calculated mathematically as

with = angle to the true wind.

An example yacht reaches a speed of 4 knots at 55 ° in the true wind, the windward speed is then 2.3 knots. At 60 ° upwind it reaches 5.0 knots, the windward speed is therefore 2.5 knots. At 65 ° the speed through the water increases to 5.2 knots, but the windward speed is lower again at 2.2 knots. So this ship sails optimally at about 60 ° upwind.

Downwind VMG of a hypothetical keel yacht. The red dots show the optimal course for different courses. At a wind speed of 10 kn the ship runs an optimal 6 kn on a course of 150 °, at 20 kn it runs 8 kn, but on a lower course of 174 °

What applies to windward speed also applies to leeward speed ( downwind VMG ), i.e. the achievable speed to leeward. Here, too, due to the aerodynamics of the ship, the maximum speed to the destination is not reached flat in front of the wind ( butterfly sails ), but must be “crossed down” on a course a little higher up the wind. On such courses, modern racing yachts achieve leeward speeds that are a multiple of the wind speed (see Sailing faster than the wind ).

Individual evidence

  1. ↑ Schult, keyword windward speed
  2. ↑ a b Püschl, page 11f

literature