Velocity made good
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Velocity Made Good, or "VMG," is a term in sailing, and specifically yacht racing, that refers to the component of a sailboat's velocity that is in the direction of the true wind. The concept is useful because a sailboat often cannot, or should not, sail directly to a mark to reach it as quickly as possible. Sailboats cannot sail directly upwind; it is usually less than optimal, and can be dangerous to sail directly downwind. Instead of sailing toward the mark, the helmsman chooses a point of sail that optimizes velocity made good.
Boats cannot sail directly into the wind, requiring the sailor to alternate between headings, which are commonly called "tacks." On a tack, the sailor will generally point the sailboat as close to the wind as possible while still keeping the winds blowing through the sails in a manner that provides aerodynamic lift to propel the boat. Then the sailor turns slightly away from the wind to create more forward wind pressure on the sails and better balance the boat, which allows it to move with greater speed, but less directly toward the mark.
Determining the velocity made good usually requires computation and instrumentation.
For example, on a heading of 60 degrees NE, the speed of the boat is 5 knots. Falling off to 65 degrees NE accelerates the boat to 5.2 knots. This data indicates the trade-off between speed and distance. The answer requires basic trigonometry. Determining the northward component of the velocity vector requires taking the cosine of the angle between north and the sailboat's heading.
cos(60) * 5 = 2.50 knots made north (vmg) cos(65) * 5.2 = 2.20 knots made north (vmg)
- The New Glénans Sailing Manual, David & Charles.
- "Ocean Sail Articles: Velocity Made Good Trading off course against speed". Oceansail.co.uk. Retrieved 2013-11-03.