Wind is the velocity of air. Like any velocity, wind is a vector: It has a magnitude, or speed, expressed in nautical miles per hour, called knots, and it also has a direction, expressed in degrees or via points of the compass. Unlike every other vector that comes to mind, however, wind is a negative vector. Its direction expresses, not which way the air is going, but rather, which way the air is coming from. An east wind therefore moves air toward the west.
We know from Einstein that velocity is always relative to a frame of reference. For a weatherman, wind is relative to the solid ground, which has zero velocity, and this is called true wind. For a sailor, who scoffs at solid ground, wind is always relative to the boat. This is called apparent wind: the velocity of air as seen and experienced by the boat and all aboard her. It is apparent wind that moves the boat, not true wind.
In physics, relative velocity is the difference between two vectors. The velocity of A relative to B is the vector velocity of B subtracted from the vector velocity of A. Because wind is a negative vector, however, we add it to boat velocity instead of subtracting. Apparent wind is thus the vector sum of true wind velocity and boat velocity. A boat motoring at 5 knots north into a 10-knot north wind sees apparent wind of 15 knots north. A boat moving south at 5 knots in that same 10-knot N wind experiences apparent wind of only 5 knots N. A boat moving west at 5 knots in a 10-knot N wind sees apparent wind of 11 knots (square root of 125) from the north by northwest (NNW or 330°). If that boat now speeds up to 10 knots westward, the apparent wind that it sees will increase to 14 knots NW (square root of 200, WNW or 315°). Apparent wind can thus be significantly higher or lower than true wind, depending on direction of travel. Since it is apparent wind that moves the boat, direction makes a big difference in boat speed.
Hull
speedThe other constraint on boat speed, at least for most sailboats, is length (at the waterline). Fast powerboats have planing hulls whose bow rises up out of the water and skims atop the surface when the boat is on plane at high speed. Sailboats, trawlers, and large ships usually have displacement hulls that remain always below the surface, displacing a mass of water equal to the weight of the boat. As a displacement hull gains speed, it creates ever bigger bow waves that push it back. Eventually it reaches a maximum speed, called hull speed, which in knots is about 1.4 times the square root of the waterline length in feet. That would be about 7.5 knots for a 30-ft boat, 8.8 knots at 40 ft, or close to 10 knots at 50 ft of waterline length. Some boats, such as catamarans, have higher hull speed than others of equal length.