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.
The speed of one knot is one nautical mile per hour. A nautical mile, in turn, is one minute (a sixtieth of a degree) of latitude, which works out to 1.15 statute miles. Speed in knots is thus 15% faster than the same speed in MPH. Charts are calibrated in degrees and minutes, and therefore also calibrated in nautical miles.
Why
“knots”?
Long ago sailors estimated their location using dead reckoning. (The “dead” has nothing to do with mortality but instead derives from “deduced,” as opposed to from a compass fix, and is marked on chart notes as DED.) From a known position, dead reckoning uses direction and speed to deduce a later position.
Direction was not a problem. It could be read from the compass. Speed, however, had to be measured. At first sailors threw a piece of wood off the bow and noted how long the boat took to pass it. Then, later, they attached to the wood a long spool of line that had a knot tied in at every fathom of its length. Counting knots as the line unspooled while an hourglass ran out gave the speed. In knots. The piece of wood came to be called a log. Recording the speed was said to be logging it.
Boat
speed
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.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.
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 (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
speed
The 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.