Sailing with the wind is surely the oldest and simplest type of sailing. In its primitive form, it is hardly sailing at all. It is logical to wonder how fast a boat can go when sailing downwind. Physics gives clues about how to make faster sailing craft for both upwind and downwind sailing. The starting point is a general discussion of speed. Sailboat speeds are determined by the wind’s force and water’s opposing force. A typical sailboat’s speed is comparable to the wind speed, so calms produce bored and frustrated sailors, while a really strong wind results in panic. A knot is the historical nautical speed unit. In the “old days” (perhaps all the way back to the Netherlands in the 1500s), a series of knots were tied on a rope, separated by about 15 m. (A meter is a little more than a yard.) The number of knots paid out to a fixed point in the water in about 30 s gave the boat’s speed, U, in knots. Wind speeds, W, are also often quoted in knots. One can use knots, kilometers per hour, miles per hour, furlongs per fortnight, or any other unit to characterize speed. The quaint units of the American system and other historical oddities can be avoided by always specifying speed in meters per second. It is easy to estimate boat speed in meters/second using an analogy of the old knots measurement. If a man overboard at the bow of a boat can be hauled up at the stern after 10 s, and if the boat is 10 m long, then you know the boat speed is 1 m/s. Desirable wind speeds (for sailors) are similar to the speeds of human locomotion. Normal walking at a little more than 1 m/s corresponds to a minimum usable wind for sailing. The world’s fastest sprinters struggle mightily to obtain the Fresh Breeze speed of 10 m/s. Ice boats are a special case. Ice boat speeds of 30 m/s or more, corresponding to automobile highway speeds, can be obtained. For simple calculations, Fresh Breeze will be taken as 10 m/s and Gentle Breeze (3 on the Beaufort scale) to be half that speed, or 5 m/s.
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