| http://www.w3.org/ns/prov#value | - Thus, starting from rest, and using only the sail, one can only reach speeds in the circle centred at with radius (i.e. the circle in Figure 4); thus one cannot sail against the wind, but one can at least reach speeds of twice the wind speed, at least in principle. (In practice, friction effects of air and water, such as wave making resistance, and the difficulty in forcing the sail to provide pur
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