There are many examples (levers, pulleys, gears, etc) where the output speed can be greater than the input speed and yet energy is conserved. It is about energy not speed. The thing to look at here is the total energy imparted to the sail by the wind, and how much speed can be obtained using that energy. Imagine two vehicles of equal weight, but one with a much larger sail than the first, and the first is able to go with the speed of the wind. The second vehicle, capturing moar energy, should be able to go faster than the first. Doing so then becomes a problem of mechanics.
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There are many examples (levers, pulleys, gears, etc) where the output speed can be greater than the input speed and yet energy is conserved. It is about energy not speed. The thing to look at here is the total energy imparted to the sail by the wind, and how much speed can be obtained using that energy. Imagine two vehicles of equal weight, but one with a much larger sail than the first, and the first is able to go with the speed of the wind. The second vehicle, capturing moar energy, should be able to go faster than the first. Doing so then becomes a problem of mechanics.