A blade assembly for vertical axis windmills (for driving use, eg for propelling boats, or for driven use, eg for wind or tidal flow generators) maintains the blades advancing towards the wind generally streamlined, the blades retreating generally bluff giving greater power outputs for a given swept area. A vertically rotatable main rotor arm 7 fixed to an output shaft 8 may carry eg two blades 1 each rotatable about a vertical axis and having a blade pulley 2. A wind direction following vane 3 is fixed to a vane pulley 4. A toothed belt passes around the pulleys 2,4 and the blade pulleys 2 have twice as many teeth as the main pulley 4 such that the blades 1 rotate at half the angular velocity of the main arm 7 and in the opposite direction to reduce the frontal area of the blades presented to the wind. Protection from extreme wind speeds may be provided by a weighted bell-crank (13) flying outwards under centrifugal force and acting via a sliding collar (11) and rods (12) to turn the blades 1 to the horizontal plane.
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