A vertical axis wind turbine (1) comprises three vertically extending sails (8) where each sail (8) comprises a strip (80) of substantially constant width. The opposite ends of each sail (8) are longitudinally twisted to have a pitch angle of approximately 90 degrees. The turbine (1) further comprises a vertically extending central core (7) and a vertically extending opening (9) between each sail (9) and the core (7). Also disclosed is an improvement in a vertical axis wind turbine (91) having at least one main blade (94) each of which has a longitudinal extent and a longitudinally extending radially outermost edge (924). The improvement comprises a longitudinally extending auxiliary blade (944) spaced from the main blade (94) to define a venturi inducing gap (99) between the main blade (94) and the auxiliary blade (944) whereby the turbine (91) has a zone of influence which extends radially beyond the maximum radial extent of the blades.