Page:Popular Science Monthly Volume 89.djvu/65

 Popular Science Monthly

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��Assuming that the hull resist- ance is not affected by different angles of keel, the course at 190^ with the wind is the one in which the boat will make the fastest headway. In the wind tunnel of the laboratory the construction is such that uniformly moving currents of air without swirls in them blow against the object tested. The mast with its sail is set up within the tunnel, and the ef- fect of the wind in the sail is measured. The sail is set at different angles to the direction of the current of air

For courses from 45° to 160° the angle between the boom and center line of ship should be one-half the angle between the fly and center line of ship. In the wind tunnel measure- ments are made at each angle of the direction of the current of air. The thrust and twist are measured, the experiments being repeated many times with different wind velocities. It is the apparent direction of the wind that concerns the sailing man

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