Abstract: Apparatus and a method for use in determining the optimum direction of progress of a sailing vessel from one point to a second point are described. The angle of leeway between the direction of heading of the vessel and the course of the vessel is measured and the course of the vessel is set to a required apparent wind direction so as to obtain the sum of the angle , leeway and the angle of the apparent wind direction with respect to the direction of heading. The distance covered by the vessel relative to the apparent wind in a given space of time is measured. Simultaneously the distance covered by the vessel relative to the water in said space of time is measured. The quotient of the measured water distance divided by the measured air distance is obtained, and a chart relating the said sum and the said quotient is used to give a figure of merit for the course. Three separate charts for use respectively in "reaching," "beating" and "running" are provided.