A method and apparatus for plotting a boat's position relative to a rhumb line when tacking. The apparatus includes three independently rotatable disk members. A first disk member presents a compass rose, the second of the disk members presents an indicator that aligns on the compass rose along with a grid, and the third of the disk members presents a rhumb line. The starting and ending positions are plotted on the rhumb line. The rhumb line is then aligned with the compass rose in accordance with the boat's true course. An initial course is set by rotating the indicator with respect to the compass rose on the first disk. The boat's first tack position is located and plotted on the grid by counting grid lines from the starting position. The second disk is then rotated and the next tack position is plotted away from the first tack position. The tack positions are plotted on the third disk to provide the boat's position relative to the rhumb line.