A sailboat racing calculator which is mounted on a sailboat adjacent a compass. A compass dial marked with compass bearings is coaxially rotatably supported below a reference disc having windows for viewing the compass markings on the bearing dial. A pair of starboard and port tack arms are pivotally mounted relative to the compass dial and reference disc. The dial and the tack arms are rotatable relative to one another and the disc to permit the sailor to utilize his compass readings as a guide for planning an optimum racing course.
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