Pelorus

Science / Tides and Currents / Pelorus: An instrument formerly used on a vessel in connection with a current line and current pole to obtain the set of the current. In its simplest form, it was a disk about 8 inches in diameter and graduated clockwise for every 5?° or 10?° . It was mounted rigidly on the vessel, usually with the 0?° mark forward and the diameter through this mark parallel with the keel. Bearings were then related to the vessel's compass and converted to true.
Search Google for Pelorus: