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.

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