Description: In appearance and use, a pelorus resembles a compass, with sighting vanes or a sighting telescope attached, but it has no magnets or independent directive properties. That is, it remains at any relative direction to which it is set. It is generally used by setting 000° at the lubber's line. Relative bearings are then observed. They can be converted to bearings true, magnetic, grid, etc., by adding the appropriate heading. The direct use of relative bearings is sometimes of value. A pelorus is useful, for instance, in determining the moment at which an aid to navigation is broad on the beam. It is also useful in measuring pairs of relative bearings which can be used to determine distance off and distance abeam of a navigational aid, or to determine whether a nearby vessel’s course is closing.
If the dial is turned so that its reading opposite the lubber’s line is the same as the reading of the ship’s compass, bearings read from the dial will be identical to those read from the compass. Similarly, if the true heading is set at the lubber's line, true bearings are observed directly. However, the vessel must be on the heading to which the pelorus is set if accurate results are to be obtained, or else a correction must be applied to the observed results. Perhaps the easiest way of avoiding error is to have the steersman indicate when the vessel is on course. This is usually done by calling out "mark, mark, mark" as long as the vessel is within a specified fraction of a degree of the desired heading. The observer, who is watching a distant object across the pelorus, selects an instant when the vessel is steady and is on course. An alternative method is to have the observer call out "mark" when the relative bearing is steady, and the steersman note the heading. If the compass is swinging at the moment of observation, the observation should be rejected. The accuracy of bearings taken by a pelorus depends on how accurately the instrument is aligned with the keel and whether or not the ship is on the course to which the pelorus is set.
A portable pelorus, also called a “hand bearing compass” , may be moved to any convenient location for sighting, provided that care is taken to position its lubber’s line properly.