Navigation and Seamanship

2.2.8. Pelorus (Dumb compass)

          A clear view in all directions may beunobtainable from the compass. Pelorus or dumb compass are used for thispurpose. The pelorus is a portable instrument additional to the compass, but insome boats a pelorus may be fixed in a suitable position giving more or lessall round visibility. It consists of a circular brass plate graduated like acompass card. This plate is capable of being rotated by hand within fixed ring,which carries lubber line. Outside the fixed ring a set of sight vanes areprovided which can also be rotated by hand. The sight vane is provided with areflecting mirror and shades for taking bearings of right objects. Thisinstrument is mounted in such a way that it remains horizontal in spite of therolling and pitching of the ship.It is used on board for taking bearings ofobjects which are not visible from the compass position and for findingdeviation and correcting them while swinging the ship for compass adjustment.To take a bearing with pelorus mount it in a convenient position on the wing ofthe bridge and align its lubber live parallel to the fore and aft line of theship.

Use of Pelorus

It is a circular metal plate, graduatedlike a compass card and mounted on a vertical axis so that it can be rotatedfreely by hand and can be clamped into any position required. It has a lubberline marked on the plate and sight vanes (front sight and back sight). Theplate is turned by hand so that the lubber line indicates the same direction asthe boat’s compass, and it is used mainly to obtain bearings of objects thatmay not be visible from the position of the boat’s compass and it would alsosave turning the boat’s head to the object as described earlier. In taking abearing by pelorus the sight vane is moved until the object is visible throughthe back sight aperture and is in line with the Fore sight cord. The bearing isread from a pointer at the foot of the Fore sight.