Pelorus (instrument)

In marine navigation, a pelorus is a reference tool for maintaining bearing of a vessel at sea.

It is a "simplified compass" without a directive element, suitably mounted and provided with vanes to permit observation of relative bearings.

This regularity simplified navigation for the Polynesians using a pelorus, or dummy compass: [Arabs] divided the horizon into 32 points.

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.

The number of degrees between the desired and actual headings is added if the vessel is to the right of the course, and subtracted if to the left.