A Pelorus is used by boats and ships for navigation.

Before gyro repeaters, a Pelorus was often used by the navigator to take bearings to fix the vessel’s position on a chart. The 0 mark is oriented to the centreline of the vessel, and a relative bearing taken by sighting the target using the pin. Then by looking more downward, to where the pin crosses the compass rose, the relative bearing can be read. By asking the helmsperson for the vessel’s head at the same time, a compass bearing to the target can be calculated.

Recording the change in relative bearing of an approaching vessel allows the risk of collision to be determined.

The Pelorus can also be used to determine azimuths to check compass error.

This model is 200mm x 200mm at full scale. The hole is sized for a 1/16th pin. I used a piece of carbon rod from another project. The pin can be stored in a hole at the side, which can then be blocked by a lanyard or if hung from a hook. A Pelorus is normally gimballed, as they need to be level when used. To keep things simple, I haven’t done this but have added a spot for a bullseye level. The cylindrical pin support would allow for the addition of sighting vanes if desired.