The compasses: gyros, repeaters and the magnetic compass

What you do on the bridge

1. The gyros and the changeover
2. The magnetic compass
3. Check the repeaters
4. A bearing at the wing
5. Gyro one fails
6. Log it

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. Compare the gyro compass and the magnetic compass. Why carry both?Often asked

   A good answer covers:

   • The gyro seeks true north, has a small and steady error, and feeds the autopilot, radar, ECDIS, AIS and repeaters
   • But it needs power, takes hours to settle after starting, and has speed and latitude errors that need correcting
   • The magnetic compass needs no power and cannot fail electrically, so it is the independent back-up
   • But it points to magnetic north, so it carries variation and deviation, and it is disturbed by steel, cargo and electrical equipment
   • SOLAS requires a magnetic compass on every ship covered, and a gyro on ships of 500 GT and over

   Show me on the bridgeThe heading changeover panel and the compass projector at the helm.

2. Explain variation and deviation, and how you go from a compass course to a true course.Often asked

   A good answer covers:

   • Variation: the angle between true and magnetic north, from the chart, changing with place and slowly with time
   • Deviation: the angle between magnetic north and where the compass points, caused by the ship's own magnetism, and changing with heading
   • Compass error is variation and deviation combined
   • Compass to true: apply deviation to get magnetic, then variation to get true; easterly errors are added, westerly subtracted
   • Deviation comes from the deviation card, and is checked by comparison and by azimuth
3. The gyro feeding the autopilot fails. What do you do?Often asked

   A good answer covers:

   • Take hand steering if the autopilot has lost its heading, and steer by the magnetic compass if need be
   • Change the heading source to the other gyro, or the magnetic heading, and check every receiver has it: autopilot, radar, ECDIS, AIS
   • Remember radar stabilisation and ARPA are upset by a heading failure until the new source is in
   • Call the master, log it, and tell the engineers or electro-technical officer
4. Before taking bearings at a wing repeater, what do you check, and why?

   A good answer covers:

   • That the repeater agrees with the master gyro heading: a repeater out of step puts its error into every bearing
   • The known gyro error, so it can be applied to the bearings
   • That the azimuth ring or pelorus turns freely and is level, and you can see the mark clearly
   • Line the repeater up with the master if it has slipped, and report it

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   We're heading west at eight knots on autopilot, late in the morning. Every heading and bearing on this bridge comes from a compass, so let's see where.

   We'll find the gyros and the magnetic compass, check a wing repeater before we use it, take a bearing, and deal with a gyro failure.

2. The gyros and the changeover

   At the helm, open the gyro compasses and heading changeover panel. We have two gyros and a magnetic heading sensor. The one selected feeds the autopilot, the radars, the ECDIS, the AIS and every repeater.

   The heading monitor compares the selected source with another one, and alarms if they drift apart. Gyro one is selected now.

3. The magnetic compass

   The magnetic compass sits on the monkey island above us, and a projector shows its card at the helm. Open it. It needs no power, so it is there when everything else has failed.

   It points to magnetic north, not true. Variation from the chart and deviation from the card turn its reading into a true heading. You will work that out properly in the compass error lesson.

4. Check the repeaters

   Each bridge wing has a gyro repeater with an azimuth ring for bearings. Before you use one, compare it with the master heading. In the changeover panel, look at what each wing shows.

   The port wing is four degrees out: someone left it out of step. Every bearing taken on it would be four degrees wrong. Line it up with the master.

5. A bearing at the wing

   Go to the port wing, key six. Aim the azimuth ring at the sun with the mouse, and press T to take the bearing. It goes in the bearing book. In later lessons the same move takes bearings of lights and headlands for a fix.

6. Gyro one fails

   Any moment now gyro one will fail. When it does, the autopilot loses its heading. In the changeover panel, select gyro two, then acknowledge the alarm.

   Check the autopilot holds the course again, and remember the radars and the ECDIS took the new heading too.

7. Log it

   Write it up in the deck log: the time gyro one failed, and that the heading is now from gyro two. Then tell the master.