Jocalia’s course computer quirk

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There is a known rare problem with Jocalia’s Course Computer, where it loses its calibration data and the heading symbol points backwards. While this looks disconcerting, it poses no hazard. It’s simply a nuisance and a distraction. First identified in 2014 in Scotland, the problem has resurfaced in 2024.

A brief compass history

Many of us will recall the days when every magnetic compass – on ships and aircraft alike – had to be corrected for two key effects:

  • Variation: the effect of the movement of the Earth’s iron core with the Earth’s crust, which is presently conveniently small in Portsmouth
  • Deviation: the magnetic effect of the boat’s equipment which adds to the Earth’s magnetic field and will change in direction and amount as the boat alters course

Deviation is often minimised by adding magnets and soft iron cores to the compass binnacle. The residual deviation can be found by swinging the boat through 360º and tabulating the results on a compass card.

What the course computer does

The Course Computer replicates this calibration process automatically. It uses internal accelerometers to measure the boat’s rate of turn, integrates this to determine change of heading, and compares that against the fluxgate compass reading. The result is a linearised heading table.

This table is then corrected to true heading using either:

  • GPS course over the ground, or
  • More accurately, by aligning the heading with a known landmark on a true bearing.

All this data is stored in the Course Computer’s memory.

What happens when calibration is lost

If the stored calibration data is lost, the heading display defaults to the fluxgate compass magnetic heading plus 180° – presumably as an indication that there is a fault.

The procedure is shown here (click/tap on the images to view full size):

Calibrating the compass Compass calibration

If conditions don’t allow for a full recalibration – including swinging the boat in Section 2 – you can skip directly to the Heading Alignment step. This will correct the display, but there may still be some heading errors.

Make a note in the Tech Log under all circumstances so that this nuisance behaviour can be monitored.

Incidentally, the Royal Navy once had specialist officers who handled compass calibration and correction. You can probably guess what they were called.