Intermittent Deviation and a Bubble in the Bowl
Alongside in a cold climate, the master of a 24 m steel pilot boat reports that the steering compass card is sluggish and a bubble about 8 mm across has appeared in the liquid. The compass was last swung eight months ago; the card then showed residuals within 2°. During a trial, the compass reads steadily on 045°C while the vessel is secured, but as soon as the starboard windscreen wiper is switched on the card swings about 6° and returns when the wiper stops. A new 12 V loudspeaker has been installed 0.8 m from the binnacle, and a steel toolbox is sometimes stowed in the console below. The owner wants to top up the bowl with distilled water and go to sea, saying the wiper effect is ‘only temporary’. You must decide whether the compass is serviceable, what causes the swing, and what must be done before the next deviation swing.
1. Assess the compass’s serviceability. What do the bubble and sluggish card indicate, and may the bowl be topped up with distilled water?
Read the worked answer
The compass is not serviceable as a navigation compass. An 8 mm bubble means air has entered the bowl, usually through a leaking seal or filler; the card is sluggish because the damping fluid is low, contaminated, or the pivot is worn. Topping up with distilled water is not acceptable: most marine compass bowls use a specified alcohol/glycol or refined hydrocarbon fluid, and water changes density and damping and can promote corrosion or freezing. The bowl should be drained, leak-tested, refilled with the approved fluid, and the pivot/card checked by a compass technician. Until then the compass may show a steady reading alongside, but its dynamic accuracy and repeatability are not reliable.
2. Diagnose the 6° swing when the wiper runs. What checks and remedies are required before adjustment?
Read the worked answer
The 6° swing that appears only when the wiper runs is a transient magnetic field from the wiper motor or its wiring, not a correctable ship’s deviation. The new loudspeaker 0.8 m from the binnacle is another suspect, as is the steel toolbox in the console. Test by running each electrical item separately on cardinal headings, then remove/relocate the speaker and toolbox, and route or shield the wiper supply. A corrector magnet cannot cancel a field that switches on and off; if it did, it would introduce an error when the wiper is off. The compass must be clean of such transient fields before any swing.
3. List the actions required before the next deviation swing and state what should be recorded on the new card.
Read the worked answer
Before the next swing: repair and refill the compass; remove the toolbox from the console; relocate the loudspeaker and check its magnet; confirm the wiper and other circuits do not produce a measurable card deflection with the compass on cardinal headings; then swing with equipment off and with normal equipment on. Record any residual change. If the wiper still deflects the card after repairs, the binnacle position or wiring must be changed, not merely compensated. The new card should record the residual deviations, the equipment states during the swing, and the fact that the compass was serviced and the magnetic sources controlled.