THE KNOWLEDGE
Amid all the technology found on cruising boats, compass accuracy can easily err. Adjuster Jo Robinson tells us why it’s still essential to check your compass accuracy regularly
Swing your compass. Why modern electronics make adjusting your compass more important than ever
Jo Robinson is a specialist in an age of generalists. Focusing on one instrument, the fixed compass, she plies her trade across almost every size of craft, from day boats to oil tankers; even the Royal Navy is a loyal customer. Her role is to tame the most fickle of navigational instruments, the fixed compass. But why is compass adjustment still so important on a relatively small modern yacht?
‘A properly adjusted compass and deviation card remains the primary source of directional information for non-electronic navigation. If you are sailing any distance, error matters. Sail 60 miles and a degree’s error will put you a mile off course. If you’re sailing any further then it really starts to make a difference to where you end up at the end of the passage!’ explains Jo.
Having served an extended apprenticeship with her father Rob Robinson, who adjusted compasses in an age before GPS even existed on big ships, Jo has been jumping from boat to boat in the Solent for the last 20 years. It’s given her both a wealth of technical experience and a feel for what’s most likely to be affecting deviation, an invisible force present on every boat. ‘How hard it is to reduce error varies enormously from boat to boat. It’s a process. First we determine what error is present, then get rid of any major anomalies, then fine tune. As a rule, anything less than 10° is considered quite accurate and it usually takes no more than three circles of the boat to calibrate each compass.’
WHAT IS COMPASS ADJUSTMENT?
Compass error is the difference between Magnetic North and the direction in which the compass is pointing. Both variation and deviation are measured in degrees east or west. The aim of the compass adjuster is to nullify the effect of the unwanted magnetic fields by placing correctors (magnets and soft iron) adjacent to the compass. These
create equal but opposing magnetic fields, thus eliminating the deviating fields around the compass, enabling it to align correctly.
Why does a compass need adjusting?
It’s not just something for big ships. ‘Most of my clients own yachts under 40ft in length. Some new boat owners make the assumption that their compass is right when it’s installed at the yard,’ explains Jo. ‘The reality is that every newly commissioned yacht needs to be properly swung once everything has been installed as part of the commissioning process. The amount of kit in proximity to the compass, particularly VHF radios, stereos and electronic equipment, can have a significant influence. If you install new equipment on an older boat, the same will apply and it’s a good idea to have the compass re-swung when you change the electronics. As a rule, every two years it’s
also worth checking as a matter of routine. People are always surprised what can throw a compass off – even a pair of glasses can have an affect, and a mobile phone can easily cause a 10° error. If your compass is suddenly not behaving, think, what could be affecting it, what’s changed?’
HOW DO YOU DETERMINE THE AMOUNT OF ERROR THAT EXISTS?
By swinging the yacht through 360°, every angle she will sail at and comparing true headings with the compass reading.
Motoring out into a busy Southampton Water, we find some space with nearby buoyage to start the process. This is always done in an area where there are no known magnetic anomalies marked on the chart. Traditionally, ships will fly flags Oscar and Quebec above each other to indicate they are swinging the compass as it necessitates a lot of steering in circles, something that can easily confuse other marine traffic.
On a yacht, it’s worth having someone keeping a good lookout whilst someone else concentrates on swinging the compass.
‘A compass adjuster is only as good as the helmsman. Being able to hold heading on a cardinal and using transits is how we make the assessment of how much error we are dealing with. A hand-bearing compass is also used in different parts of the boat (to mitigate the influence of on-board magnetic anomalies) to cross-check,’ says Jo. ‘With the yacht held steady on each of the eight primary compass points, the existing compass bearing and headings are then compared with what we know the headings should be; this then gives us a deviation in degrees. As a rule of thumb, anything below 10° is usually considered acceptable. We will knock out error above 8°.’
To check the difference of true angles, a pelorus is used. Effectively a dumb sighting compass, it accurately measures the difference in degrees but provides no magnetic information.
To take the error out of a modern yacht compass, the outer casing must be removed revealing the magnet housing. Initially, a box of loose magnets is used to calibrate, then small magnets are installed in the casing at various points around the compass so error can be compensated for. On the new catamaran on which the corrections were being made, the biggest error found was 30°.
Taking the error out of an electronic compass is the same process. However, electronic plotter designers have made it easy to do yourself, largely because an electronic compass in calibration mode will remember the error that it encounters as you swing the yacht round in circles. Most will ask you to swing the boat around at least two or three times before manually adjusting the heading. So, it’s well worth adjusting the manual compass before you do this. Fluxgate compasses are just as susceptible to magnetic interference as their traditional counterparts.