Compasses & Magnetism
The compass is the one instrument that keeps working when everything electronic dies — but only if you can correct what it tells you. This module covers the full IYT Module 18 syllabus at Working depth: how the magnetic compass works, the three norths, variation and deviation, and the TVMDC drill for converting between true, magnetic and compass courses — with a live calculator to practise on.
The Magnetic Compass & the Lubber Line
A steering compass is a magnetised needle on a near-frictionless pivot, fixed to a circular card graduated 0°–360° (000° and 360° are the same mark — north). The needle holds the card aligned with magnetic north no matter which way the boat turns. The compass is mounted on or parallel to the boat's fore-and-aft centreline, and it serves two jobs: steering a course, and taking bearings of objects for the chart.
The lubber line is a fixed reference line on the compass housing, aligned with the centreline. Because the card stays put and the boat turns around it, the number sitting against the lubber line is always your heading. Most compasses carry a second lubber line aft to show the reciprocal — the exact opposite direction, 180° away (so a heading of 040° has a reciprocal of 220°). Try it — turn the boat:
Watch what moves: the card and needle stay locked on magnetic north — only the boat and its red lubber line swing. Students who grasp this stop "chasing the card" when they steer.
Three Norths, Three Courses
True North is the geographic North Pole — the fixed point where the meridians of longitude meet, and the datum for everything printed on the chart. Magnetic North is where the needle actually points: a wandering spot in the Canadian Arctic (historically near Bathurst Island), roughly 1,600 km from the true pole — and it moves every year. Because the two poles don't coincide, almost everywhere on Earth the needle points somewhere other than true north.
That single fact creates three versions of every course and bearing — and a number is meaningless until you say which one it is, so always suffix (T), (M) or (C):
Referenced to the geographic pole. The chart's latitude/longitude grid and the outer ring of the compass rose are True. All chartwork is done in True.
True corrected for variation only. A hand bearing compass used away from the boat's own magnetism reads Magnetic.
Magnetic corrected for deviation as well — what the steering compass actually shows. All helm orders are given in Compass.
Variation
Variation is the angle between true north and magnetic north at your location. It's a property of the planet, not the boat — the same for every vessel in the harbour. If magnetic north lies east of true north, variation is named East; if west, it's named West. Drag the slider and watch the magnetic rose swing against the true rose:
You never memorise variation — you read it off the nearest compass rose on the chart, which prints the variation, the year it was measured, and the annual change. "Working it up" means applying that annual change for the years since. Cross-check any location with NOAA's declination calculator.
Deviation
If variation is the planet's error, deviation is the boat's own error — the needle deflected east or west of magnetic north by magnetism aboard the vessel itself. It's named East or West just like variation, but unlike variation it changes with the boat's heading (and can change as she heels, if the keel is iron or steel).
- Ferrous metal near the compass — engines, iron/steel keels, tools, even a pocket knife left by the binnacle
- Electrical gear — motors, cookers, instrument clusters (a motorboat's helm often has several of these close together)
- Speakers in radios and VHF sets — powerful magnets, a classic install mistake
- Steel-framed spectacles against a hand bearing compass
- Steel hulls are worst; fibreglass and timber are far more forgiving
- A professional compass adjuster places corrector magnets to remove most of it
- What remains goes on a deviation card — deviation tabulated against compass heading, kept at the chart table
- When to check: at the start of any passage, at the start of the season, after fitting any new equipment near the compass, and on any new, chartered or borrowed boat
Applying Error — the TVMDC Drill
Corrections must be applied in sequence, never in one lump: variation sits between True and Magnetic; deviation sits between Magnetic and Compass. IYT teaches the box order with the mnemonic — this is the exact phrasing to memorise:
And the sign rule, in the exam wording — going from True toward Compass: "Error west, compass best (add on); error east, compass least (subtract)." Going back from Compass to True, reverse it: add easterly error, subtract westerly. Draw the boxes, fill what you know, and the blanks solve themselves:
Magnetic Anomalies
In a few places the seabed itself is magnetic — iron-rich volcanic rock (magma) close under the bottom. Sailing over one, the needle can wander or spin continuously and the compass is simply unusable for a few miles until you're past. These zones are surveyed and marked on the chart as magnetic anomalies — the fix is awareness, not correction: note it in the passage plan, hold your course by other means (GPS, transit, steady helm) and trust the compass again once clear.
The Hand Bearing Compass
A hand bearing compass is a portable compass with a sighting notch or prism: point it at a charted object — a lighthouse, a headland, a tower — and read the bearing. Take bearings of two (better, three) objects, convert them for the chart, and plot: where the position lines cross is your fix. The full plotting drill belongs to Module 19 and the practical course; here you need the instrument itself.
Used standing away from the boat's engines and instruments, it sits outside the vessel's deviation field — so it reads Magnetic, needing only variation applied to reach True — unlike the steering compass, which also carries deviation.
Brace against a shroud, hold it at eye level, breathe out, average the swing. Keep it away from winch handles, phones and radios — and remember steel-framed spectacles can pull the reading right at your eye.
Compasses & Magnetism — Knowledge Check
11 questions at IYT exam standard (75% to pass), drawn from the IYT Module 18 knowledge review — including two conversions to work by hand before you peek.