Deviation and the steering compass

The steering compass

In Lesson 9 you saw that a magnetic compass points to magnetic north, not true north, and that the difference — variation — is printed on the chart’s compass rose. You converted directions between true (°T) and magnetic (°M) using the top of a ladder: T, V, M. This lesson adds the last two rungs: deviation and compass.

The steering compass is the one the helmsman steers by. It is fixed where it can be seen from the helm: on a pedestal (a binnacle) in front of the wheel or, on many tiller-steered yachts, on the cockpit bulkhead beside the companionway. Inside a liquid-filled bowl, a card carrying magnets and the degree scale floats on a pivot; the liquid damps its swings. A mark on the bowl, the lubber line, lines up with the boat’s fore-and-aft line.

As the boat turns, the card keeps pointing north and the bowl turns around it, so the figure against the lubber line is the boat’s heading by compass. Compass directions are written with a C: 047°C.

Deviation: the boat’s own magnetism

A compass responds to every magnetic field near it, not just the Earth’s — and a yacht carries plenty:

• Iron and steel: the engine, an iron keel and its bolts, steel fittings, and tools or tins stowed near the compass.
• Electric currents: a wire carrying a current makes a magnetic field, so deviation can change when equipment near the compass is switched on.
• Magnets: loudspeakers (in radios, stereos and phones) and many instruments contain them.

Together these pull the compass a little away from magnetic north. The angle between magnetic north and the direction the card’s north actually points (compass north) is the deviation. Like variation it is named east or west: easterly if compass north is pulled to the east of magnetic north, westerly if it is pulled to the west.

Variation comes from the Earth and is the same for every boat in the same place. Deviation comes from your own boat — and it changes as she turns.

Why deviation changes with heading

Picture the boat’s magnetism as a single magnet fixed in the boat — say in the engine, a little ahead of the compass and to one side — pulling the compass towards it, while the Earth’s field pulls it towards magnetic north. On a heading where the engine lies on the same line as magnetic north from the compass, the two pulls act along one line and the compass isn’t pushed sideways at all. Turn the boat a quarter of the way round and the engine lies east or west of the compass: now its pull is across the Earth’s, and the compass is dragged towards it — east on one heading, west on the opposite one.

A real boat has several sources, so the pattern is less tidy, but it changes smoothly as the boat turns. That is why a single deviation figure is no use: you need the deviation for the heading you are on, called the ship’s head.

Swinging the compass: the deviation card

To find the deviation, the compass is swung, usually by a professional compass adjuster. The boat is turned slowly through a full circle, steadying on a series of headings, and on each one the compass is compared with a bearing known to be right (transits, which you meet in Lesson 13, are one way of getting one). Where possible the adjuster reduces the deviation with small correcting magnets, then writes whatever is left on a deviation card for that boat and that compass. Here is the card for the course yacht:

On this card, ship’s head is the heading by the steering compass (°C) — the figure you read at the lubber line.

Reading the card

When ship’s head falls exactly on an entry, read the deviation straight off. Between entries, interpolate: assume the deviation changes steadily from one entry to the next.

The same method works across north. 330°C is between 315° (4°W) and 000° (2°W): a change of +2 over 45°, and it is 15° along, so +2 × 15/45 = +0.7. The deviation is −4 + 0.7 = −3.3, which rounds to 3°W.

The full ladder: T V M D C

You can now finish the ladder from Lesson 9. Write it down the side of the page whenever you convert a direction:

Count easterly errors as plus and westerly errors as minus. Then the whole ladder is one line:

Going up the ladder, from compass to true, you add the errors. The memory aid is CADET: Compass ADd East → True — add easterly errors, subtract westerly ones. Coming down, from true to compass, do the opposite. Lesson 9’s rhyme is often given in a general form that covers both errors: Error West, Compass Best; Error East, Compass Least. A westerly error — variation or deviation — makes the figure lower down the ladder, nearer the compass, the bigger one. They are only memory aids; the signs are what really do the work.

Compass to true

You convert a compass heading to true whenever you want to put it on the chart — for example, the heading you have been steering, to work out where the boat has got to (Lesson 14). You know the compass heading, so read the card for it directly.

True to compass: the course to steer

Coming down the ladder there is a catch. The card is listed by compass heading — but the compass heading is what you are trying to find. Deviation is small and changes slowly, so the compass heading will be only a few degrees from the magnetic one. Read the card at the magnetic heading, work out the compass course, then glance at the card for that compass course. If it gives the same deviation to the nearest degree — as it usually does — you are done. If not, use the deviation for the new compass course and check once more.

The hand-bearing compass

A hand-bearing compass is a small compass you hold up to your eye to take the bearing of a landmark — the subject of Lesson 11. Because you can take it to a spot on deck well away from the engine, the steering compass, the electronics and anything made of iron or steel, the deviation where you use it is usually too small to matter.

Keeping the compass honest

The card is only right while everything magnetic on board stays as it was when the compass was swung.

• Keep magnetic things away from the steering compass: phones, handheld radios, speakers and headphones, torches, knives, tools and tins of food. Left beside the compass, many of them can throw it out by several degrees — an error that is on no card.
• Watch electrical items near it: deviation can change when they are switched on.
• After changes on board, have the compass checked or swung again.

Checking the card with a transit

A transit is two charted objects in line (Lesson 13). With the bow pointing straight at them in line, the boat’s heading is the transit’s charted bearing, so you know the true heading exactly and can compare it with the steering compass. The Port Kerran leading lights make a transit on 000°T.

Common mistakes

• One deviation for every heading. Deviation belongs to a ship’s head; read the card for the heading you are on.
• Applying an error the wrong way. Say CADET every time: going up to true, add east and subtract west.
• Reading the card at the true course. The card is by compass heading; coming down, use the magnetic heading as your first guess, then check.
• Applying deviation to hand-bearing compass bearings. By our convention they have none.
• Old variation. Update it to the year from the compass rose (Lesson 9): 2°44'W in 2027, which is 3°W to the nearest degree.
• Forgetting to wrap round north. 358 + 4 = 362, which is 002°; 002 − 4 = −2, which is 358°.

T V M D C, east plus and west minus: going up, add; coming down, subtract. Read the card for the ship’s head.