Compass Adjustment & Correction

Magnetic compass adjustment and correction

Magnetic compass adjustment and correction, often called a compass swing, reduces the deviation caused by the ship's own steel and electrical equipment so that the magnetic compass gives a reliable heading. The magnetic compass works without electrical power and remains the backup for the gyro and electronic heading systems. After dry-docking, structural work or new equipment on the bridge, its deviation can change enough to make it misleading.

vslSafety coordinates qualified compass adjusters at the vessel's port or anchorage to adjust standard and steering compasses, issue a new deviation card and inspect the bowl and binnacle, often during a short port call or before sailing from a shipyard.

Regulatory requirements and standards

SOLAS regulation V/19.2.1.1 requires every ship, irrespective of size, to have a properly adjusted standard magnetic compass, or other means independent of any power supply, to determine and display the ship's heading at the main steering position. Regulation V/19.2.2.1 requires ships of 150 gross tonnage and upwards and passenger ships to carry a spare magnetic compass or equivalent, and ships of 300 gross tonnage and upwards need a properly adjusted transmitting heading device. IMO resolution A.382(X) covers magnetic compass carriage and performance, including a table of residual deviations kept near the compass.

Many flag administrations and class circulars refer to ISO 25862, which sets out when a magnetic compass should be adjusted: on installation, when it becomes unreliable, after repairs or structural changes, after electrical or magnetic equipment near the compass is added or altered, when records are not kept or deviations become excessive, and at intervals of no more than two years. Guidance commonly calls for adjustment when deviation on any heading exceeds 5 degrees. The adjustment should be made by a qualified compass adjuster or, where the flag allows, by the master, with verification by an adjuster at the next opportunity.

Inspection and servicing intervals

Item	Requirement	Interval	Reference
Standard magnetic compass	Adjustment and new deviation card	At least every 2 years, and after triggering events	ISO 25862, flag guidance
Deviation monitoring	Compass error checked and recorded in the compass observation book	Regularly, often each watch and after large alterations of course	Resolution A.382(X), company procedures
Excessive deviation	Adjustment when deviation on any heading exceeds 5 degrees	When observed	Flag and class guidance
Compass bowl and binnacle	Inspection of liquid, bubbles, card, gimbals and lighting	At adjustment and at surveys	Maker instructions

What the service includes

• Inspection of the compass bowl for bubbles, discoloured liquid, sticking card or damaged gimbals
• Check of binnacle correctors, including Flinders bar, quadrantal spheres, heeling magnet and fore-and-aft and athwartships magnets
• Swinging the ship through the cardinal and intercardinal headings with bearings from the sun, transits or a gyro reference
• Adjustment of correctors to reduce deviation
• Preparation of a new deviation card or curve for the compass
• Check of the transmitting heading device and compass lighting
• Check of the spare compass bowl and its stowage
• Topping up or replacing compass liquid, or bowl replacement, where needed
• Guidance to officers on recording compass error

Equipment types covered

Compass adjusters work on reflector and projector standard compasses, steering compasses, spare bowls and magnetic compasses with transmitting heading devices. Makers frequently seen on board include C. Plath, Cassens & Plath, Lilley & Gillie, Saura and Sestrel. Heading sensors such as gyro and satellite compasses are handled under navigation equipment services, and adjustment is often planned with other work at the end of a ship repair and maintenance period.

Common defects and PSC deficiencies

• Deviation card missing, illegible or more than two years old
• Compass error not recorded, or no compass observation book kept
• Large air bubble or discoloured liquid in the bowl
• Compass lighting not working, so the card cannot be read at night
• Deviation above acceptable limits after dry-docking or new equipment installation
• Spare compass bowl missing or not interchangeable

Certificates and documents you receive

After the adjustment you receive a new deviation card or curve signed by the compass adjuster, and a compass adjustment report or certificate showing the date, ship's heading references, residual deviations and corrector positions. Some flags or class societies have their own forms. Keep the deviation card near the compass and file the report with the ship's navigation records.

How to prepare the vessel

A compass swing needs the ship to turn through all headings, so plan it for a period when the ship can manoeuvre in open water or at anchor with tugs, or during departure from port. Tell the adjuster about any structural work, new equipment or magnetic cargo since the last adjustment. Keep the previous deviation card and compass observation book ready, and make sure the gyro is settled and working as a reference. Good weather and visibility make the work quicker.

Information needed for a quotation

• Vessel name, IMO number, flag and class
• Port, anchorage or sea trial area, ETA and ETD
• Type and maker of standard, steering and spare compasses
• Date of the last adjustment and current deviations
• Reason for adjustment, such as dry-dock, PSC deficiency or new installation
• Whether the adjuster will sail with the ship to the next port or pilot station

vslSafety coordinates a suitable compass adjuster at the vessel's port and confirms scope, timing and cost before attendance.

Frequently asked questions

How often must a ship's magnetic compass be adjusted?

ISO 25862, widely referenced by flag administrations and class societies, calls for adjustment at least every two years and after events such as structural repairs, dry-docking or changes to nearby equipment. Adjustment is also needed when deviation on any heading exceeds 5 degrees. Some flags publish their own rules, so check your flag's circulars and the company's procedures.

Can the master adjust the compass?

Some flags allow the master to adjust the compass and issue a deviation card when a qualified adjuster is not available, with verification by a compass adjuster at the next opportunity. Other administrations require a qualified adjuster. Check the flag requirements before relying on a master's adjustment, especially after a port State control deficiency.

What is a deviation card?

A deviation card is a table or curve showing the remaining compass deviation on each heading after adjustment. It allows officers to convert the compass reading to a magnetic heading. IMO resolution A.382(X) expects the table of residual deviations to be kept on board near the compass. A new card is issued after each adjustment.

Can the compass be adjusted while the ship is alongside?

A full swing needs the ship to head in different directions, which is not usually possible alongside. Adjusters often sail with the ship from the berth to the pilot station or carry out the swing at anchor using tugs. Inspection of the bowl and binnacle can be done alongside.