Marine Magnetic Compass construction and siting :IMO Resolution A382(X);

Of course. Here is a detailed breakdown of the requirements for marine magnetic compass construction and siting as laid out in IMO Resolution A.382(X), supplemented with context from SOLAS regulations which mandate its application.

Overview of IMO Resolution A.382(X)

Adopted on 14 November 1977, IMO Resolution A.382(X) - “Performance Standards for Magnetic Compasses” establishes the minimum requirements for the construction, performance, and installation of marine magnetic compasses to ensure they are reliable, accurate, and function effectively as a primary means of navigation.

These standards are referenced by the International Convention for the Safety of Life at Sea (SOLAS), Chapter V, Regulation 19, making them mandatory for most commercial vessels.

The resolution defines two main classes of compasses:

• Class A: The standard magnetic compass, which must be carried by all ships of 150 gross tonnage and upwards. It is used for steering and taking bearings.
• Class B: A secondary compass used for steering purposes (e.g., at an emergency steering station) or as the primary compass on smaller vessels and lifeboats.

The following details focus primarily on the Class A standard magnetic compass.

Part 1: Construction & Performance Standards

The resolution outlines specific technical requirements for how a compass must be built to ensure its accuracy and durability in the harsh marine environment.

1. Compass Bowl and Card

• Diameter: The compass card of a Class A compass must have a diameter of at least 125 mm.
• Graduation: The card must be graduated in 360 single degrees. The cardinal points (N, E, S, W) and inter-cardinal points (NE, SE, SW, NW) must be clearly marked. Every tenth degree must be numbered.
• Lubber Line: A clear and distinct lubber line must be provided to indicate the ship’s fore-and-aft line. Its reading error against the card should not exceed 0.5 degrees.
• Readability: The markings must be clear and easily readable under all lighting conditions.

2. Materials and Liquid

• Non-Magnetic Construction: All materials used in the construction of the compass, its binnacle, and correcting devices must be non-magnetic and resistant to corrosion from seawater and sea air.
• Compass Liquid: The bowl must be filled with a transparent liquid (typically a mixture of alcohol and distilled water) that:
  • Remains clear and free from bubbles.
  • Has a low freezing point (e.g., below -30°C).
  • Does not become viscous at low temperatures.
  • Is not corrosive to internal components.

3. Gimbal System

• The compass bowl must be mounted on a gimbal system to ensure the card remains horizontal, regardless of the ship’s rolling and pitching motions.
• The gimbals must be robust and move freely, allowing the compass to remain effective up to an angle of heel of 40° in any direction.

4. Binnacle and Correcting Devices

The binnacle is the stand that houses the compass bowl and its essential correcting devices. Its construction is critical for correcting magnetic deviation.

• General Construction: The binnacle must be a robust, watertight structure capable of protecting the compass and housing the correctors securely.
• Correctors for Coefficient B and C (Semicircular Deviation): The binnacle must contain permanent magnets to correct for the fore-and-aft and athwartships components of the ship’s permanent magnetism. These are typically adjustable magnet bars placed in trays.
• Quadrantal Correctors (for Coefficient D): It must be fitted with soft iron spheres or equivalent devices (often called “Kelvin’s balls”) to correct for deviation caused by magnetism induced in the ship’s horizontal soft iron. They must be adjustable.
• Flinders Bar (for Coefficient D induced by vertical soft iron): A holder must be fitted on the binnacle (usually on the forward side) to accommodate a Flinders bar. This is a bar of soft iron used to correct for deviation caused by magnetism induced in the ship’s vertical structures, which varies with magnetic latitude.
• Heeling Error Corrector: A device, typically a central vertical magnet below the compass pivot, must be included to correct for deviation that occurs when the ship heels over.
• Illumination: The binnacle must be fitted with a suitable electric lighting system that does not create a magnetic field. A dimmer should be included to adjust brightness for night vision.

5. Performance Requirements

• Settling Time: When deflected by 40 degrees, the compass card must return to within ±1 degree of its original settling position in a specified time.
• Durability: The compass must be able to withstand vibrations, humidity, and temperature fluctuations typically experienced at sea without its accuracy being affected.

Part 2: Siting & Installation

Proper siting (location) is as crucial as proper construction. The goal is to place the compass where it is least affected by the ship’s magnetic fields and where it can be used effectively for navigation. These requirements are reinforced by SOLAS Chapter V, Regulation 19.

1. Primary Location

• The standard magnetic compass (Class A) must be installed on the ship’s centerline.
• It should be placed where the magnetic field is as free from interference as possible. This is typically on the monkey island (the uppermost deck above the bridge).

2. Field of View

• From the standard compass position, the horizon must be visible all-round (360 degrees). This is essential for taking terrestrial bearings to determine the ship’s position and compass error.
• If this is not possible, the compass must be positioned so that the view is as uninterrupted as possible for its primary function.

3. Magnetic Safe Distances

• The compass must be sited as far as possible from masses of steel and sources of electrical or magnetic interference. This includes:
  • Steel superstructures, funnels, masts, and derricks.
  • Electrical equipment such as motors, radar scanners, and power cables.
  • Radio antennas and high-power communication equipment.
• Manufacturers of shipboard equipment are required to state a “compass safe distance” for their products. Navigational equipment (e.g., radar, ECDIS) must be installed outside this minimum safe distance from the magnetic compass.

4. Accessibility

• The compass must be easily accessible for reading, for taking bearings (e.g., using an azimuth mirror), and for the compass adjuster to carry out corrections.

5. Compass Adjustment and Deviation

• After installation, the compass must be adjusted by a certified compass adjuster to reduce deviation to the smallest possible values.
• A deviation card or curve must be created, showing the remaining residual deviation for the compass on various headings. This card must be prominently displayed near the compass and at the steering position.
• The compass must be re-adjusted whenever there are structural changes to the ship that could affect its magnetic field, or if the deviation is observed to be unreliable.

Summary: The Interplay of A.382(X) and SOLAS

• IMO Res. A.382(X) provides the “what”—the technical and performance specifications that a compass and its binnacle must meet. It is the “type-approval” standard.
• SOLAS V/19 provides the “why” and “where”—it mandates that ships must carry a compass that meets these standards, specifies where it must be sited, and requires that it be properly maintained and adjusted.

In essence, a ship cannot simply have any magnetic compass. It must have a compass built and tested according to A.382(X) and then sited and maintained according to the principles outlined in both the resolution and SOLAS, ensuring it remains a reliable, non-power-dependent instrument for safe navigation.