Marine Magnetic Compass construction and siting :General principles (size and relative position; soft iron correctors; types of correctors used on ships; etc.,)
Of course. Here is a detailed explanation of the general principles of marine magnetic compass construction, siting, and correction.
Marine Magnetic Compass: Construction and Siting
The magnetic compass is one of the oldest and most reliable navigational instruments. Despite the prevalence of GPS and gyrocompasses, a properly adjusted magnetic compass is required on virtually all commercial vessels as a crucial, non-power-dependent backup. Its accuracy depends heavily on its construction, its location (siting) on the ship, and the correction of magnetic interference.
The core problem is that a ship, being made mostly of steel, has its own magnetic field that interferes with the Earth’s magnetic field. This interference, called Deviation, must be minimized and compensated for.
1. The Problem: The Ship’s Magnetism
A ship’s magnetic field is composed of two types of magnetism:
- Permanent Magnetism (Hard Iron): This is a fixed magnetic field acquired by the ship’s steel during construction (due to hammering, welding, and riveting in the Earth’s magnetic field). It is like having permanent magnets fixed within the ship’s structure. This magnetism is constant in strength and direction relative to the ship.
- Induced Magnetism (Soft Iron): This is temporary magnetism induced in the ship’s soft iron components by the Earth’s magnetic field. Unlike hard iron, its strength and polarity change as the ship changes its heading or its magnetic latitude.
The goal of compass siting and correction is to counteract the effects of both hard and soft iron.
2. General Principles of Compass Construction
A marine magnetic compass is more than just a magnetized needle. It’s a robust instrument designed for stability and accuracy in a dynamic sea environment.
- Compass Card: A lightweight, circular card graduated with 360 degrees (and sometimes cardinal points N, E, S, W).
- Magnetic Needles/Elements: Attached to the underside of the card are several powerful, parallel magnetic needles. Using multiple needles creates a more stable and powerful magnetic directive force.
- Compass Bowl: The card and needle assembly are housed in a sealed bowl filled with a liquid (typically a mixture of distilled water and alcohol or a clear oil). The liquid serves two purposes:
- Dampening: It slows the card’s oscillations, making it steady and readable in rough seas.
- Buoyancy: It reduces the weight on the pivot, minimizing friction and wear.
- Pivot and Jewel: The card assembly is balanced on a sharp, hardened pivot (often made of iridium) that rests in a jeweled cup (like sapphire or ruby) to allow the card to rotate with minimal friction.
- Lubber’s Line: A fixed vertical line marked on the inside of the compass bowl that is aligned with the ship’s fore-and-aft line. The helmsman steers the ship by keeping the desired course on the compass card aligned with the lubber’s line.
- Gimbals: The compass bowl is mounted in a set of rings called gimbals, which allow the compass to remain level as the ship rolls and pitches.
- The Binnacle: The entire compass assembly is housed in a non-magnetic stand called the binnacle. The binnacle is not just a housing; it is a critical part of the compass system because it holds all the correctors.