Understanding Earth’s Magnetism, Navigation, and Maritime Rules

Posted on Jan 26, 2025 in Other subjects

Earth’s Magnetic Field

The Earth itself acts like a huge magnet. The field is thought to be a result of swirling molten rock in the Earth’s core, which causes circulating currents, and thus an electromagnetic field.

Magnetic and Geographic Poles

There are two ‘Norths’: True North and Magnetic North. True North is the center of rotation of the Earth. Bearings and lines of longitude given on charts are based on True North. Earth’s magnetic field Magnetic North is the position of the magnetic pole. The Earth acts like a giant magnet and just like any other magnet, it has different polarities, which attract the opposite and repel the same.

Variation

The difference between the bearing of True North and Magnetic North is called ‘variation’. This changes according to a vessel’s location on the globe.

Deviation

If a magnetic compass is put in a vessel, the presence of iron and steel will cause the compass needle to deviate from the magnetic meridian. The angle between the magnetic meridian and the direction in which the needle actually points is called the ‘deviation’. If compass north lies to the east of the magnetic meridian, the deviation is said to be easterly; if west, westerly. The ship’s hull, keel, machinery, boats, cranes, and electrical machinery affect the magnetic compass because of:

  • Permanent ferromagnetism of the ship’s structure caused when the ship was built or when berthed for a prolonged period on one heading.
  • Magnetism induced by the ship’s “soft iron”.
  • Magnetism induced by electromagnetic equipment (wires carrying an electrical current, e.g., aerials).
  • Magnetism induced by magnets (loudspeakers, analog instruments, etc.).

Minimizing Deviation

To correct deviation, or at least minimize it, the compass adjuster breaks down the ship’s magnetism into longitudinal, athwartships, and vertical magnetism. Each component is corrected using the appropriately aligned correctors.

Longitudinal Magnetism

To reduce permanent magnetism, fore-and-aft ‘longitudinal magnets’ are placed beneath the compass to cancel out the effect of the ship acting as a magnet lengthwise.

Athwartships Magnetism

‘Athwartships’ magnets are placed to cancel out the effect of the ship acting as a magnet crossways. This will leave the compass needle free to be influenced only by the Earth’s magnetic field.

Reasons for a Change of Deviation in the Long Term:

  • Protracted docking or refit period, especially if major works are undertaken, causing vibrations to the hull.
  • Long, repeated voyages on a continuous heading.
  • Major changes of latitude.

Reasons for a Change of Deviation in the Short Term:

  • Changes in the position of large ferrous objects, such as a crane trained outboard.
  • Changes in electrical equipment, such as a new radar or repositioning of the same.
  • Modifications of a ferro-nature close to the compass or leaving tools, beer cans, etc. in the vicinity.
  • Heavy shocks to the vessel, such as grounding or collision, or repeated vibration, such as pounding in heavy seas.
  • Lightning strikes and heavy current short circuits.
  • Heavy rolling or leaning for a period in one direction due to weather.
  • If the lubber’s line is incorrectly aligned.

Light Ranges

Geographical Range

Assumes unlimited visibility. It is the range at which a light can first be seen and takes into account the height of the eye of the observer and the height of the light above sea level.

Nominal Range

Is shown on the chart next to a light. It is the range that light can be seen when the meteorological visibility is 10 nautical miles, irrespective of the horizon.

Luminous Range

Is the range at which a light can be seen, taking into account only the nominal range and the prevailing visibility.

Steering & Sailing Rules

Section I – Conduct in Any Condition of Visibility

  1. Rules in this section apply in any visibility.
  2. Proper lookout by all available means appropriate to make a full appraisal of the risk of collision.
  3. Safe speed in prevailing conditions; affected by:
    • Visibility
    • Traffic density
    • Maneuverability
    • Background lights
    • Weather and navigational hazards
    • Draft regarding depth
  4. Use all available means to assess the risk of collision. Use bearings and radar.
  5. Take avoiding action positive and early. Maintain a safe distance. Reducing speed is an option.
  6. Narrow channels: Stay on the starboard side. Sailing or vessels under 20 meters should not impede larger vessels.
  7. Traffic separation zones: See regulations. Main points: Avoid crossing if practicable. If crossing a zone, the heading should be at right angles.

Section II – Vessels in Sight of One Another

  1. This section applies to vessels in sight of one another.
  2. Sailing vessels:
    • On opposite tacks, the port tack gives way.
    • On the same tack, the windward boat keeps clear.
    • If in doubt, assume the other is on the starboard tack.
  3. All vessels: If overtaking, keep clear until past and clear. Overrides all other rules in Part B, Sections I & II. If in doubt, assume you are overtaking.
  4. Two power vessels head-on: Both alter course to starboard.
  5. Two power vessels crossing: The vessel with the other to starboard gives way. Avoid crossing ahead.
  6. Give-way vessel to take early and substantial action.
  7. Action by stand-on vessel:
    • Stand on with caution.
    • May take action if the give-way vessel is not acting.
    • Must take action if a collision is inevitable.
  8. Priorities: Not Under Command (NUC) & Restricted in Ability to Maneuver (RAM), Constrained by Draft (CbD), Vessel Engaged in Fishing (VEF), Sailing Vessel (SV), Power-driven Vessel (PV).

Section III – Conduct in Restricted Visibility

  1. This rule applies to vessels not in sight of one another.
  2. Maintain a safe speed. Have engines ready.
  3. Due regard to conditions for Section I.
  4. Radar alone: Take avoiding action, but not:
    • Alter course to port for a vessel forward of the beam unless overtaking.
    • Alter course towards a vessel abeam or abaft the beam.
  5. Fog signal forward of the beam: Slow down or stop, and navigate with extreme caution until the danger is over.

Sound & Light Signals

When Maneuvering:

  • I am altering course to starboard: *
  • I am altering course to port: **
  • I am operating astern propulsion: ***
  • I do not understand your intentions: *****

In a Narrow Channel:

  • I intend to overtake to your starboard: –*
  • I intend to overtake to your port: –**
  • Agreement by overtaken vessel: -*-*
  • Warning, I am coming! (& reply): –

In Restricted Visibility:

  • Power vessel making way (every 2 mins): –
  • Power vessel underway but stopped: —
  • NUC, RAM*, CbD, SV, VEF*, Tug (*even when at anchor): -**
  • Towed vessel (last): -***
  • Vessel at anchor (every 1 min): Bell
  • Vessel > 100m at anchor (every 1 min): Bell + Gong
  • Additionally, to warn approaching vessels: *-*
  • Vessel aground, before and after bell: 3 rings on Bell
  • Pilot vessel (optional in addition to above): ****

NB. Underway: Towing vessels carry masthead lights, sidelights, and sternlights when underway, even if RAM; it’s important to know their heading at all times. Trawling vessels > 50m carry an additional masthead light when underway. Fishing (not trawling) vessels do not.

Making way: Trawling, fishing, NUC, and other RAMs all carry sidelights and sternlights, but only when making way. They may stop in the course of work, so this is a useful distinction. RAM & dredgers carry masthead lights as well, only when making way; their direction of movement is more predictable, so it could be useful to see masthead lights.

Q1: State 5 Actions in Reduced Visibility

A:

  • Assume the possibility of poor visibility to landward.
  • Start a fog signal, switch navigation lights on.
  • Call the Master.
  • Bring the engine room to standby.
  • Reduce speed.
  • Post a lookout.
  • Close up helmsman with steering in hand mode (follow-up).
  • Commence plotting of radar contact.
  • VHF: Listen on Channel 16 and Coastal/Port Control Channel.

Q2: Target Detected on Radar, Steady Bearing

A:

  • Don’t assume that he can see you. Take early action to avoid a close-quarters situation developing.
  • Call the Master.
  • Reduce speed.
  • Sound fog signal and take actions as for a) above.
  • Rule 19 applies.

Tides

GPS

Geographical Datum and Datum Shift

A horizontal datum is a base reference system for specifying position in Latitude and Longitude; it is a mathematical model of the Earth’s surface. Nearly every country had its own datum, to be found in the chart title panel. The World Geodetic System 1972 (WGS72) was introduced and was replaced by WGS84 datum.

System Description

The satellites orbit the Earth at an altitude of 20,000 kilometers, continuously broadcasting data on their positions. The user is equipped with a receiver capable of tracking these signals and determining the range from a particular satellite by measuring the time it takes for a signal to travel from the satellite to the receiver and multiplying this by the speed of the radio wave. Measuring the distances from four such satellites allows the user to fix his position uniquely in three dimensions and verify the receiver clock. There are 3 major components to the system: the Control Segment, Space Segment, and User Segment.

The Control Segment

The control segment consists of five ground monitoring stations for tracking all visible satellites. They also receive data concerning the conditions of the atmosphere and clock data obtained from the caesium frequency atomic clock. This data is then transmitted to the master control station where the ephemeris data and clock status predictions for each satellite are generated. This data is then used in formulating the navigation messages to be uploaded to the satellites every eight hours from the three ground antennas at earth stations.

The Space Segment

The space segment consists of the 21 satellites plus 3 spares. The height of the satellites is approximately 20,200km, and they have a sidereal period of 12 hours. Each satellite transmits data on two frequencies onto which is superimposed a coded signal. This signal contains information on satellite positions.

User Segment Range Position Fixing

Signals with extremely accurate time and satellite position are broadcast by each satellite. The GPS receiver takes 3 signals and can use them to triangulate an approximate position. The position must be approximate because the clock in the receiver on board is relatively inaccurate. However, the receiver applies corrections to the onboard clock so that all 3 position lines come together at one point. The onboard clock is then in time with the satellite clocks, and an accurate position is obtained. So, 3 position lines are required for a 2D fix. A 4th satellite is required for a 3D fix.