OOW Nav & Radar

THE MAGNETIC COMPASS

The basic principle of the magnetic compass has remained unchanged through the centuries.
- Composed of a magnetized needle mounted on a compass card to pivot freely.
- Advancements in understanding magnetism and precision manufacturing enhance modern compasses.

Basic Magnetism

Magnetism Definition: The phenomenon by which materials exert attractive or repulsive forces.
Materials: Common magnets include nickel, iron, and their alloys; all materials are affected to varying degrees by magnetic fields.
Any magnetized material develops poles: regions of concentrated magnetism with unlike polarities.
Magnetic Lines of Force: Connect one pole to another.
Electrons act as tiny magnets; in un-magnetized materials, they are randomly oriented, whereas in magnets, they align directionally, creating a magnetic field.
Magnetic Field Intensity: Number of field lines per unit area.
First Law of Magnetism: Like poles repel, unlike poles attract.
- Magnets Formation: Possible by stroking soft iron with a magnet or placing it in a strong magnetic field.
- Demagnetization: Can occur if a permanent magnet is heated or impacted in differing magnetic fields.

Types of Magnetism

Permanent Magnetism: Retains magnetism after being removed from a magnetizing field.
Induced Magnetism: Loses its magnetism once out of the magnetizing field.
Retentivity, hardness of iron, and stress affect magnet retention.

Earth’s Magnetism

COMPASS VARIATION

The angle between the true meridian (geographic North) and the magnetic meridian (magnetic North): known as variation.
- Variation changes by location and over time (annual changes).
Example: North Magnetic Pole located approximately at 83°N and 115°W.
Determining Variation:
- Variations are expressed in degrees, and the error is east/west.
- Variation is illustrated on Admiralty charts as compass roses with values and dates.
- Example: Var. 23º 45ʹ W (1999) 8ʹW.
Calculating Future Variation:
- (23º 45ʹ W) + (8ʹ imes 11 ext{ years}) = 25º 13ʹ W (rounding as needed).

COMPASS DEVIATION

Deviation: Error arising due to the vessel's magnetic field affecting the compass.
Causes: Steel and iron vessel structure and location near machinery.
Types of Iron:
- Hard Iron: Difficult to magnetize, retains magnetism.
- Soft Iron: Easily magnetized, loses magnetism quickly when field changes.
Two magnetic effects affect compasses on vessels:
- Permanent magnetism (static structural) and induced magnetism (dynamic).
Correction Techniques:
- Permanent correction (aligning magnets around the compass) versus soft iron correction (around compass position).

CHECKING DEVIATIONS

Despite corrections, deviation checks should be regular, ideally once per watch or after course changes.
- Methods of determining true bearing:
- Aligning two fixed landmarks, known fixed position bearings, comparing with gyro compass readings, celestial navigation.
Retentive Magnetism: Can cause deviations that last from minutes to days due to changes in the vessel's structure or operation.

SITING OF MAGNETIC COMPASSES

Important considerations include:
- Visibility from steering position
- Placement on centerline to reduce magnetic interference
- Avoid proximity to any magnetic sources like machinery or steel.

PRACTICAL VARIATION AND DEVIATION CORRECTIONS

Conversion of courses:
- True → Magnetic ± Deviation → Compass
- Compass ± Deviation → Magnetic ± Variation → True
Mnemonic: Timid Virgins Make Dull Company (T/V to C), Error WEST compass BEST/Bigger, Error EAST compass LEAST/Smaller.

SAMPLE DEVIATION CARD

Example shown in the transcript with inner changes in deviation based on direction and angles which need to be prepared and plotted accordingly.

GYRO COMPASS

Basic Principles
- Free gyroscope spins on 3 axes indefinitely if undisturbed, remains stable due to rigidity in space.
- Uses precession to adjust and stabilize to true North.
Gyro Compass Errors
- Affected by latitude, course, and speed of ship. Errors range from 3° to 4° typically.
Determining Gyro Error
- Check by visual transits, plotting fixes or azimuth from celestial bodies. Operate ideally alongside manual checks against magnetic compass readings.

ADVANTAGES & DISADVANTAGES OF GYRO COMPASS

Advantages: Not influenced by magnetism (no variation/deviation), can integrate into other navigational systems.
Disadvantages: Requires power, may take time to recalibrate after failures, may accumulate errors during maneuvers.

Nautical Charts: Represent parts of Earth's surface graphically mapped onto flat planes. Current practices include Raster and Electronic Navigation Charts (ENCs).
Admiralty Charts: Include water depths, topographic features, navigational hazards, etc. Different color codes indicate varying water depths and features. Understanding the symbols and charts such as Chart 5011 is crucial for mariners.
Chart Projections: Mathematical projections translate the 3D Earth onto 2D charts. Mercator projection favored for navigation, ideal for rhumb lines.
Essential Calculations: Distance, Time, Speed calculations are routinely performed (D=Speed × Time). Various examples demonstrate the application of these calculations.

CORRECT CHART WORK SYMBOLS & POSITION FIXING STANDARDS

Using symbols uniformly while working on charts is essential for clarity.
Techniques for fixing position include deduced reckoning, lines of position, estimated positions, and triangulating distances.

Notices to Mariners: Ongoing updates to navigational charts and information published weekly. Critical for maintaining safe navigation, including temporary notices, updates to Admiralty Sailing Directions, etc.

Navtex: System for providing safety-related messages to ships regarding urgent navigational information; required for certain ships under GMDSS regulations.

Tide Theory: Effects of Moon and Sun gravitational pull cause tides. Understanding types of tides (e.g. Spring and Neap tides), tide tables are essential for mariners and navigation operations.