23/12/2025
1. – Speed Over Ground
Definition: The actual speed of your ship over the Earth’s surface.
Unit: Knots (nautical miles per hour).
Importance: Shows real speed including effects of current or tide.
Example: You might steer 12 knots through water, but with a 2-knot current, your SOG could be 14 knots or 10 knots depending on direction.
2. – Course Over Ground
Definition: The actual track your ship follows over the ground.
Unit: Degrees (0–360°).
Importance: Shows the real path, affected by wind, current, or tide.
Connection: If your heading (direction of the ship’s bow) is different from COG, you’re experiencing drift and must correct to maintain intended route.
3. – Bearing
Definition: The direction of another object or vessel relative to your ship.
Types:
True Bearing → relative to True North
Relative Bearing → relative to ship’s bow
Importance: Used to determine position of other ships, land, or navigational marks.
Connection: Bearings are essential for calculating collision risks and monitoring crossing traffic.
4. – Closest Point of Approach
Definition: The minimum distance expected between your vessel and another vessel if both maintain current speed and course.
Unit: Nautical miles (or meters).
Importance: Predicts risk of collision.
Connection: CPA tells you how close another ship will come — smaller CPA = higher risk.
5. – Time to Closest Point of Approach
Definition: The time remaining until the CPA occurs.
Unit: Minutes or hours.
Importance: Gives you time to act to avoid collision.
Connection: Calculated using SOG, COG, and target vessel’s speed/course. Short TCPA = immediate action may be needed.
6. – Bow Crossing Time
Definition: The time it will take for another vessel to cross in front of your ship’s bow if both maintain current courses and speeds.
Importance: Helps in crossing situations to know exactly when a vessel will pass ahead.
Connection: Like TCPA, but focused on crossing ahead of your bow. Used together with CPA and TCPA for collision avoidance decisions.
How They Work Together
Navigation and collision avoidance is like a puzzle of position, speed, and time:
1. SOG + COG → Tell you where your ship is moving in reality.
2. BRG → Shows direction to nearby vessels or objects.
3. CPA + TCPA → Warn you of potential collisions.
4. BCT → Tells you when another vessel will pass in front of your bow in crossing situations.
Example Scenario:
You are steering COG 090°, SOG 12 knots.
Another ship is on radar at BRG 045°, COG 045°, SOG 10 knots.
CPA = 0.2 NM → dangerously close
TCPA = 8 min → time until closest point
BCT = 7 min → exact time the other vessel will cross your bow → you can decide when and how to maneuver to maintain safe separation.
