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SPEED OVER GROUD VS
SPEED THROUGH WATER
By: Gian Gabriel P. Ibo
BSMT student
 Is the speed of the vessel relative to the surface of
the earth
SPEED OVER GROUND (SOG)
15 knots speed
2knots current
13 NM covered in an hour
SPEED OVER GROUND: 13 kts
 Is the speed of the vessel relative to the water
SPEED THROUGH WATER(SW)
15 knots speed
2knots current
13 NM covered in an hour
SPEED THROUGH WATER: 15 kts
OTHER EXAMPLES:
17 NM covered in an hour
15 knots speed
2knots current
SPEED THROUGH WATER: 15 kts
SPEED OVER GROUND: 17 kts
Is it possible to have zero STW while you have SOG?
YES!!!
7 knots current
Vessel is dead
On water
0 knots
Speed over Ground: 7 knots
Speed through Water: 0 knots
This is possible when a ship is drifting and there is a current allowing the vessel
To move 7 knots
Is it possible to have zero SOG while you have STW?
YES!!!
7 knots current
7 knots
Speed over Ground: 0 knots
Speed through Water: 7 knots
This is possible when a ship has a speed the is the same with the opposing current
Opposing current
 Speed over Ground
 Speed through
water
APPLICATION
Doppler Log (Ground Tracking mode)
Used in Navigation
Electro Magnetic Log
Doppler Log (Water Tracking mode)
Used in Collision avoidance

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Speed over groud vs speed over water

  • 1. SPEED OVER GROUD VS SPEED THROUGH WATER By: Gian Gabriel P. Ibo BSMT student
  • 2.  Is the speed of the vessel relative to the surface of the earth SPEED OVER GROUND (SOG) 15 knots speed 2knots current 13 NM covered in an hour SPEED OVER GROUND: 13 kts
  • 3.  Is the speed of the vessel relative to the water SPEED THROUGH WATER(SW) 15 knots speed 2knots current 13 NM covered in an hour SPEED THROUGH WATER: 15 kts
  • 4. OTHER EXAMPLES: 17 NM covered in an hour 15 knots speed 2knots current SPEED THROUGH WATER: 15 kts SPEED OVER GROUND: 17 kts
  • 5. Is it possible to have zero STW while you have SOG? YES!!! 7 knots current Vessel is dead On water 0 knots Speed over Ground: 7 knots Speed through Water: 0 knots This is possible when a ship is drifting and there is a current allowing the vessel To move 7 knots
  • 6. Is it possible to have zero SOG while you have STW? YES!!! 7 knots current 7 knots Speed over Ground: 0 knots Speed through Water: 7 knots This is possible when a ship has a speed the is the same with the opposing current Opposing current
  • 7.  Speed over Ground  Speed through water APPLICATION Doppler Log (Ground Tracking mode) Used in Navigation Electro Magnetic Log Doppler Log (Water Tracking mode) Used in Collision avoidance