This provides the difference between speed over ground and speed through water and its application

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