What is the difference between speed through water and speed over ground?

• A. Speed through water is relative to the water; speed over ground is relative to the seabed; currents affect the difference.
• B. Speed through water and speed over ground are identical in all conditions.
• C. Speed through water is relative to the seabed; speed over ground is relative to the water surface.
• D. Speed through water is the rate of drift due to wind.

Answer: (C)

Speed through water is how fast the vessel is moving relative to the water around it. Speed over ground is how fast the vessel is moving relative to the seabed (the ground beneath the water). The difference between these two speeds comes from currents in the water. If water is moving in the same direction you’re going, your speed over ground increases; if the current is against you, your speed over ground decreases. You can think of it like this: ground speed is the combination of your speed through the water and the current, viewed from a fixed point on the seabed.

For example, if you push the hull through the water at 6 knots and the current is moving 2 knots in the same direction, your speed over ground is about 8 knots (and your track tilts with the current). If the current runs against you at 2 knots, your speed over ground drops to about 4 knots. If the current is at a right angle, your ground speed is found by combining vectors, and you’ll drift off your intended track.

The other statements either swap the reference frames (speed through water isn’t relative to the seabed) or imply they’re identical (currents always change one but not the other), or confuse drift with wind rather than water currents. The essential idea is that through-water speed is relative to the water, while over-ground speed is relative to the seabed, and currents cause a difference between the two.