Speed Over Ground
The speed of a yacht through the water will vary from its speed over the ground (SOG) because the water itself is moving over the ground.
Imagine that we are sailing at 5 knots and the tide is moving towards us at 3 knots. We may feel happy that we are making good speed through the water but our SOG is not as impressive because it will be a paltry 2kts.
Bear in mind that if we plan our passage carefully so that we are sailing in the same direction as the tidal flow then not only will we not have our SOG reduced but we can have it increased. Our boat speed of 5kts plus the tidal speed of 3kts will give us a total of 8kts speed over ground.