Navigation and Chart work - Speed Over the Ground(SOG)
Boat speed over the ground
It is important to realise that the speed through the water will always be different to the speed over the ground if there is a tidal stream.
This is easy to imagine if the tide is travelling directly with or against the boat, but not so simple when it is travelling at right angles to it.
In each of the above diagrams, the boat travelled 5.0M through the water. In the first, the tide is directly against the boat, so the boat only covers 3.0M over the ground, in the second example the vessel would cover 7.0M over the ground. In the third, the boat covers 5.3M over the ground-but in a different direction.
The distance travelled over the ground is the distance made good. If it took one hour to make the passage the boat's speed over the ground in the first example would be 3.0kn whilst its water speed was 5.0kn, whilst in the second example the boat's SOG is 7.0kn.
In the last example the boat's speed over the ground would be 5.3kn and its water speed would still be 5.0kn.
The direction the vessel travelled over the ground is the ground track. This is always marked with two arrows and usually given as a true bearing.
Normally you would not draw in the ground track when working up an EP. You have already travelled along it, if there was a hazard you would have already found it!
Always use the tide if possible
The examples above show why it is vital to sail with the tide whenever possible. Frequently people think "it only 2 knots, it won't make much difference". In fact, if you can not avoid the tide the difference is between 3kn and 7kn, a huge difference in passage time.
In a power vessel that can travel at much higher speeds when the sea is flat enough, this is less important. It may even be faster to go against the tide if this means that the wind and tide are in the same direction, a situation that produces flatter sea conditions, and may allow the vessel to travel at a higher speed.