Speed through water VS speed over ground

Which one is more critical for ship’s navigation and collision avoidance: the speed through water or the speed over ground?


Speed through water is required to be used for collision avoidance, and not necessarily for navigation.

By using speed over ground, a navigator is more aware of the situation than otherwise. For example, if the ship is drifting towards a danger, the navigator will know it better if he has speed over ground in radar. This is because, with speed over ground, radar will show a vector of course made good. (Vector need to be selected as true in this case).

Let me give you an example,

Your vessel is moving on a true course of 000 Deg. You have another vessel dead ahead on a course of 180 Deg. There is a really strong easterly current and because of that your vessel is making good a course of 040 Deg. The Other Vessel is making good a course of 140 Deg.

If you think you would take action as per crossing situation, you would be wrong. Collision avoidance rules and situations are based on how we see the ship and not on how they are moving.

For example, as per ROR, Head on situation is when you see a ship dead ahead or nearly right ahead. That is when you can see both her Mast headlights in line or nearly in line and both of her sidelights.

Now just visualize the situation I described. Will the definition of head on situation fit in this situation ?
Yes it would

But the problem is that if you follow just the radar, It will give you a false sign that the situation is a crossing situation. This is because the radar would be showing the course made good of both the ships.

This problem is tackled by using the speed over water in Radar. And that is the reason vetting companies require use of speed through water for collision avoidance.

When we use speed through water, the same situation would look as a head on situation. That is because when we use Speed Through Water, it does not consider the effect of current on the vessel.

Hope that helps :slightly_smiling_face:


The answer is the Speed through water. For a collision avoidance between two vessels under way, the current is a factor which affects both vessels.


@Nick @ptoul61 Excellent answers, thank you both very much!