When it comes to speed over water vs Speed over ground, many of us get confused. May be, it is a confusing topic. What do you feel ?
And the confusion is not only what is the difference but also where and when these are used ? Why we need to have input of speed over water in radar ? Why do we have doppler log on board ? And many more questions like these.
So today i want to put all these doubt and confusions to rest once for all. But before i start, I need you to ask yourself this question. Do you know the answers of questions I asked above ?
Let’s test it. Can you tell what is the speed over ground and speed over water in these three conditions ?
Condition A: No wind, no current, absolute ideal conditions. The GPS speed of the ship is 15 Knots.
Condition B: All other conditions being same but now we have 2 knots current from ahead. What will be speed over water and over ground in this case ?
Condition C: All other conditions being same but now we have 2 knots current from astern. What will be speed over water and over ground in this case ?
Write down your answers. Tick tick….Tick tick.
Ok, I hope you have your answers. If your answers for speed over water for all the three conditions is same, most likely you have got it. But if you have different speed of water for all three conditions, you must read on.
Speed.. What it is ??
There is nothing absolute in this world. Everything is relative to something. Speed too is measured with respect to something adjacent. While travelling on a train you might feel increase in speed when another train passes on opposite direction. Or Sometime on railway station, we suddenly feel our train moving even when it is other train adjacent to ours which has just started moving.
So what is the speed of your train in this case. You may say zero but I may disagree. Depends on with respect to what we are talking about ?
Speed with respect to station will be zero, but with respect to other train will be equal to the speed of that train.
In the same way, the ship’s speed is either measured with respect to water or ground.
Speed Through water & Speed over ground
Speed over water is the distance traveled in one hour with respect to water. Similarly speed over ground is the distance traveled in one hour with respect to ground. Now what does that mean ?
Let us understand this and look for the answers of our three conditions. Imagine your ship is moving from an island and there is a floating boat near to the island. Sea conditions are perfect with no wind and current and your ship’s GPS is showing speed of 15 knots.
In one hour you would have covered 15 NMs with respect to island as well as floating boat. This is because the boat would have maintained its position as there is no wind/current.
So in Condition A, speed over ground and speed over water will be 15 knots.
Now as in Condition B, rest of the things being same, we have 2 knots current from ahead. In this case ship would only cover 13 NM in one hour because of adverse current. So when measured from island, vessel has only moved 13 NM. So the speed over ground will be 13 Knots.
Because of 2 Knots current, the boat would move 2 NM away from the ship. The distance covered by ship with respect to floating boat will be 15 knots. So the speed over water will still be 15 knots.
Now finally let’s assume that there is 2 Knots current from astern as in condition C. In this case ship would cover 17 NM in one hour because of favorable current. So when measured from island, vessel has moved 17 NM. So the speed over ground will be 17 Knots.
But because of 2 Knots current, the boat would move 2 NM in the direction of the ship. The distance covered by ship with respect to floating boat will still be 15 knots. So the speed over water will still be 15 knots.
What were your answers ??
Interpretation of Speed Through water
As you would have noticed, current has nothing to do with speed over water. Irrespective of how much current you have, the speed over water will remain same.
How can we use this information ? Let’s see.
Speed through water for collision avoidance
If you have been sailing on tankers you would already know how much emphasis vetting inspections put on this fact. They want us to use speed over water in radars for collision avoidance. Do you know why ?
To understand this, let me give you a situation. I know many of us are fed up with ROR situations during competency exams but this one would be interesting.
Your vessel is moving on a true course of 000 Deg. You have another vessel right ahead on a course of 180 Deg. There is strong easterly current and because of that your vessel is making good a course of 040 Deg. Other Vessel is making good a course of 140 Deg.
Your radar screen would look something like this.
If there is risk of collision, what action you will take and under which rule ?
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 right 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 visualise the situation I described. Will the defination 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 over water for collision avoidance.
When we use speed over water the same situation would look as a head on situation. That is because when we use speed over water, it does not consider the effect of current on the vessel.
But in both the cases, CPA and TCPA would not change. That is because CPA and TCPA are calculated with range and bearing of the target between two or more intervals. Remember Radar plotting techniques ??
Is the GPS speed required in RADAR ?
There is no debate that GPS feed is required in Radars for position. But if we have to use speed over water in radar, do we require GPS speed then ?
Yes, we do. Infact i have come across some vetting inspector who ask to put the radar on standby. They then check what source of speed it is showing. If it shows GPS speed, they have an observation such as..
“Vessel did not use speed over water while approaching the present port as was evidenced from the radar screen in standby mode.”
In my view, this is really incorrect observation. Navigators need to use both speed over water as well as speed over ground judiciously. Speed over water is only 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, 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).
As you can see in above, with speed over water navigator may get an impression that ship is moving clear of the danger. In actual it may be drifting towards the danger.
So Navigators should use both the speed to their benefit and as a tool to better navigation.
Log speed as Engine Speed
As we have seen, current has no effect on the log speed. For this reason, in normal wind force, log speed is very close to the engine speed.
Engine speed is calculated by Propeller distance divided by the time. Propeller distance is obtained by multiplying total revolution to a constant. This constant is proportional to the pitch of the propeller.
Engine speed = (total revs x constant) / Time
In normal wind conditions, this engine speed will be equal to the speed over water (or speed obtained from log).
If it is usually not same on you ship, there are few important conclusions that can be drawn.
1. Log is erratic
This can be a good indication of you log giving erratic readings. The conclusion however need to be verified with other means too. These include comparison with the GPS speed in calm weather with no current.
2. Foul Hull
There is another conclusion that can drawn from the difference in actual engine speed and log speed. There are chances of hull having considerable amount of marine growth. This will increase the resistance to the ship and ship’s speed over water will reduce to that with the clean hull.
This can give considerable difference in engine speed and speed over water. There is another way to confirm this finding. If the hull is fouled, ship may not be able to run on NCR rpm because of excessive torque. In run on NCR with foul hull, it is very likely that you will get torque limit alarm on the engine.
Speed over water and speed over ground are two distinct tool. One does not replace other. Navigator should make full use of these speeds where these best fit.
For example, navigator should use speed over water for collision avoidance. Speed over ground should be used for navigation.
Using speed over water, a navigator can be sure of his assessment of colreg situation. Using speed over ground can give an early signs if the ship is drifting towards a danger.
About Capt Rajeev Jassal
Capt. Rajeev Jassal has sailed for over 19 years mainly on crude oil, product and chemical tankers. He holds MBA in shipping & Logistics degree from London. He has done extensive research on quantitatively measuring Safety culture onboard and safety climate ashore which he believes is the most important element for safer shipping.