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Why Speed over Water

"What is your speed?"
"Well it depends."

"Depends on what? On what it is referenced to."
"If you are in a train, and I asked you what your speed, then how would you answer it?"


Yes, it depends. Depends on what you reference your speed to.

Your speed relative to the train is zero, your speed relative to the ground is equal to the speed of the train, and your speed relative to the sun is about 108,000 kph ( Let us ignore if you are going west or east) . See!

The gleeful child by the railroad waving at you in the train would see you pass by him at 100kmph or so. He is referencing you to the ground. The child in the seat opposite yours sees you perfectly still. She is referencing you to the train.

Speeds are always relative. For landlubbers it is always referenced to the ground and so is easily understood. Whatever speed is spoken of, is relative to the ground. But for us sailors, we are on water. And water moves. So when we are speaking of the speed of a ship, it is important to know what the ship’s speed is referenced to.

Water may have its own speed. When a ship is stopped still on the water, its speed referenced to water is zero. But since the water is moving over the earth (ground), the water has some speed with reference to the ground. If I were floating on the water by the ship, I wouldn’t realise that the ship is moving.

When this ship starts its engines ahead, it begins to push itself forward. Now it moves at a speed different from that of the water and so develops a speed relative to the water. If I were floating on the water by the ship, I would now see it moving. This speed maybe different from the speed of the ship relative to the ground. The ship now has two speeds one relative to the ground and another relative to the water. This is in addition to the speed that its engine is theoretically expected to deliver- The Engine speed.

Well, so far so good.

“Speed over water should be used on the RADAR for collision avoidance”. For the previous generation of navigators, this statement was drilled into them by the various Oil major inspectors who came on board and asked the question – “What speed do you use on the radar for collision avoidance?” We were drilled to answer “Speed over water” without actually thinking through why. Many among us used Speed over water for collision avoidance just because!

And then the next generation of seafarers continued the practice without actually questioning it. Many senior masters fear that this incessant drilling of speed over water will make junior officers reluctant to use speed over ground at any time. They may feel that speed over water is somehow better than speed over ground and should be used no matter what. This is a genuine concern because it is speed over ground that will help you determine if you are moving towards that shoal or the buoy.

What speed you use as a reference on your radar should depend on the danger you want to avoid. In open seas when collision avoidance is the prime concern, there should be no debate. Speed over water wins hands down.


Why Speed over water?

The use of speed over water or that over ground would not make much of a difference on the ARPA so far as its calculation of CPA and TCPA are concerned.

However the reasoning for the advice that speed over water be used for collision avoidance is an altogether different one .

The collision regulations don't require you to take action basing it on CPA and TCPA information. The rules that require you to take a certain action expects you to base your decision on the aspect of the target.

It is with the aspect of the ship that you determine whether you need to give way or can stand on. You will alter course for a crossing ship on your starboard side. You will remain stand on for a crossing vessel on your port side.

All collision avoidance actions are based on the aspect. For an officer on watch to determine whether he needs to act or to stand on, he will have to look at the target. And in order for him to be able to correspond the actual scenario outside the window and that on his Radar PPI, be will need the speed referenced over water.

If referenced over ground, he will see a vector on the screen which is different from the actual heading of the ship and so MAY be disoriented.

But if you are navigating a narrow channel or a buoyed one, referencing your speed to the ground will help you see whether the ship is moving towards a buoy or a shoal. And here the greatest danger is that of running aground or allision with a buoy. It also shows the drift while turning the ship. And so speed over ground works best here.

While in narrow channels, if you have two radars, then the best option would be to have one referenced over water for collision avoidance and the other over ground to determine set and drift.

Originally Published here: in 2014

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