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