5 Confusing Terms of Ship Navigation and its Clarification
If it wasn’t about safe navigation, no ship would reach its destination safely. But safe navigation is not only about steering a ship. It is also about navigators being crystal clear about rule of the roads and all the terms used in it.
But not all the terms used in COLREGS are simple or easy to understand. For example while COLREGS has defined the responsibilities in a narrow channel, it fails to define narrow channel.
If navigators do not know what is a narrow channel, how can they apply the rule ?
There are many such confusing terms and I am here to discuss some of these.
1. Underway or Making way
These are simple terms but if we go in details of it, it might not look so simple. But what makes it important to differentiate between these two terms is the inclusion of these in COLREGS.
For example, a vessel not under command is supposed to display different lights when underway and when making way.
Also a vessel need to sound different sound signals when underway and when making way.
So what is the difference between a vessel that is underway and when it is making way.
As per COLREG
The word “underway” means a vessel is not at anchor, or made fast to the shore, or aground.
In simple words when a vessel is afloat and in no way is touching the ground or seabed, she would be underway.
Now COLREG does not give any definition for a ship making way. It is assumed that it is easy to understand when a ship would be making way through water.
So let us see if we understand the difference with these two situations.
- Situation 1: A vessel was at anchor and it just picked up its anchor.
- Situation 2: A vessel is moving in open sea
These two are easy. In situation 1, the vessel is underway while in situation 2 vessel is making way. Now let is see 3 more situations.
- Situation 3: A vessel is moving while approaching a port and it stopped its engine but it is still moving with a speed of 8 knots.
- Situation 4: A vessel is moving up river with 5 knots speed and with 5 knots against current. So in this situation even though vessel’s engine is running but the GPS speed would be showing zero.
- Situation 5: A vessel is heading downriver with 5 knots following current. The vessel has its engines stopped but would be moving with 5 knots GPS speed because of following current.
Now what do you think the vessel is doing in these three situations.
A vessel is “making way through water” or not is not defined by whether or not she is using engines. It is defined by whether or not ship has any speed through water.
There is another factor that we need to take into consideration while deciding if the vessel is underway or making way. We need to know if the vessel is controllable by helm or not.
So in situation 3, vessel still has speed through water. Till the time vessel can alter her course with the rudder, the vessel is making way. But when the vessel’s speed is reduced below which the rudder is not effective and ship cannot alter her course, she would be underway.
In situation 4, vessel’s speed over ground is zero but she is still moving with respect to water. So in this case the vessel is making way through water.
In situation 5, even though vessel is moving with respect to ground, she is stopped with respect to water. Hence vessel is underway but not making way.
2. Speed over ground or speed through water
Speed over ground and speed though water are most misunderstood terms.
In simplest of terms, speed over ground is the speed of a ship with reference to ground. For example how fast a ship is moving with respect to an island, a vessel at anchor or with respect to seabed.
Speed through water is the speed of the ship with reference to a floating object.
Let us understand this with these examples
1. Calm weather with no current and wind
Let us say in calm conditions, a vessel is moving with GPS speed of 15 knots. There is one floating boat parallel to the ship at one point.
In one hour how much distance ship would cover with respect to boat and ground (island).
As there is no wind or current, boat would maintain its position. So the distance covered in one hour with respect to boat and island will be 15 NM. So in this case speed over ground and speed through water will be 15 knots.
2. Same condition with 2 knots adverse current
Now consider the same situation but with 2 knots adverse current. The ship would move only 13 NM in one hour. But the floating boat would also move astern by 2 knots in one hour because of adverse current.
Now how much distance the ship has covered with respect to island (ground) and with respect to floating boat ?
Yes you got it right. In this case speed over ground would be 13 Knots and speed through water (with respect to floating boat) would again be 15 knots.
You can do same exercise with a situation with 2 knots favourable current and I am sure you will get your answers.
So any speed that is calculated with reference to something on the ground is speed over ground. And speed that is calculated with reference to floating object on the water is speed through water.
Let us work out one more situation. A vessel is made fast to a jetty in river which has 4 knots of head current. What will be the speed over ground and speed through water in this case.
I leave it up to you to calculate.
Hint: What do you think will be the speed of the ship with respect to jetty and with respect to a piece of paper floating in river.
3. Narrow Channel or Open sea
While COLREG 1972 was being drafted, Indonesia proposed to define “narrow channels”. Most of the other countries like Finland, US and Germany rejected the proposal and proposed to keep it undefined.
The people who drafted the rules must have either assumed that the meaning is too obvious or too difficult to give a concise definition.
But the navigational incidents because of unclear definition of narrow channel points out to the fact that the meaning of narrow channel is not too obvious.
On 22nd of March 2008, an Ukrainian flagged oil rig supply vessel NEGTEGAZ 67 collided with Chinese flagged panamax bulk carrier Yao Hi. The collision took place in the western approaches of Hong Kong harbour.
The supply vessel sank because of this collision and 18 of the 25 crew died.
Captain of the supply vessel was found to have breached rule 9 (Narrow channel) because he did not keep on the starboard side of the channel.
There were number of debates on the court’s finding which considered this to be a narrow channel. Many experts did not agree with the court’s decision that it was a narrow channel.
The experts argued that even outside this buoyed channel, there was sufficient sea room for the bulk carrier.
Now if in this incidents, so many experts could not agree on narrow channel, it would be unfair on seafarers to expect them to decide correctly on this.
Even COLREGS do not give any definition for Narrow channel. It is left up to the seafarers to decide if something is a narrow channel or not.
But still seafarer would need some base to decide on if they are in narrow channel or not.
The dictionary meaning of channel is
“A navigable passage in a stretch of water otherwise unsafe for vessels.”
Mariner’s handbook defines the channel as
A comparatively deep waterway, natural or dredged, through a river, harbour, strait, etc, or a navigable route through shoals…”
Now this is the definition of a channel, as in the English channel. But how narrow a channel need to be for it to be called narrow channel.
Some define the narrow channel as
- Channel of 2-3 miles navigable waters
- Channel limited by buoys
- Channel at the river bed
- Approach to a port
But if it was so easy to define, IMO would have included it in COLREGS. While we can consider above points for assessing if a channel is a narrow channel or not, while assessing we should alway err on the safer side.
4. Restricted visibility or good visibility
What visibility will you consider as a good visibility ? Or rather what visibility will you consider as restricted visibility ? I mean how many nautical miles ?
This is another term that colregs has not defined clearly and there is a reason for that. COLREG defines what restricted visibility is but navigators may get confused if they do not have a number.
So visibility of how many miles can be considered as restricted visibility ? To understand that we need to understand what is the purpose of COLREGS ? What do we want to achieve with so many rules ?
I am sure your answer would be “not to collide”. So at what minimum distance you can discover a target and still would have time to take action to avoid collision.
But before you think of a number, consider that you have same visibility all around. For example let us say you think you can alter to starboard for a target that you spotted head on at 2 NM to avoid collision.
In this situation you need to consider that on your starboard side too, you can only spot targets in 2 NM range. So would you be sure that if you have few ships at 2.5 NM on you starboard side which you cannot spot visually and if you take action for a vessel head on at 2NM, you would be safe ?
Now if I give you same situation in open sea with not much of traffic, you may consider 2NM as safe distance to take action. This is because you may not have much traffic to analyse in open sea. But in traffic dense areas, 2NM may not be enough to analyze the situation and take action.
Usually 3NM is considered to be safe distance to take action. For a distance lesser than 3NM specially in head on situation, there may not be enough time to take action.
So this is the visibility at which you should extra look out so that you can spot the target early.
Depending upon factors particular to your own ship, you may post the extra look out earlier than that.
But if we talk about action for collision avoidance, it does not matter so much if you take action as per restricted visibility or not.
Need to know why ? That is because if you take action as per “restricted visibility” or “in sight of one another”, your action would be same. Even if it is not exactly same, it would not contradict with each other.
See these examples.
Conclusion: 3NM is usually considered to be safe distance for considering if it is restricted visibility or not. We need to consider ship’s local factors like speed and traffic density to assess the time when we should have extra look out.
For collision avoidance action it does not matter if the action is taken as per restricted visibility ot not as both does not contradict with each other.
5. Safe Speed or unsafe speed
Does safe speed always means slow speed ? Well not always. So what is safe speed ?
It is all in the name. Safe speed is the speed at which vessel will be safe. A vessel would be safe if it has the time to respond to an emergency situation.
Remember the collision of MSC Chitra in Mumbai ?
Here is the excerpt from the investigation of this incident. See what it say.
It is always about having ample time. If we have ample time, we can assess the situation and take action.
Have you seen the passenger ferries crossing the strait of Gibralter at a speed of 35 knots ? Do you think they are proceeding at unsafe speed ?
I do not think so. These passenger ferries can reduce their speed from 30 knots to 5 knots in no time.
Now imagine at same location, a ship moving at a speed of 15 knots. But to start reducing its speed, the engine room would need 30 minutes notice. That is what we call unsafe speed.
When we talk about safe speed, it is not how slow a ship is moving but how fast she can respond to a situation that require reduction of speed.
There are whole lot of other factors that need to be considered to assess what is the safe speed. These factors are outlined in the rule 6 of the COLREGS.
These factors mentioned in rule 6 basically ask us to consider two things to decide safe speed
- How early the target can be detected
- How effective the action of the vessel will be
If the target can be detected easily, for example in good visibility, we can proceed at more speed. But if the target cannot be detected easily then we should move at slower speed and have our engines ready.
There are lot of factors that can reduce the effectiveness of the target detection. Some of these are
- The visibility
- presence of background lights
We also need to consider how effective the avoiding action that we take would be. For example in an area of dense traffic, we cannot just alter our course for a developing situation. We need time to analyse the traffic.
If we are moving at higher speed, the time is what we would not have, specially for head on situation.
There is one theory that says that Titanic did not have the binoculars as the navigating officer could not find the key of the store in which these were kept.
Absence of the binoculars meant that targets could not be detected early. So the ship should be proceeding at slower speed. But the titanic was still proceeding at full speed and the result is the history.
There are many terms in ship navigation that are not clearly defined. This is either because the terms are too obvious or the scope of a term cannot be defined in few words.
Whatever the reason, navigators should be clear about these terms. While assessing their own definition, navigators should err on the safer side.