One night in September 2003, a loaded Panamax vessel headed north trying to cross the east-west coastal traffic had a collision with a small chemical vessel. The weather is fine and clear. The small chemical vessel half submerged on the sea surface with ship bow pointing the sky. All four crew were able to get out on time, although had some wound more or less. The chemical vessel sunk very quickly after the collision. Luckily enough, the 400 tons chemical loaded is Xylene that is a highly evaporable substance. The impact to the ecosystem is minor. The relative low bunker quantity remained on the small chemical vessel won’t pose another threat to the fishery resources. 

Before the collision, the Panamax vessel is steering course  with 19 Kts and the chemical vessel is steering  with 11.5 Kts. The Panamax vessel had detected the chemical vessel 6 N. M. away with CPA of 0.4 N.M.. The master had to take care of other east-west traffic with even less TCPA. Two minutes before the collision, master found the chemical vessel’s CPA had reduced to 0.2-0.3 N.M. and order STARBOARD 10. When the ship responded to the rudder and commenced the turning, the chemical ship had vanished into the blind area. Captain estimated the time interval needed to see the chemical vessel again on port bow. But the chemical vessel didn’t show up as captain’s expectation, captain order STARBOARD 20 to accelerate the turning rate. The chemical vessel still cannot see at port bow in this stage. Captain order HARD STARBOARD to clear the chemical vessel and the collision happened. 

By all the useless actions of the starboard side turning, the captain assumed the chemical vessel should be hit by port side. From the statement of seaman rescued from the wreck, the chemical vessel had made the round turn to port side before the collision. The chemical vessel was hit at starboard side pump room which is located between third cargo hold and the engine room. The collision flooded the pump room and the engine room behind it causes the vessel sunken. 

There are several issues of this case. We will try to discuss here with our concept of collision avoidance.

Ample sea room

Two ships occupy the same space at the same time is a collision. Each point along own ship course line across other vessel’s course line is a Possible Area Of Collision (POC) point. Keeping this in mind, we will put every leg of the course line very carefully and try to avoid crossing other ship’s course line as minimum as possible. This Panamax vessel’s course crossing the east-west traffic at full sea speed. The safe and comfortable distance of the coastal vessel is 0.5 N.M. comparing with the one N.M. of the ocean going vessel. The difference comes from the ship’s length. Coastal vessel has 150 Meters length over all with 6 times ship length for necessary advance to avoid collision. The safe distance is 150 M x 6 = 900 M which is 0.5 N.M. When the traffic is heavy, all the coastal vessel proceed at 0.5 N.M. spacing or even less for traffic in reciprocal direction. The ocean going vessel need one N.M. to pass the coastal traffic comfortably. 

Course line crossing with other traffic

In this case, there is no comfort. The Panamax vessel’s course line is crossing every east-west traffic coastal lane at almost right angle, Ocean going vessel will have to have alter course to starboard to make ample sea room for stand on vessel. When the coastal vessels come in a bunch with 0.5 N.M. spacing, there is no safe distance left for captain to pass through directly. If the Panamax vessel can not adjust the timing to arrive at these junction point with suitable spacing from east-west traffic, the solution from the route planning point will be roundabout.  

Possible area of collision

The possible area of collision is two ship’s length. If the Panamax vessel can go round this area earlier, the collision with the brown vessel can be avoid. The problem is Panamax vessel had to stand on her course and speed to let the blue vessel to pass.      

The drawing above show the possible scenario when the Panamax vessel took one ship’s length earlier action. Panamax vessel cannot avoid the possible area of collision with the blue vessel.  

But, the situation may be ease off if the Panamax vessel can round off the possible area of collision even earlier, the collision risk will much more reduced. These early actions can adjust the suitable point to cross the heavy traffic. More or less, reduce the original speed. However, this is not the standard solution according to the COLREG. By the rule 10, vessel should proceed in the general direction of the traffic flow. In this case, it is east-west direction.

Panamax vessel circle outside of the general traffic route and change the approach direction. to the east-west traffic. This circling has two purposes. One is to adjust the crossing spot. The other is to change the approaching direction.  The Panamax vessel cannot change course to starboard side directly, as it will block the blue vessel’s way and against the COLREG. By circling outside the traffic, the collision risk to the chemical vessel is diminished. However, the collision risk still cannot reduce. So, the Panamax vessel needs to go some extra miles to the east and find the suitable position to cross the east-west traffic. If the starboard side has not enough sea room, the Panamax vessel will have to change the approach direction to the east and wait for the right time to cross. 

This circling maneuvering is not a new idea, only now we know the specific purpose is to cross the traffic 0.4 N.M. starboard side from the original course line. If this circling has to be done, it is then better to do the circling 6 miles away when necessary collision data has been gathered. The early action can make sure own ship has enough sea room to adjust to the correct spot to cross. Even own ship has to change the approaching direction to east, the early action will not confuse the blue vessel .

DTC concept

As we had discussed, the minimum distance to take the avoidance action is 7 SL. 7 SL is 1,995 meters(285 m x 7 = 1,995 m) By the speed of 19 Kts, 7 SL advance takes about 3.5 minutes(TCPA). Two minutes before the collision, master took avoidance action. If the collision target is also a Panamax vessel, this late action is already in fault. 

The action distance is 1,172 meters (19 kts x 2 /60 x 1852 m = 1,172 m) which are 4 times ship’s length. Even for avoiding a smaller target, the 4 SL is not enough unless the ship use the hard over rudder. By this knowledge, the captain will not use only starboard ten rudder order at the first opportunity.  

2 SL advance

When the ship responded to the rudder and commenced the turning, the chemical ship had vanished into the blind area. By the statement above, we know the vessel had advanced two SL and the chemical vessel already sail inside two SL from Panamax vessel’s bow. This coincide with the captain’s statement “take avoidance action two minutes before collision” which by our calculation is 4 SL. Once the collision target get into this distance, captain had no choice but continue the turning with more rudder angle. If the captain try to use the counter rudder to clear the target, it is surely in vain by the knowledge of previous chapter..

Full rudder turning

In the heavy traffic, full rudder turning must use with caution. If the hard over rudder is used, it should not be used to make the full swing. It should only be used to initial the turning. But, the full rudder can only accelerate the turning rate after initial 2 SL advance. In the first 2 SL advance, the ship is still immobilize in her original course. Using the full rudder to avoid the smaller target is only useful when the target is within 4 to 6 ship’s length. After the ship response to the rudder, the hard over rudder should be reduced in time to avoid the ship lost control.

4 SL Range

Once the small target already inside the 4 SL range, by our discussion before, there is no cue by using the rudder along. But, this 4 SL rule also apply to the small vessel. 4 SL For a Panamax vessel.285 meters x 4 = 1140 meters. These 1140 meters is 7.5 SL for a 150 meters long vessel. That’s why when the Panamax vessel waiting anxiously, the chemical vessel had conducted a full swing to the portside and be hit at starboard midship section causing the sinking. 

Keep the target in sight

If by using the rudder along cannot avoid the collision, the Panamx vessel should stop the engine immediately. This can give Panamax vessel some extra time to access the situation. But, it’s still cannot reduce the collision risk for the chemical vessel is still no where to see. This is another very important concept keep the target in sight all the time. Remember the steering rule of the COLREG is two ship in sight of each other. Once the Panamax vessel lost visual contact with chemical vessel, the rightness of changing course to starboard side will no longer exist. 

Change conning position to shipside

The bearing change of target is only accelerating after the Panamax vessel had began the swing. From the statement of Panamax captain, the chemical vessel vanished into the blind area after the ship response to the rudder. Before the Panamax vessel response to the rudder, the bearing change is going very steady. Ship’s blind area won’t change every minute. The captain knows by heart when the chemical vessel will sail inside the blind area and he should change the conning position from the bridge to ship side to keep the chemical vessel in sight.

Blind area as wide as the ship’s width

No matter how many meters ahead of the ship bow is block by any reason. If captain can check the visual contact from each side, the limitation of the blind area is only 32 meters wide as the Panamax ship’s width. The chemical vessel is 150 meter long. There is no escape from sighting unless the target is end to end with Panamax vessel which is not the initial case. If only the captain can spare some time to check of ship’s side. The collision risk will much more reduce.  .  .  .          

Possible area of collision

At the 6 N.M. range’s assessment, the CPA is 0.4 N.M. What if the Panamax vessel alter course 4 degree to the starboard side at 6 N.M. range ? The transverse distance by this course change is 0.4 N.M.(). The Panamax captain’s statement “Two minutes before the collision, master found the chemical vessel’s CPA had reduced to 0.2-0.3 N.M.”  The difference is due to the limitation of the navigation aids. The captain of Panamax vessel decide to take action is out of the consideration of Possible Area of Collision which is 2 SL. Two ship’s length of Panamax vessel is 0.3 N.M. 

The way to treat the coastal vessel

The collision is due to the chemical vessel take action to avoid the possible area of collision. The decision is to alter course to port side. Where is the blue vessel when the chemical vessel alter course to port side? To the observation of the chemical vessel captain, there is a ample sea room between Panamax vessel and the blue vessel. The Panamax vessel did not take action to sail as close as possible to the blue vessel, i.e. follow the blue vessel’s track. As the discussion earlier, the coastal vessel needs much lesser sea room than the Panamax vessel due to the ship length difference. Reducing the sea room between the Panamax vessel and the blue vessel is necessary due to coastal vessel’s spacing is much more narrow. If the Panamax vessel follows the blue vessel’s track, to the chemical vessel’s observation, the decision to alter course to port side will have to avoid the collision to the blue vessel at the same time..