DEFINITION OF COLLISION

              TWO OBJECTS PASS THE SAME PLACE AT THE SAME TIME.

This is quoted from JRC radar operation manual. In the court precedent, there are many cases of damage to the ship or property categorized as collision. In this article, we will only refer it to physical contact between two vessels. 

This definition serve to illustrate two important features of collision: at the same place and at the same time. If we want to avoid a collision at sea, we can have two choices. 

First: maneuver own ship to avoid the same place. 

Second: make sure own ship will not pass the collision space at the same time. 

From these two concepts, we can develop two collision avoidance techniques as spatial awareness and timing lapse. 

Spatial awareness: this mainly consists of the proper usage of the rudder to steer clear of all danger area. We need to get the feeling of own ship's whole body as the collision can be any point of the vessel. The space occupied by a vessel is its length over all (LOA) times max. breadth. There will be a collision space on the high sea, not the collision point only. 

Timing lapse: knowing that the collision can be avoided by varying the ship's speed (use main engine) to make necessary time lapse to pass the collision space without other ship's present.  The duration of time lapse needed for avoidance is to wait for other ship's length over all pass the possible area of collision. 

                                        RISK OF COLLISION

exists when another vessel's CPA is zero. CPA is the closest point of approach and this point is the contact point or collision point.  If two ship have enough sea room, there is nothing wrong to regard other vessel as a point in radar or visually.  For we take other vessel's bearing as one point in the compass card only and access the risk of collision by observing "does this bearing is changing clockwise or anti clockwise ?" if the bearing change is obvious in ample time and distance away, the risk of collision may deem not existed. This is the daily practice on a navigation watch. However, 

 "Such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or a vessel at close range". 

When approaching a very large vessel or a vessel at close range 

A large vessel takes more space and a small vessel at close range needs some special techniques to clear of. The collision point could be any point on a ship's length, bow, midship or stern.  If we use any point on a large vessel's hull as the bearing point, we may clear this part of ship, but hit another part.  So we should have the spatial awareness of the whole ship's length of a large vessel. We can say all clear, only after every part of other ship is clear. 

For a small vessel in a close range, we should maneuver our vessel's bow, midship and stern to clear it. For a single rudder movement, ship's bow and stern response in opposite direction which means we may turn one part of ship away from danger, but bring another part up to it. 

 These three figures illustrate that two vessel have same relative position may have different consequence with different stage of turning. 

In 1st stage: the purple vessel is sliding ahead; about to hit other vessel's break of forecastle. 

In 2nd stage: the purple vessel is hitting right in other vessel's midship section and with the tendency of ripping it open. This is the most unfavorable situation at sea. 

In 3rd stage: the purple vessel is in full swing, possible angle of blow already reduce to zero. If over-swing maybe hit other vessel by own ship's stern 

Why two vessels in the same relative position will have different collision consequence. This is the situation a ship’s master need some extra knowledge to handle it. We will explain it fully in next chapter. 

                               SPACE AND TIMING

Once we have the concept of spatial awareness, we know collision avoidance is to clear a whole ship's space. Now let's take 300 meters as a vessel's length over all (LOA) and say that we need 300 meter to clear other vessel. The collision risk is based on two major factors unchanged: course and speed. Altering course is to change the sailing space to avoid the same collision area. Changing the speed is to pass the SAME collision area at different time. The time difference needed to clear the collision area depend on ship's speed reduction. 

  time interval x speed changed  =  clearing distance 

 58 sec x 10 kts = 2 min x 5 kts = 4 min x 2.5 kts = 300 meter 

By this simple fact, we can see that the less speed changed, the more time interval we need to clear the same distance. 

It is very important to know that it’s the speed difference that make the necessary clearing distance (300 meters). 

The 10 kts for 58 sec is the speed vessel had reduced. If you can only reduce 5 kts from your original speed, you need 2 min time interval to clear another vessel whole ship's length. 

If two ships meet in cross situation, nobody will expect his ship can increase 20 kts speed at once and last a 29 sec. So, the only option is to reduce 20kts speed for 29 sec. But the instant speed reduction is also impossible for any vessel, we should make some allowance for it.  
  
  

For example : 

One 15 kts vessel steady on 270(t) course, about to collide with another vessel after 10 minutes. It is decided to reduce to 5 kts for 3 minutes. Find out how many distance this vessel's position away from its original collision position. 

Ans. : 

Distance going for 10 min is:  15 kts x 1852/60 x 10 min = 4630 meters 

Distance with 3 min reducing: 15 x 1852/60 x 7 min+ 5 x 1852/60 x 3min = 3704 meters 

The distance off is 4630 - 3704 = 926 meters # , which 

Speed difference is 15 - 5 = 10 kts and keep this speed for 3 minutes, 

10 x 1852/60 x 3 = 926 meters # (end of answer) 

10 kts difference for 3 minutes can make so much room for us (about three ship's LOA). But, ship's speed does not response so quickly as we wish. Once the engine telegraph is rang,  there is always a slow reduction in ship's actual speed and the reducing duration for this is depended on a lot of factors like, sea force and direction; wind force and direction; ship's size and construction; original speed;  draft; trim; ship hull's fouling..... . 

Speed reduction is unpredictable and it's have the side effect of "titanic effect" (lost rudder effect when propeller revolution is reduced) which will reduce the ability to steer away from our original track. 

              ASSUMED SPEED AND ACTUAL SPEED

But we have to do some estimation to give the reader a roughly idea of speed reduction process : 

Original speed is 15 kts and required speed is 5 kts. There must be have a process that the speed will slow down from 15 to 14,13,12.....then 5 kts. 

If the speed reducing need 3 minutes to complete, then actually ship's speed is 10 kts (we take average speed of (15+5)/2 = 10kts) at this 3 min. 

If the 15 kts is the full ahead speed and 10 kts is the half ahead speed. 8 kts is the slow ahead speed and 5 kts is the dead slow ahead speed. In captain's mind is to keep going in dead slow ahead(5 kts) 3 min to avoid collision, but by the slow process of speed reduction. Actually, ship is actually running in half ahead speed(10kts) for 3 min. 

This means captain have to double the time in dead slow ahead, i.e. 6 minutes to get same result. 

This is the reason why COLREG asking ship to reduce speed to barely steerage when there is a risk of collision. For this make the biggest speed difference to avoid the collision. 

From the example above, we can further calculate the time lapse we made for avoid the collision: 

After 10 kts speed reduction, ship's position is 926 meters behind her original position (in the red ship's position) and ship's speed now is 5 kts only.  

926 meters / 5 kts x 1852/60 =  6 min. 

We then will arrive the same collision spot 6 minutes late than other vessel. 

THREE STAGES OF OBLIGATION VARIED

From the definition of the collision, we had derived two concepts to avoid the collision. The first is to avoid getting into the space where other vessel will occupy (spatial awareness). The second is to avoid entering the possible area of collision in the same time (timing lapse). 

There are three stages regarding what action should be taken by give-way vessel and stand-on vessel when there is a risk of collision involved. 

In the first stage (12-8 n.m.), both vessel have their own will to do the maneuvering as they needed, regardless the collision risk. At the second stage (8-4 n.m.), if both vessel maintain their course and speed from 12 miles away, the give-way vessel have to do something (alter course or reduce speed) to give way and the stand-on vessel should keep its course and speed. At the last stage (3-1 n.m.), if the collision risk is still present, both vessel have to do the best thing to avoid the collision or abate the collision damage. 

The COLREG is aimed to avoid the collision, not to restrict the normal ship’s routing. Although the collision risk is present, with ample distance away, both vessels can alter course or speed in their normal route. Only when the range of two vessels had reduced to    less than 8 miles, the stand-on vessel’s action may impede the give-way vessel’s avoidance action. This is the time when the COLREG begin to apply. In this second stage, stand-on vessel can still do his own maneuvering as long as the stand-on vessel can be sure of both vessels’ safety and the give-way vessel had not yet take any avoidance action. Once the give-way vessel had taken the avoidance action, the stand-on vessel have to maintained his original course and speed until the collision risk is no longer exist. If the collision happened in the later stage, the stand-on vessel will be blamed for not keeping original course and speed. In the third stage, both vessels have the obligation to take action to avoid the collision. It is obvious that the avoidance action is needed by both vessels at some stage when the collision risk is present. 

However, "what is the proper avoidance action in each stage" is still not fully explored in our industry. This is the main object of this book to provide the necessary knowledge to best aid the avoidance action. Especially in the last stage, take the best aid to avoid the collision.  

But some navigator just shakes their head and says "in my twenty year's sailing, I never run my ship into the situation like that". In English, they say "Apple falls not far away from the tree". In Chinese, we say "Water jugs broke not far away from the well; General sometimes die in the battle field." the daily practice to avoid the collision at the second stage to maneuvering the vessel to clear any kind of dangers nice and clearly is a necessity for any prudent navigator. But, if you caught unguarded by any chance to face the last stage, you have to do the best thing to avoid the collision. This is the risk we have to face in our life-time career. Are we prepared well enough for this? Is their any necessary knowledge we must have as a responsible ship's master to face the most crucial situation? 

In the voyage planning, it is importance to avoid our course line to intersect with other vessel's possible route, especially when planning the inbound route into a harbor or sailing outside a harbor area. This is the first concept in collision avoidance; spatial awareness. 

If the course line intersection is inevitable, make sure we will not pass the intersection point in the same time. This is the second concept in collision avoidance; timing lapse.