A 300 meter long 32 meter wide vessel sail over the sea surface will take some space at any minute. Another 150 meter 20 meter wide vessel will also take some space. If these two vessel’s spaces don’t overlap at a specific moment, there will be no collision risk between these two vessels. If the collision risk do exist, the spaces took by these two vessel will due to overlap at some time lapsed. To avoid the overlapped space will help to reduce the collision risk. This is the first concept of collision avoidance. In this chapter, we will study the relative position change, i.e. occupied space change, when give-way vessel takes the avoidance action. 

Linear space change 

A ship takes some actions to avoid the target 150 meters dead ahead. After the ship successful move 200 meter ahead without collision, she still collide with the target. 

This 150 meter is the linear measurement dead ahead from the ship's bow. If her length over all is 285 meter, she still has the chance to hit the target 50 meter abaft ship's bow. So, this ship has to maneuver successfully moving ahead 150+285 = 435 meter without collide with the target then she can say "all clear of the target".  

The whole ship's body is the space we shall aware of when we take the collision avoidance actions. Only take the linear measurement into account is not enough, the avoidance action of the give-way vessel has many restrictions. The vessel’s body moves to opposite direction when the rudder put over to initial the turning.  

If we were to use the rudder to avoid the dangerous space of collision, we should have some basic knowledge of the ship body response to the rudder been used. Let's go through the turning characteristics of a particular vessel with different rudder angle been used. See the diagram below, then 

We may divide the turning process into three stages: this turning diagram is a single screw, fixed pitch propeller and single rudder ship. Other vessel construction may have different characteristics. Reader should use their knowledge to apply these points we discussed here.   

First stage: the rudder is put over, there will be a still time which seems the ship does not response at all then she will incline to the side where rudder is put over. 

Second stage:  ship's heading changed rapidly but her body still roughly follows the original track and ship flip over to the opposite side which rudder is put over. 

Third stage: ship's heading changed in regulated rate and ship's body actually leaves the original course line. This is the stage ship can gain abeam distance from her original course line. 
These stages had simplified deliberately for easy memory. 

Now we can examine the turning curve closely. The four black curve line is the track of pivot point of vessel with 5,10,20,30 degrees rudder order been used. The straight line connected the red dot in black curve is the position of pivot point along the turning curve which heading had changed from original course. Each straight line is spaced with 10 degrees difference of heading. 

This is a 285 meters long container vessel (Panamax size) and recent research show the ship's speed is irrelevant to the turning curve. (the ship's speed can only affect the time needed to finish the turn.) Each vessel's characteristics of maneuvering are different and captain have to use his own knowledge to evaluate the actual distance of these stages. 

In this diagram, each stage is using 0.25 nm to represent. This will be too handy and may lose the real connection of vessel's characteristics. Nevertheless, the ship’s length over all is close related to the necessary advance to accomplish each stage. To simplify the discussion, we will use two ship's length to represent each stage of turning. 

      INITIAL TURNING WITHIN 2 SHIP'S LENGTH

In this stage, represented by the first red line vessel shape in the 0.25 nm range, the turning curve are almost identical regardless how much rudder we have used to change the heading. But, remember this is only the track of pivot point. The ship body is conducting a serious change from the bow to the stern. The ship bow will turn more inside the track of pivot point. The ship stern pushed by the rudder plate will always be leaved outside of the track of pivot point. The pivot point is the point neither moving inside nor moving outside when the ship body rotated by the rudder effect.  

SIDE KICK

Inside this advance (2 ship's length), when the rudder been used, ship's pivot point (pp) almost occupied the original course line regardless how many rudder angle been used. But, ship did response to her rudder. The stern will sheer away from the side rudder been used, this is called side-kick of the stern. The ship bow will rotate to the other side. Since the pivot point is located one fourth ship's length from bow, the whole ship's body will look like moving sideway to the opposite side of rudder order. 

The amount of the side-kick depend on the rudder angle been used. The more rudder angle used, the more side-kick will be. This is the only effect under ship’s master control in this stage.

As we say spatial awareness, our concern is the whole ship’s body. If one small fishing boat is found dead ahead suddenly, that is the usual case when fishing boat is just too many or just come out from his hand-held torch which light up at the last moment in the night time. 

If we rely on a simple turn to avoid the collision like we usually do at sea, we may avoid the collision in ship's bow. But, hit the target by ship's stern, like the drawing above. 

It means we need more considerations than just one simple turning action at this close range to avoid the collision. 

Serpent swing

If a small rock is dead ahead of a snake, the snake can still pass it without change its original heading. It body is not going in the same direction as its heading. Its body constantly sways to both side leave some ahead ground along its track untouched. 

Since the ship's body won't leave its original track at the initial stage as we can see in last paragraph. After two ship length’s advance, the pivot point will begin to leave it's original course line. From here up to 4 ship's length, the ship's stern will not well clear of the original course. If the small target’s range is in 2-4 ship's length ahead, we have to sway it like the serpent to clear. 

The proper maneuvering in this situation should use the rudder to swing off the ship's bow first, and then at some stage put it hard over to opposite side to kick off the ship stern out of the collision space. 

At what stage is appropriate to put the counter rudder over? 

In theory, the ideal position should be when the target is abeam of our pivot point. But, we should make some allowance for the over-swing when the counter rudder put in use. For the picture above lost control due to hard port turning, the big vessel hit a small vessel in anchorage, the maneuvering should use the counter rudder 10 or 20 degrees before own ship parallel to the anchorage vessel to stop the swing. Then, see if own ship have the tendency to continue the turn and finally parallel to it. If not, apply the rudder again to parallel. If the swing is too fast and about to over-swing to hit the target by stern, the counter rudder should apply more to check the swing and keep the same heading as the anchorage vessel.     

It is easy for us to discuss "the perfect maneuvering" here. In the real case, this kind of maneuvering need lots experiences and practices. Above all, it is the knowledge of this possibility to handle the ship’s movement of the bow and stern. No body prepares us before we stand before the mast. 

 Let's take another factor into consideration.  

IMO BLIND AREA

Two ship's length or 500 meters whichever is lesser is the IMO resolution about the minimum visibility requirement, which means the blind area from bridge looking ahead of ship's bow should not exceed this distance. The reason is clear from the discussion above. The ship is actually immobilized in this range to create any clear space to avoid the collision. The only technique we can use is the side-kicks of aft part of the vessel when target is inside these two ship's length range. The side-kick effect has very limit usage to avoid the collision. We will see the actual distance calculation later.  

Thus, it is prudent to keep the dangerous target in sight during all the watch and maneuvering. The blind area ahead of own ship is the area beyond ship handler’s control. Make some estimation of the ship's blind area distance before ship’s departure is to understand our weakness in ship-handling. 

 
If the target is 2-4 ship's length ahead, the initial rudder used to avoid the target may or may not clear of the target. It depends on "how many distance remained after the first swing?  In another words, does the target already within two ship's length distance? If it is so, the target is already cannot see from the bridge cunning position. The only means we can use to avoid the collision will be the side-kick of the stern and the swing of the bow about the pivot point. 

If the target cannot see in the center cunning position doesn't mean it cannot see on the wing of the bridge. If the master can change the cunning position to the side of the bridge wing like he usually do when the ship is berthing. The visible range will vary along the ship side. The blind area will be on the starboard side and forward of the ship when master stands on the port wing. The blind area is actually enlarged, but we can get some extra visible sector along the port side and the blind area will reveal as the ship’s bow continue the swing. From which side of bridge wing we should go to check the target's position? Usually, it will be the opposite side of the bow swing. What happen if we still cannot see it from the bridge wing? We may have to run to another side to see "does the target have change the course or stop the engine at the last moment?" Or, the option will be assign another officer to check the other side to verify the target’s movement. By checking on both side of the bridge, the actual blind area will be reduced.   

In very rare case, the target cannot see from both side of bridge which means the target is dead ahead. At this moment we may have no choice but continue the swing or order more rudder angle to accelerate the swing. If the target have changed the course or stopped the engine at this actual blind area, the use of counter rudder will not have any effective use. For the initial turning will have to take two ship's length distance to be effective. 

Other than this, keep the visual contact of the target is vital in the collision avoidance. Like the perfect maneuvering we say above, keep the target in sight will give us the correct picture of the target’s movement and even more the tendency to collide the target by stern. For the ship’s stern will sweep over the opposite side of the bow turning. 

Let see the 10 degree heading change line in the diagram. It is obvious ship's bow will change only about 10 degree in initial stage of turning. Because every ship's deferent in this regard, this is only the assumption we use to begin our discussion. 

Ship is turning on her pivot point. The pivot point is located at about one fourth ship's length from the bow when ship going headway. 

Ship's length is 285 meter. Pivot point will be 285/4 = 72 m from bow. 

With 10 degree turning ship bow will leave the original course line 

By (72 m x sin 10 deg =) 12.5 meter 

The ship's stern is three fourth ship's length from pivot point. The transverse displacement of the stern will be 

285 m x 3 / 4 x sin 10 deg = 37.5 meter 

And the pivot point will go along almost it's original track. 

Now we got the idea of how is the transverse displacement of three parts (bow, pp, stern) of the vessel. At the end of the turn after two ship's length, the bow will shift 12.5 m, the stern will shift 37.5 m and the one fourth from the bow (pp) will remain on original course. 

If the midship section is the widest part of the vessel, the side-kick of midship shell plating will also have 12.5 meter (one fourth from pp) which make the whole ship's body seems like kick away by the rudder effect. The 12.5 meter from the bow will only be wide enough for passing clear of a 12 meter long vessel. 

But, it is enough for avoiding collision with a beacon or a buoy. In the emergency, it's worth trying to avoid fixed object by these side-kicks. 

              TURNING UP TO 4 SHIP'S LENGTH

IMO TURNING ABILITY RESOLUTION

Heading change will accelerate at this distance(2 to 4 ship's length) after initial stage. How many degrees it would turn depend on many factors like rudder angle used, rudder area, draft, trim, water depth, wind, current, bottom contour…,etc. All these factors will have some effect on the pressure field around the ship’s body which are not our intention to discuss here of the turning mechanism. The IMO turning ability requirement set out a standard to exclude those factors may affect and specify the sole use of the full speed and full rudder angle, ship should be able to complete a 90 degree turn within 4.5 ship's length advance. 

After the first stage, the heading change is about one compass point (11.5 deg) as we see in last paragraph. With two more ship's length advance, the pivot point advance direction could change from 15 to 45 degree from her original course at this stage. And her drift angle which can be varied from 15 to 30 degree (drifting angle is the angel between her heading and the direction of pivot point advanced). Together, the ship heading change varied from 30 to 75 degree. 

The whole ship's body movement measure from the pivot point still have the component to advance along the original course line and this component slowly decrease to zero when the pivot point advance direction change to 90 degrees (heading will be 90 degrees plus drift angle). The ship's stern is 3/4 ship's length abaft from its pivot point and still have not clear the original course line in this stage.  

Let's take a second look of the diagram above. The pivot point track is depending on how many rudder angle we have used. The pivot point position look like had take a ship's length transverse distance if the rudder order is hard starboard. If the rudder order we used is starboard ten, the pivot point will off the track only about half the ship's length. With the ship stern 3/4 abaft the pp, there still have 1/4 ship's length inside the original ship's course line. 

Using more rudder order can get out the danger area more quickly. 

POINT OF NO RETURN

With 4 ship's length distance advance, ship can change her heading from 30 to 75 degree. This is usually enough to cope with the need of course change in the fairway and river stead unless it's a sharp turn. By this analyst, it is obvious that any sharp turn must have the radius of at least 4 times of ship's length. In ship handling, this is the point of no return where vessel should not get any closer than this distance to any danger, otherwise vessel won't have enough sea room to turn back to sea. By the full rudder turning, a complete turn will need 4.5 ship’s length advance. However the full rudder turning is not a preference in normal ship handling. We need more distance to turn comfortably. The knowledge of the turning can be completed within 4.5 ship’s length is a mantle requirement to conduct the turn. The actual advance needed to complete a 180 degree turn will depend on the ship’s maneuvering ability. Master should make the correct estimation before the time when this maneuvering without any tug's assistance is needed.  
  
   

                      FINAL STAGE UP TO 6 SHIP'S LENGTH

Ship' can almost have enough transverse distance after 6 ship's length advance regardless how many rudder order (minimum 5 degree) been used. This is demonstrated on the turning curve drawing represented by the yellow vessel crossing the original course line. The main interest in this distance is what the ship’s track is through different rudder angle been used. Her heading may vary from 45 to 120 degree. The turning radius is varied from 4.5 to 9 ship’s length advance. The turning curve of 5 degree or 10 degree rudder angle been used is not fully illustrated on the drawing. The observation here is the transverse distance made out by 6 ship’s length advance is enough for avoiding the possible area of collision by using the rudder angle along.  

For a 280 meter vessel, 6 ship's length is 280 x 6 = 1680 meter. Usually we take other ship distance from our own bridge, so 1680 meter plus some distance from the bridge to the bow is about one nautical mile. Some recommendation of collision avoidance state the minimum distance allowed to get close into other vessel in confined water is 7 ship's length. Based on the discussion above, this is quite reasonable.  For easy memory, one nautical mile is our dead line for any vessel have collision risk can get close to us before we take necessary actions. 

In one N.M. distance, it is clear that the ship's avoidance action by the rudder along should take three separate parts of the ship body movement into consideration whenever she conduct a emergency turn to avoid collision. These are ship's bow, pivot point and her stern. 

At the first stage, the whole body advance is unavoidable and the side displacement is not apparent. Heading change about one point and can make a transverse away from its original course line about 15 meter for a 300 meter long and 30 meter wide vessel( 300 meter x 1/4x sin 11.25 = 15 meter). Ship's stern can create a side displacement more vivid and quickly than the ship's stem ( 300 meter x 3/4 x sin 11.25 = 45 meter). For any target size bigger than 15 meter, any action you take to avoid the collision is too late because you cannot clear the possible area of collision completely. 

In the second stage, the heading changed considerably and the side displacement is varied as vessel’s turning ability, but the ship's body still cannot create enough side displacement more than own ship's length. The whole ship body still has the residue speed to move along the original course line and flip over to other side of the rudder order used with 5 - 15 degree list. The ship's stern swing out toward the possible area of collision and the distance to it still reducing, but the side displacement to avoid danger is not enough for a large target. 

In the final stage of turning, ship's stem and body moving sideway is usually enough to avoid the possible area of collision. But," does the ship's stern can also clear of the possible area of collision?" 

We can roughly say that the turning action taken to avoid the target inside one N.M. range. The more closer to the target will have more parts of the ship's body have to take into account. 

The possible collision parts of own ship in the collision 

In first stage, will be bow, midship section and stern. 
In second stage, will be midship section and stern. 
In the last stage, will be the ship's stern.