Risk of collision

Every navigation watch starts from lookout and ends in lookout. Lookout should comprise of all available means, visual, hearing or other navigation aids. Today, the ARPA systems are compulsory by SOLAS. It is always handy to get the collision data we need in ARPA. But ARPA is only better than radar because it have the ability to do the calculation of ship's speed vector. The ARPA can help us in memorizing the data of target and the machine can even show the speed vector of each target been picked up. But these ARPA data are only useful when the target is positive identified and the traffic is light at sea (heavy traffic tend to confuse the target identification). Any limitation adverse to radar detection is holding true in ARPA.

Limitations in radar detection such as: 

 (a) Small targets cannot be detected in long range. 
 (b) The radar scanner is often not installed in an ideal location. 
 (c) The number of targets frequently exceeds the ARPA's capacity. 
 (d) Radar resolution is not good enough to distinguish a small target from a big one. 
 (e) Sea clutter swamps a small target at close range.

ARPA cannot do everything to fulfill our lookout duty and it tends to lost target as the radar does. Even the target been all picked up, the most challenge part is the positive identification of the correct target. This situation happened mostly in the night time when a ocean going vessel sails close to a school of fish boats and one of them want to go home early and sailing the different course as others on the sea. The tendency of the OOW in this situation is: give up the visual contact and concentrate on the ARPA screen. Determining the proper course by rotating the ARPA electric bearing line to a direction seems free of the most probably collision area attached on the end of speed vector of the targets. Giving up the visual contact of the targets is because the mariner has no time to ascertain the target by comparing the gyro compass bearing. If we have a OOW dedicated to the this specific job(ARPA observation and target identification) will be a great help, for ARPA observation alone need systematic attendance and consist of lots experiences to master the CRT picture. But, young officer may just distract by other errands on the bridge too often. Sometimes, ship master has to take it all. Imaging even the navy will shoot down their own airplane regardless how many OOW on CIC(naval bridge : Combat Information Center) and how many electrical identification aids they have. We need some visual techniques to help us to overcome the radar limitations and take the correct actions to avoid the most dangerous target. 

COLREG Rule 7 Risk of Collision (a) Every vessel shall use all available means appropriate to the prevailing circumstances and conditions to determine if risk of collision exists. If there is any doubt such risk shall be deemed to exist. (b) Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects. (c) Assumptions shall not be made on the basis of scanty information, especially scanty radar information. (d) In determining if risk of collision exists the following considerations shall be among those taken into account: (I) such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change; (ii) such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range.  

In 7(a) If there is any doubt such risk shall be deemed to exist. The point is mainly aimed to encourage the give-way vessel to take proper action to avoid the collision when other vessel’s bearing change is not obvious. It is interest to note that in a study from the aviation industrial of the so called mid air collision. There is a phenomenon called blossom effect. If the image of a target getting bigger and bigger, it means the risk of the collision is imminent. When the image augment is more than the bearing change, the OOW will confuse of the risk of collision. This is the time rule 7(a) should be applied.    

The workload composed by the COLREG 7(b) is replaced by the ARPA functions if properly set up. The COLREG rule 7(c) stated the importance of make sure of the risk of collision by cross reference of all available means. 

The COLREG 7(d) gives us the rules of ascertaining collision risk by checking a target's true bearing change that we can easily take from a gyro repeater. By using the short term's memory, man can only remember two or three set of bearing reading. Most people have the problem to remember the eight or less figures of a telephone number. If these two or three set of bearing readings belong to one target only(taken by consecutive observations), it will be fine as long as one OOW concentrate on memorizing these bearing change and use it to access the collision risk. If there are three targets at the sea, one can only use his short term memory to remember the first sets of bearing reading of each target. The problem now is that you need to remember a further two or three sets of bearings to compare with the first sets to ascertain the collision risk. A prudent mariner may take out his scrap paper to note three targets bearing set by set and establish the collision risk assessment. However, this is quite time consuming and in the night time he will need a flesh light to help his work. By the restriction of human’s working memory, we can say the visual technique by using the true bearing take from the gyro compass is only useful with one target. Multiple targets situation needs more sophisticate technique.     

At some stage of our career before the mast, we will have to have the ability to pick up the most dangerous target's bearing and distance visually from multiple targets situation. If all the necessary data to avoid the collision have to come from the ARPA, mariner will rely on the machine always. If mariner can practice some visual techniques to ascertain the collision risk, these techniques will become part of his intuition (his six sense of the danger) after many years accumulated sea experience. From the first chapter, the intuition comes from experience. The experience comes from practicing. The practicing comes from knowledge. Here is the knowledge.

Visualizing the target’s bearing change

In the COLREG rule 7 (d): In determining if risk of collision exists the following considerations shall be among those taken into account: (i) such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change. (ii) such risk may sometimes exist even when an appreciable bearing change is evident, particularly when approaching a very large vessel or a tow or when approaching a vessel at close range. The rule 7(d)(i) stated the risk of collision is determined by the compass bearing change.  Rule 7(d)(ii) is demonstrated on the drawing above. At close range is the reason why these situations can be happened. It is the true bearing (TB) changes that specified in the COLREG gives a very good indication of the collision risk. For a vessel in a steady course, the relative bearing (RB) reading take out systematically can also serve this purpose. . If the heading is steady (heading as a constant), the TB change is equal to the RB change. (). The true bearing change is verified from taking the compass reading. The relative bearing change can be taken from visual technique to ease the process. Any kind of change will need a reference point to compare. The true bearing change’s reference point is the compass bearing first been taken. The reference point of the relative bearing change is taken visually from some conspicuous point on the ship’s deck.

For the vessel type without deck cargos, deck fitting’s relative bearing are always the same. For instance, the fore mast's relative bearing is always zero if we take it from the ship's centerline gyro repeater (as are mostly found on Japanese-made vessels). The sheer deck plate on the break of fore castle may be 10 degrees relative bearing to each side from fore mast, and the pilot boarding station's stanchion may be 30 degrees to each side, etc. However the concept is that, the precise bearings of reference points (fore mast, sheer plat corner…) on deck is not important, it is their relative bearing to the ship's bow is always the same. And, they can be used as reference points to ascertain the bearing change of the targets. For those ship have deck cargo, the specific deck cargo's shape, color, cargo ends, etc, may serve the same purpose as reference points. For example, a container positioned in the outer row of a certain bay may be coloured orange, one foot lower than the row next to it and the target is located on its fore corner post direction. These characteristics of the colour (orange), gap (different height between containers), and position( fore-end of the box) comprised a vivid and compound visual impressions of the target's bearing which is easier to remember and check with. The picture memory is more stable than digital reading from the gyro compass for the visual data processing in the brain need more complicated and wide connecting which vitalize more parts in our brain cells. For just one look, the visual data are always overflow for us to memory. The mariner need to chunk out the data into some more meaningful hints. The mark been choose along the relative bearing line need to have specific characteristics like mentioned above,    

Inside the bridge, take your usual lookout position if not in the fore-aft center line. The center position of the bridge is usually taken by the master or pilot because they assume more responsibility of the ship's safety than an OOW. Image a relative bearing line starting from your eyes in your lookout position to the target on the sea. Lower down your eyes and pick up one mark on deck along this line as a reference point. In the picture above, the lookout is not in the fore-aft center line of the vessel and the target is in dead ahead of own ship. If the lookout position is in the fore-aft center line, the red line will pass the fore mast. The bearing for the target ahead can use the point marked by the white cross as the reference. Now, the mark of this target’s bearing is the corner between blue and red container or the far left end of the white container above, it is up to the lookout’s feeling which one is easier to memorize for later comparison or to avoid the confusion from another target’s reference mark. General speaking, the mark locate far away from the lookout can get more precise detection of the relative bearing change. The little difference due to the forward white container mark is not located precisely along the relative bearing line is not so important to our purpose. “such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change” For the relative bearing has to be appreciably changed to eliminate the risk of collision. It is important to maintain the lookout position on the bridge to avoid any parallax of the reference point. However, the lookout needs not to stand still on the bridge. The lookout only needs to remember where he stands before he carries out other duties on bridge. Once other things have been done, the lookout should stand on the original position and look at the target again. The lookout should lower his eyes to check the reference point.  If the target's relative bearing line is changing, the reference point (white cross) wil1 not on the target relative bearing line (red line) again. In the picture below, the target’s bearing is shifting away from the fore-aft center line. The relative bearing of the target is increasing. When the target’s relative bearing line moves aft the reference point (RB increasing), it is probable that the target may pass astern of own ship. If the first observation is the picture below and the observed sequence is reverse. The reference point of the target’s bearing line may be chosen as on the blue container or the point between two refeer containers (white around with green color) on deck. After some time passing, the target is on fore-aft center line. The RB of the target is decreased to zero. The relative bearing line moves ahead of the reference point closer to the bow (relative bearing getting smaller), the target may pass the ship's bow. When the bearing line remains over the reference point (RB unchanged) or does not move very much, a risk of collision shall be deemed to exist. . For a vessel in a steady course, Relative bearing change = True bearing change. Due allowance must be taken for ship’s yawing when the sea is rough. This is the technique used to establish the risk of collision. Once the vessel has altered course to avoid the collision, the procedure needs to exercise again when the vessel had steady on her new course to access the risk of collision. 

Practical lookout procedures 

There is a concept to say “The relative bearing change should not be used to access the risk of collision”. It is illustrated in the drawing above. The reason is the vessel A is altering the course and the vessel B is within its turning curve. For a vessel in a steady course (Vessel B), the risk of collision still can be detected by the relative bearing change. Even the vessel is altering the course (Vessel A), the relative bearing change can be used to access the risk of collision if the target is outside of the vessel A’s turning curve. The diameter of the turning curve is different from each vessel’s length; usually 7 SL will be enough for this purpose. The mariner can calculate the 7 SL of his own vessel to determine the applicable distance of this relative bearing technique. How to develop the skill of using relative bearing as the aid to avoid collision? It is better to start the practicing when you are only a cadet. For no one will bother you to stand on the bridge idle looking outside without taking any compass bearing or using RADAR observation. You can check does the relative bearing line is moving ahead or astern of the reference point. Does the vessel's relative bearing line moving ahead of the reference point can really pass our ship's bow? How about the one relative bearing moving astern and the other one relative bearing remain stationary? After few try, you will get it. 

If you are already an OOW, there is no chance for you to stand and look only. Before you got the chance to practice the relative bearing check, take the true bearing of crossing vessel or checking the ARPA data is still your responsibility. Actually, there is more than one duty in bridge for a deck officer. You have to response to the calls frequently. The basic point shall never forget after you had run for other errands. The position where you stand on the bridge to establish the relative bearing line and the reference point which you pick up from the deck, remember these two basics. Now you can answer your call, whatever it is. Check the true bearing, talk to the VHF, fix the position, change chart, attain the engine telegraph or else. After all these, return to the position where you stood and establish new relative bearing lines. Where is the target now, ahead or astern of the reference point? Does the relative bearing changing result coincide with what you get from the ARPA or true bearing? 

Pinpoint the most dangerous target

When navigating in heavy traffic area, there may have many ships involved in the risk of collision. What is the most dangerous target, even the ARPA has limitations. Let's define the most dangerous target is the target that have collision risk and the DTC is smallest. The DTC is close related with TCPA. The smaller DTC, the shorter TCPA. The smallest DTC target has risk of collision is the first target we will collide with. Once again, the DTC is brought up to our attention. For the same range vessel, the overtaking case has the longest DTC, the end-on case has the shortest DTC, the crossing case has the DTC in-between. Several targets have risk of collision in the same range; the priority should be the end on case first, the crossing case second and the overtaking case last. For the end-on case, we have the tendency to take the action in the early stage (4-6 miles away). For the overtaking case, we may be waiting too long and almost forget the risk is still exist, the action range should be 1-2 miles. The most obscure case is always the crossing situation. 

Risk of collision in crossing situation

In mathematic expression, the collision risk is determined by the speed vector of both vessels. In the radar observation course, the speed vector use 6 minutes as the time interval. The time interval for calculation can be 6 minutes or 15 minutes or else, the basic is all vessels use the same time interval. In the figure 1, the DTC of own ship (OS) is the distance cover by OS speed over a specific time interval. The length of each line from the target to the point C is the distance cover by the each target’s speed at the same time period. For the same time interval, these lines represent the speed vectors of each vessel. The point C is the collision point if every vessel maintains their course and speed. The black semi sphere is the possible position in different course of OS speed vector. The red semi sphere is the possible positions of same speed vessel (point C is the radius. Line CT = line OC) in different course have collision risk with OS.

This is a special case. The range of the red target is equal to the DTC of OS. The red target is on the black semi sphere which every point has same radius. OS has the same speed as red target, two vessels are on the red semi sphere. The triangle of OS and red target and point C is an equal side triangle. The relative bearing of red target is 60 degrees. 

In this case, the DTC will equal to the range of two vessels under the conditions of

a)     Two vessels have same speed.

b)     The RB of the target is 

For a same speed vessel has collision risk with own ship,  
if it's RB is 60 degree, then the DTC is equal to the range. (red vessel)
if it's RB is less than 60 degree, then the DTC is less than range.(blue vessel on the red semi sphere) 
if it's RB is greater than 60 degree, then DTC is greater than range. (black vessel) 

Assumption 1: For a same speed vessel, the lesser RB target is more dangerous. 

The reason is the DTC is lesser than the range which let the OOW have lesser time to react. 

For a slower vessel in the same range has collision risk with own ship, 
if the DTC is equal to the range, then it's RB is lesser than 60 degree.(blue vessel) 
For a faster vessel in the same range has collision risk with us, 
if the DTC is equal to range then it's RB is greater than 60 degree. (black vessel) 

Assumption 2 : In the same range, the faster vessel is more dangerous than a slower vessel. 

The reason is the faster vessel have wider RB arc than slower vessel which may be neglect by the OOW.

From the assumption 1, lookout duty should always begin from lesser RB target. At first, the OOW should check the ship's bow area for any vessel in a reciprocal course, then to the starboard side for crossing vessel, then to the port side crossing vessel. Finally, from the astern direction to check any vessel is overtaking us. 

From the assumption 2, the faster vessel has more wide range of danger in crossing situation. For the same range, lookout should have the ability to identify the fast vessel and access its risk of collision correctly. Usually, we will assume the ship's speed by what type of ship it is. 

 The RB of no care

Here we will discuss the mathematic plain triangle principle first, by which the vessel’s actual length and size had not taking into consideration. These discussions are used to derive the concept of no care sector and the usage of this technique should apply to small coastal vessel only.    

For two same speed vessels have the collision risk (point C), the red semi sphere is the possible positions. The red semi sphere’s RB range from OS is . If own ship finds other vessel's RB is over 90 degrees and she is in the same speed with OS, then there is no risk of collision. If the RB is more than 90, the target vessel is overtaking OS and target has same speed with OS, then the target behind us can only keep up the distance with OS and can never catch up own ship. 
For the vessel speed faster than own ship ( the red semi sphere has greater radius than the figure above), the RB is  . There is no escape. Risk of collision comes from every direction.  
For a vessel slower than own ship, there is a line of no care and a sector of no care. If this vessel's RB is greater than this no care RB, the risk of collision is not exist regardless what course she is steering. 

In figure above. If a slower vessel has the risk of collision with OS, there is a collision point or possible area of collision. For a specific time interval, we can draw two speed vector meet in the point of collision. The orange circle represents all the course which the target vessel may steer to the point of collision. At this time interval, the target must be located in the orange circle otherwise these two speed vector will not meet at the collision point (not pass the same place at the same time). Draw one line tangent to the orange circle from own ship will get the no care RB(). The orange circle is smaller, if the target is slower, which will have smaller no care RB. 

For practical usage, due allowance must be made for own ship length and target vessel’s overall length as we discuss in the chapter before. 
This no care RB is only useful when apply to the target which speed is really slow. If target vessel does not have too much speed difference, the no care RB will be useless. We can estimate the target speed by the experience we get from our usual trading route and area or simply from the ARPA. Since it's a RB, we can pick one visual reference point on deck for a specific speed of vessel. 

When we navigate in some heavily fishing area, this sector of no care is useful to decide the proper course we shall take to avoid a whole school of fishing vessel regardless what action they are taking. This technique is also widely used in the high speed craft. The speed difference makes other vessel’s relative motion line more parallel to the high speed craft’s course line make other vessel’s chance to cross the track of the high speed craft lesser. The circle around the vessel is the possible position with the vessel’s current speed in the same time interval. If the high speed craft has higher speed than the targets around, the speed difference will make the circle around the target smaller. In the high speed, any sharp turn is a dangerous maneuvering. Armed with the high speed, there is a strong tendency to go her own way instead of following the rulings of COLREG. There are already some suggestions in the IMO concessions to make a special ruling for the high speed craft.   

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Visualizing the target's range

The most dangerous target is the target has collision risk and the shortest DTC. Now we know the technique of verifying the bearing change by eyes which can save the precious time in ascertain the risk of collision. What about the ability to verify the shortest DTC? For the same range vessel, the overtaking case has the longest DTC, the end-on case has the shortest DTC, the crossing case has the DTC in-between. Before we have the ability to verify the shortest DTC, we need the ability to verify the target's range by visual to establish the practice, experience, skill, intuition.

Target at 4 to 8 N. miles range

What is the most likely scene we see when we look out the bridge window? It is the sea horizon when the visibility is good. This is the line that inspired the idea that 'the earth is a sphere'. A ship's mast appears first when she comes over the horizon. The horizon can serve as a great reference for range verification. By human nature, we are used to compare the foreground of each target to judge the distance of each target or by the size of the target in our memory. This is true, for we neither become taller nor shorter each day when we stand on solid ground and know the height of each object around us. But it may not be useful when we are on a ship's bridge with the eye height changed by draft or trim and each target's height is unknown. 

In clear weather, with a height of eye of 42 meters, the horizon lies about 13 nautical miles away. If a ship's full silhouette is visible, the water-line of the ship is the position of the ship on the water. If the water-line of ship coincides with the horizon, we might say the distance to the target is 13 nm. In fact, we cannot say this for sure, because nobody's eye can see the water-line so far away. Once the ship comes closer, its water-line will descend under the horizon. The closer of the ship the lower the water-line descend from the horizon. We can verify this by the ship's silhouette cutting the horizon deeper and deeper. By comparing the water-line position of several ships against the horizon, we can pick up the vessel closest to us regardless the target’s size. The lower water-line to the horizon, the closer distance to us. This is the technique for verifying the long range vessel. 

Target at 4 N. Miles range

A target vessel's waterline will undergo some changes as it approaches OS. We begin to see some wave form in the bow water line at about 4 n. Miles away, although this depends on the height of eye of the observer and target's bow shape and the contrast to ship’s hull color. This is an important sign which signify the necessary action range (4-6 N. Miles) shall be taken by give-way vessel. Precious time must not be wasted for doing other errands. Bearings must be taken now, if not taken earlier, to ascertain the risk of collision. This is our object: pick up the dangerous target by some useful sign. The water form at the target’s bow is a vital sign. When the target's bow wave (or wake current of a small ship) becomes more vivid, this means the target is much closer.

Target at 1 N. Mile range

The most dangerous range is when the white splashes are seen at the ship's bow, which means the distance is down to one or two miles only. Remember the 7 SL is about 1.2 n miles and inside this range (1-2 n. Miles) lay the responsibility of stand-on vessel to take some avoidance action if there is risk of collision.

The lost horizon

If a clear vision of the horizon is lost, we actually lose most of our ability to judge the distance. This is the time radar needs to be turn on to evaluate the approaching vessel's range against the distance we thought. In a foggy day, the first vessel we see is our visibility. The range of this vessel should take out as a reference. Bearing in mind that the fog density is not fixed all times, so the visibility may be varying by the weather pattern. Should the vessel in fog come into our visibility range, the bow wave form (4 N.M.) and white splash (1 N.M.) is still a useful sign of our distance judgment.

Nightfall at sea

During the night time, the horizon may still be visible if some time is spared to achieve the best night vision. Usually, merchant vessels are constructed according to the class rules, and navigational lights are well separated. In the long range, we can still compare the relative height of the target's navigational light from the horizon to estimate the range. 

The limitation of human eyes

There are two kind of light reception cell in human eye. One is the cones shape which is good in detecting the texture, color and minor movement of object. This cones shape cell need highly illuminated environment to function properly. The other is rods shape which is sensitive to the highly contrast light and shadow movement. These two cell work together in daytime, the cones cell concentrate on the dedicated job and the rods cell is vigilant on any object's movement in our subconscious. 

In the night time, we looking outside the bridge thought we do our best to safe guard the ship and our friend on board. Actually, the cones shape cell is not working because the illumination is not enough. We can still sense some target's light is moving through the function of the rod cell. By this time, we may think we are still keeping good lookout, but we don't know what kind of danger it is for the cone cell is not functional. Small target may deem as big target in long range for the navigation light spacing look like the same or the stern light of a big vessel may deem as a small target’s mast light. These kinds of misinterpretations may come from the experience of past success maneuvering which lead us to over complacent on our judgment. The complacency will reduce the conscious of danger. Hence the vigilance is relaxed which finally lead to disaster. The solution for this is to use the telescope to verify the target's nature. 

The most dangerous situation for those rely on visual lookout is the confusion of the ocean-going vessel's navigational lights with the fishing boat's station lights. To solve the problem, we will have to depend on the proper setting of the radar or have to have a dedicated OOW to verify these targets very carefully. Even the ARPA/radar cannot tell us the target's size/ type. We have to check the target by the visual. Using the visual in the night time, we have to beware of the human visual limitation.   

The proper setting of ARPA for detecting the ocean-going vessel in one Radar usage

The possibility of the confusion of the ocean-going vessel's navigational lights with the fishing boat's station lights should be detected by skillful use of the radar equipment. To distinguish the big vessel from surrounded fishing boat should reduce the gain of receiving echo slowly to eliminate the fishing boat echoes in the PPI. After reducing the gain of radar, the echo of big vessel will still remain on the PPI. For distinguishing the stronger echo of the ocean going vessel from the fishing boats’, the 3 cm Radar is better than 10 cm Radar. The visual contact of these big vessels should be established by checking the bearings on radar and the compass bearings of the targets. The gain should be tuned back to normal level to pick up all targets at sea after reduced for ocean going vessel detection. This routine should be carried out periodically at lots fishing boats area to detect the fast speed ocean-going vessel.    

The proper setting of ARPA for detecting the small target in dual Radar system

If the big and small vessels are mixed together, two radar systems should be put in use. The 10 cm radar better in detecting small target should be used for this purpose. For small target, the TCPA largely depend on our own speed and the target may well out of number of the ARPA capacity. The detecting range should put in 6 miles to enlarge the radar echo and reduce the number of targets. The afterglow should turn on and set in relative mode. The origin of EBL (electric bearing line) should set in the PPI center, then we can turn the EBL to the target in question to check "Does the EBL coincide with the afterglow of target's echo?". If the EBL coincide with relative motion afterglow in relative motion display, there is a risk of collision. If the number is still too many in 6 miles range, the radar range should set to 3 miles to concentrate on the most dangerous target. If you can identify the target type positively, the RB of no care can be determined to further reduce the number of targets in question.

The proper setting of ARPA for detecting the ocean-going vessel in dual Radar system 

The 3 cm radar should use to detect the large vessel's echo, if two radars are used. For the large vessel, the relative speed is fast, the range should be set in 12 miles. The gaining of PPI should adjust to reduce the echoes of the small targets. By doing this, we can use the ARPA automatic acquisition function to acquire the danger target among the small target. This is a useful technique when OOW concentrate on the job to avoid the fishing boat. The afterglow of target should set in true motion to know the course of other fast vessel at the first glance. If the target number is over the ARPA auto acquisition capacity or the display on the PPI is very confusing, we should reduce the radar range to reduce the target number to those more dangerous to us(in closer range). The target's speed vector should set to relative motion to know the possible close counter situation and the collision risk. The proper setting of the ARPA can save the precious time in heavy traffic to access the risk of collision.  

 Station lights in 4 N. Miles range

For those depend on visual technique in the night time, verify the range of target by comparing the horizon against the ship’s silhouette is for long range target. For close-by target, there is another technique. Beside the navigational lights, if we can see the accommodation light or some other station light on deck, the distance of other vessel should be around 4 miles away (the normal household visible range of these lights). Once again, this is the range of the last suitable distance to take avoidance action for a give-way vessel. 

For a small fishing vessel, the first noticeable light usually shows at 7 miles range (same range as the target's echo can be detected by radar). Regardless how many lights she has on board, she will illuminate as one light only. For the lights distance to each other is small, in the long range all lights merge as one. As the target getting closer, this one light will become two or more lights. This means she has come to normal household light visible range (4 n. Miles range). 

Reflection of lights on 1-2 N. Miles range

When the navigational light’s reflection is seen on the water, the target’s range is only one or two miles off. This feather holds the same for large and small target. If the risk of collision is present; bold action shall be taken in both stand-on and give-way vessel. One or two miles distance is about the 7 SL. However, the reflection on the water in only visible in inland water or where the sea is calm. Another useful sign of the close range is the cross shape of light on the bridge's glass window. The length of the cross shape of light on the glass will grow longer when the range is closer. 

Summary of the lookout

Now, we have the visual ability to verify the distance of the target and make sure the risk of collision by RB. 

Together with these visual abilities will help us find out the most dangerous target. Beside the readily identifiable target in the Radar, the visual lookout procedure is as following:  
1. Search any vessel in 1 or 2 miles range by bow splash/stern wake current or light reflection on water/cross shape on glass. 
2. If any, give way to the end on vessel/take RB of the crossing vessel/make sure the type of the overtaken vessel. 
3. Verify the target in 3-4 miles range by bow wave/water line compare to the horizon or household/stationary light/ separated lights in small target. 
4. If any, give way to the end on vessel/give way to starboard side crossing vessel if RB has not change much/take RB of the port side crossing vessel. 
5. Look for the target has full silhouette inside the horizon. Comparing the water line of the target with the horizon to identify the closest target and memorize the reference point of the RB. Pick up the target has the fast relative speed and memorize the reference point of the RB. Set out the most dangerous target and keep close monitor its movement.

Using all available means to ascertain the risk of collision is including the proper use of the radar equipment and visual techniques. The visual techniques may established from many aspect and become part of initiation of experienced seaman. The reaction time response to the challenge depends on many factors. COLREG 7 Risk of collision (c) Assumptions shall not be made on the basis of scanty information, especially scanty radar information. Human nature tends to believe what we see by eye. However, the visual information can also deceive our judgments. Cross reference should always exercise to make sure the situation we met, including the opinions of other OOW in the bridge. This is what they call Bridge Resource Management.