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 the rule 7(a) apply.    

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.

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 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.