Stand on vessel’s measure in stand on stage

COLREG Rule 17 Action by Stand-on Vessel (a) (i) Where one of two vessels is to keep out of the way the other shall keep her course and speed. (ii) The latter vessel may however take action to avoid collision by her manoeuvre alone, as soon as it becomes apparent to her that the vessel required to keep out of the way is not taking appropriate action in compliance with these Rules. (b) When, from any cause, the vessel required to keep her course and speed finds herself so close that collision cannot be avoided by the action of the give-way vessel alone, she shall take such action as will best aid to avoid collision. (c) A power-driven vessel which takes action in a crossing situation in accordance with subparagraph (a) (ii) of this Rule to avoid collision with another power-driven vessel shall, if the circumstances of the case admit, not alter course to port for a vessel on her own port side. (d) This Rule does not relieve the give-way vessel of her obligation to keep out of the way. 

The stand on vessel’s measure is regulated by the COLREG step by step. At the first, When one of two vessels is to keep out of the way the other shall keep her course and speed. From the Rule 5 Look-out :            Every vessel shall at all times maintain a proper look-out by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision. When the stand on vessel shall keep her course and speed, the rule 5 look-out is still applied. The look-out duty shall maintain at all times. 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 and speed, the relative bearing (RB) reading take out systematically can be used to access the risk of collision. . If the heading is steady (heading as a constant), the TB change is equal to the RB change. (). 

Assessment of risk of collision

Let's take a look at the fig c-2 which is a bow colliding scenario.  The initial RB of the overtaken vessel is about 25 degree on the port bow. At the time of collision, the RB is reduced to zero. The true bearing reading increased 25 degree. If this 25 degree bearing change happened at a certain distance ahead of ship's bow, the collision will not happen. The collision situation here is that the distance between two vessels is reduced to zero and the true bearing change is just equal to the RB. 

So the success of an avoidance action can be described as follow 

The amount of relative bearing change should be more than the first observed RB. 

Rule 7 risk of collision: (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 concept of risk of collision from the (d)(I) is compass bearing of an approaching vessel does not appreciably change at long range and from the (d)(ii) is such risk may sometimes exist even when an appreciable bearing change is evident at close range. 

The concept of compass bearing change is applicable in all situations. For the junior OOW, the problem is what is the appreciably change at long range? And, why the appreciable bearing change is evident at close range, the collision risk is still present? 

True bearing change of give way vessel

For a give way vessel sailing in constant speed ( full sea speed), any course change will deviate from the original point of collision. Although the course change dose not guarantee give way vessel have get rid of the possible area of collision, there are lots point of collision in the possible area of collision. 

If the course change is made in ample distance and large enough, give way vessel will clear the possible area of collision. What is the ample distance for a course change in give-way vessel?  4-6 n.m. range from the stand on vessel. What is large enough? In order to avoid the possible area of collision, the give way vessel will have to change course to let the stand on vessel's RB change well clear of ship bow or alter course to let the relative bearing change more than the first observed RB. 

The COLREG Rule 7 risk of collision is applicable to stand on vessel and give way vessel. The bearing change in accessing the risk of collision is in two ways. One is to let other vessel passing own ship’s bow which will have the observation of other vessel’s relative bearing change more than the first observed RB. The second one is to have the other vessel passing own ship’s stern which will have the observation of other vessel’s relative bearing change more than 180∘.The difference of stand on vessel and the give way vessel in accessing the risk of collision is the stand on vessel shall keep her course and speed when the give way vessel shall take early and substantial action to keep well clear. 

Despite the COLREG obligation, the give way vessel may or may not have taken some actions to avoid the collision. The course change taken by the give way vessel will have some transverse distance along the original course line. If the course change is θ, the transverse distance is.  

   . 

If the give way vessel make out one SL (say 300 m) passing distance.  
In 4 N. Miles away, the true bearing change θ is 2.32 degree. 
In 3 N. Miles away, the true bearing change θ is 3.09 degree 
in 2 N. Miles away, the true bearing change θ is 4.64 degree. 
In 1 n. Miles away, the true bearing change θ is 9.28 degree 
in 1/2 n. Miles away, the true bearing change θ is 19 degree 

These bearing changes should measure in the give-way vessel's midship section to avoid the blossom effect, the image of a target getting bigger and bigger. 

The possible area of collision is two SL. The bearing change need to be double to clear it. From the table above, it is clear that for clearing the possible area of collision  
the give way vessel's RB have to change over 5 degree before 4 n.m. Range.  
The give way vessel's RB have to change over 7 degree before 3 n.m. Range. 
The give way vessel's RB have to change over 10 degree before 2 n.m. Range. 
The give way vessel's RB have to change over 20 degree before 1 n.m. Range. 

This RB change is not the heading or course change. The RB is taken from own ship's heading. Remember that the true bearing change is equal to RB change as long as own ship is in steady course. If ship have some yawing of 2 or 3 degree in the rough sea will make the true bearing change not so obvious. 

If this RB change is not so noticeable in the 4-6 n.m. range, the give way vessel may have not take any avoidance action or the actions give way vessel take is in vain. If in the 2-3 n.m. Range, the bearing change is still not noticeable. The give way vessel has no option but to take some action. 

The RB of give way vessel

It is recommended that whenever take the bearing of give way vessel should take give way vessel's midship section as the reference point to avoid the parallax. As the give way vessel is coming from port side and the most threaten part of give way vessel is her bow, stand-on vessel have to take close watch on the give-way vessel bow's relative bearing.  

The approaching vessel's RB can also tell us some story. No matter what avoidance actions has been taken by the give way vessel. If the true bearing had not change after some time, this only means the distance is shorten between two vessels. Usually this case happened during the maneuvering of the give way vessel. The effectiveness of the give way vessel's avoidance actions have to wait for the give way vessel's course or speed have steadied. Then we could check the true bearing change is enough or not in the specific distance away.  
How much is it? 2 miles-10 degree or 1 miles-20 degree. 

The effectiveness of the give way vessel's avoidance measure is depend on it's approaching speed and direction and maneuvering ability. Even if the give-way vessel have taken some action, the effectiveness of its action is indicated in its bearing change rate. The approaching vessel has to pass our bow or stern in ample distance. If there is no such ample distance available, the stand-on vessel has to use the change of its RB as reference to the best aid of avoidance action. 

@@@@@   If a give-way vessel RB is 25 degrees at the port bow  

What is the ample passing distance is up to navigator's discretion. Anyhow, the minimum should one SL to pass each other. This can guarantee whichever part of other vessel the bearing is taken will pass well clear. Even the true bearing change of a specific part of other vessel is more than RB, the collision can still happen. For example, if own ship take other vessel bow's bearing as reference point and the bearing change have more than the first RB( which means OS had clear the target’s bow), may still collide her stern. It is obvious that in close range, the bearing change can not take as the only standard for access the risk of collision. The second concern in close range should be the ample distance between two vessel's bow and stern. Actually, at close range, watch out the distance change is more important than the bearing change. Like the vessel turning the ends in the harbor area, the ship handler should have the ability to estimate the nearest distance between ship's bow and port facilities when the vessel is conducting a swing. So, the master should do in collision avoidance.
 

Feel the distance change in bow and stern

If we are the stand-on vessel and the give-way vessel at port bow have not taken the necessary action to keep clear and OOW have check its compass bearing since 10 miles away. Although OOW have use all possible means to give the collision warning to it and may be with some in vein course change like add 5 or 10 degree away from original course, but  it finally come into 2 miles range without apparent change of approaching vessel's course and speed. 

The OOW decide to take action best aid to avoid the collision. The initial action may be "starboard ten" or starboard fifteen". After the rudder order been given, we should check the rudder indicator of the correct rudder been used by quarter master and feel the ship body's inclination (this is one kind of indication of rudder effect). Before the ship incline to her full extent, take the time to check the distance of approaching vessel again because this will affect our following actions. 

Any action we take to avoid collision should always bear the distance of other ship in mind, not only the miles, but down to the meters or yards. This awareness should be including two ship's bow and stern distance change during the maneuvering of both vessels. We may not have all these experiences to feel what it like been around by other ship in close range. Nevertheless, this is the most needed skill for ship maneuvering technique. We can get this kind of feeling when vessel is inside the harbor area while the pilot at the bridge conducting the ship's movement. There are two benefits in this situation. One is the vessel is close enough to the point of interest, the other is the maneuvering pressure on us is less stressful. 

From the figure above, we can see that it is the stage of our turning that will make the big difference in the angle of blow. The blue arrows are used to represent the position of bow and stern. The purple arrow is used to represent the whole ship body movement direction.

In stage one, OS had used the rudder for the first 1 SL, the heading is still in its original course. OS might hit the target at midship section. After striking, if the rudder does not midship or hard over to other side, OS bow have the tendency to ripe off the target. The angle of blow is 60 degree to the starboard.

In second stage of turning, OS heading is beginning the swing, OS might hit the target’s bow area. Depend on target’s type and size, OS may have the tendency to scrape other parts of target by OS stern. OS should try to steady on the same course of the target. Due to the interaction of both vessels (suction/ cushion of bow/stern), the ship’s main engine should stop for easy steering.

In the third stage of turning, OS had the full swing, OS almost reach the same heading as target. The collision may be avoidable if OS can steady on target’s heading. Once OS had steady on the safe heading as target, the distance between vessels should estimate to decide whether the usage of main engine is appropriate or not to avoid the interaction.        

Even in the emergency, the feel of OS turning is still very important for the correct actions that should be followed. 

In the stage one, the master’s feel of distance change is the OS bow distance to target midship part getting closer and the target bow and stern RB are almost the same.

In the stage two, the port RB of the target stern is increasing quickly and the starboard RB of target bow is decreasing slowly.

In the third stage, the RB of target bow had passed OS bow. 

When I was a c/mate stand-by at fore station, I report the ship's bow distance to prominent object as a good practice for forward party's duty. Sometimes I distracted by other work going on and forgot report the distance to the bridge. The captain called from the walky-talky asks for it. Later, when I became a captain, I realize that the ship's bow position should bear in mind by picking up a reference point along the ship's outfitting before mooring line been cast off. The reference point of the ship's bow position can be estimated from the bits position of head line and spring line. Even if ship bow is invisible from the bridge, by checking this point on the outfitting moving into or away from port facilities, we can have a clear picture of the ship's movement in mind. It is good for the mate to estimate the distance visually and watch out the distance carefully. Nevertheless the captain has to have the ability to estimate the ship bow movement by himself alone. 

By feeling the distance change between two ship's bow and stern, we can decide which action to take in the following stage. 

First, the possible collision scenario should be correct estimated. 
Let's look at the fig.c-5 again. Is give-way vessel's true bearing has the tendency to increase (own ship's course in the sector of )? If so, the possible collision situation is our bow will collide target in some position. If own ship's bow is due to collide other ship with some part, the most unwanted portion is other ship's mid-ship section which might cause the total loss of other ship. 

The minimum action distance for stand on vessel

By the British court precedent, the give-way vessel should take proper actions to give way at least 4 n.m. Away. The MCA recommend 3 n.m. as the distance for the stand on vessel to take action. However, the stand on vessel has no obligation to act in 3 n.m. range. The COLREG stated the stand on vessel should take action only when "collision cannot be avoided by the action of the give-way vessel alone". 

In the open sea, target is plotted or observed from 13 n.m. Away. If own ship is stand on vessel and have wait in patient to see the give way vessel did not take any actions to avoid the collision. It is very reasonable to take some action in 3 miles range to avoid the collision. 

In a heavy traffic area, lots vessel come and go in different direction. The action we take to avoid the give way vessel in 3 n.m. range may lead to another collision risk with other target. We may have the reason to wait to 2 n.m. range to decide what to do with the give way vessel. The extra time of waiting should use for more precise prediction of the possible scenario of collision. 

DTC : distance to the possible area of collision

This usually refers as the last stage action in a collision risk of a stand-on vessel. The distance appropriate for taking the best aid to avoid the collision is 3 miles by MCA recommendation and 2 miles by Dailan maritime college's study. However, through the understanding of the turning characteristics, the minimum distance can be assumed as 7 SL. 7 SL advance distance can create one SL’s transverse distance to clear the possible area of collision without drastic rudder action if stand-on vessel had correctly estimated the situation.

The stand-on vessel needs 7 SL advance distance to maneuver. This distance is not two ship's distance away which is range. This distance is the own ship’s distance to the POC (possible area of collision) which we called DTC. The effectiveness of avoidance action largely depends on the DTC that is how many maneuvering room left. If 3 miles recommended by MCA is the range between two vessels, then different meeting situation will create different DTC. Whatever two ship's speed is, the end-on case will always have shortest DTC and the overtaking case will have the longest DTC. It is obvious the end-on case should take the avoidance action earlier. The prudent master usually give the standing order to the junior OOW as: take early action to avoid the end-on vessel at 6-8 n. miles range, take avoidance action to give way to crossing vessel at 4-6 n. miles range and give way to the overtaken vessel at 2-3 n. miles range. OOW now can relief from the embarrassment of Nando ( South African chicken) when they take action to avoid the end-on case in 6 miles range, for this action distance is as brave as he take avoidance action for a crossing case in 3 miles range. 

Give-way vessel's effective give way actions

Ship speed plays a big role in avoidance actions. When two ship meet each other, the fast ship have more effective means to avoid the collision. For the fast ship have more DTC in collision situation, any course change she made will create more sea room than the slower one. If she has to reduce the engine, the fast ship will have more speed reduction advantage. 

Angle of blow between two vessels          

If the RB of give-way vessel has decreased, give-way vessel has the tendency to pass stand-on vessel's bow. The problem is "does the give-way vessel has enough time to pass? Anyhow, this situation is benefit to us. As the give-way vessel's bow is moving away from own ship's midship and stern area 

If the give-way vessel cannot clear our bow, own ship have to take some action. The best aid will be “take the same/reciprocal course as give-way vessel”. These two courses have 180 degree difference. It will up to the angle of blow between two vessels. If the angle of blow is less than 90 degree ( red ship below), stand-on vessel ( OS) may have to alter course to starboard to reduce the angle of blow to zero. 

If the angle of blow is more than 90 degree (blue ship), the stand-on vessel alter course to starboard will have to over 90 degree. Any ship want to make a turn more than 90 degree have to take at least 4.5 SL (full rudder) to accomplish it. This heading change is huge and may exposure stand-on vessel's vulnerable midship section to give-way vessel's impact. This action will have to have very strong confidence to do it, or should not try it to further endanger stand-on vessel. The confidence is coming from the suitable distance away.  The suitable distance is enough maneuvering space for stand-on vessel which is at least 6 SL DTC. In the long range (2-3 N.M.), the stand-on vessel’s action to avoid the collision didn’t have to change the course all the way. Early course change by the stand-on vessel will create some ample transverse distance to clear the POC. Thus, the RB change of the give-way vessel will clear OS stern. The minimum course change of the stand-on vessel to clear 2 SL POC will be 

The effectiveness of the give way vessel's avoidance actions 2 miles-10 degree or 1 miles-20 degree. 
The stand-on vessel course have to change over 7 degree before 3 n.m. Range. 
The stand-on vessel course have to change over 10 degree before 2 n.m. Range.

The minimum amount of course change of stand-on vessel and the minimum amount of RB change of give-way vessel is the same at 4 N.M. ranges, 5 degrees. These two are derived from the same assumption.  . 

As stand-on vessel monitor on the give-way vessel's RB and the RB is decreasing, the final RB of give-way vessel's bowmust be estimated by stand-on vessel. If the final RB of give-way vessel's bow is reduced to zero, this means the give-way vessel’s bow will hit by stand-on vessel's bow.  If the final RB of give-way vessel's bow is not likely reduced to zero, the stand-on vessel will hit by the give-way vessel. If the final RB of give-way vessel's bow is reduced to zero and grow in starboard side, the give-way vessel may have the chance to clear stand-on vessel's bow or hit by stand-on vessel. 

The final RB of give-way vessel's bow is important to the result of collision case. Where is this final RB of bow come from? The final RB of give-way vessel's bow is different from the vessel's RB taken from the midship section. It is hard to tell the final RB of bow by the initial RB of the give-way vessel's midship part. The stand-on vessel will have to estimate by the approaching situation. 

If the final RB of give-way vessel's bow is around stand-on vessel's midship or stern section and the angle of blow is greater tan 90 degrees, the best aid action to avoid the collision may have to take the reciprocal course of give-way vessel in close range.  That is to alter course to port side to avoid the possible collision on own ship's midship or stern section. In any cause, the stand-on vessel should avoid collide the give-way vessel's midship or stern part to avoid major casualty of total loss. 

Give-way vessel's RB is increasing          

When the give-way vessel's RB is increasing, the give-way vessel may have alter course or reduce speed. After these actions been taken, the give-way vessel has the tendency to pass the stand on vessel stern. If the relative bearing change is too late, the give-way vessel have the tendency to hit the stand on vessel's midship or stern. 

In any collision case, the utmost responsibility of master is to preserve own ship's life, vessel and cargo. So, the RB of give-way vessel increasing is more dangerous than the RB decreasing. If give-way vessel hit own ship's midship or stern, the casualty will be too devastating. To avoid the collision, the final RB of give-way vessel bow should take into consideration. The best aid action to avoid the collision will depend on which part of own ship the give-way vessel is going to hit. 

If the distance is ample and stand-on vessel just not so sure of the effective of give-way vessels avoidance action. Altering course to starboard side will be the safer action to take, for stand-on vessel is taking action to sail away from the possible area of collision. 

If the give-way vessel's bow is going to hit stand-on vessel's midship or stern section, the stand-on vessel should alter course to port side to swing out the aft part of vessel for two reasons: 

1. May have the chance to avoid the collision in last minute, or   

2. To reduce the collision impact angle and momentum. 

From the figure above, we can see the correct timing for the stand-on vessel to alter course to port side is 2-3 SL to the collision point. In another word, the DTC is 2-3 SL. (come again, can be decided from TCPA?) In this short range, it is not impossible for the radar set to detect the distance. The ship handler cannot see the rudder order been used by give-way vessel. He must be concentrate on the movement of the give-way vessel now. This is another reason for the ship handler should have the ability to estimate the distance between two vessels. 

As the RB of stand on vessel decrease in starboard bow then increase in port bow, the give way vessel can return to original course by keeping the RB of stand on vessel well on the port bow. 

To avoid any action stand on vessel taken conflict with give way vessel's attempt. We should use the clearing tendency of the give way vessel. If the give way vessel has some slight RB change, stand on vessel might be able to access the risk of collision by the RB change. The RB change depends on the distance between two vessels

Summary

The avoidance actions taken by stand-on vessel are in the last stage of collision. As I was a third mate in Europe trade, sometimes the North Sea pilot in the English channel just ignore the traffic come from port side. Even the COLREG have no specific ruling for the stand-on vessel, one requirement is not to alter course to portside as situation permit. Now, we can safe guard ourselves with these extra knowledge:

1. 1.     The stand-on vessel take action range is 2-3 N.M.

. The amount of relative bearing change should be more than the first observed RB.

1. 2.     At close range, watch out the distance change is more important than the bearing change.
2. 3.     The effectiveness of the give way vessel's avoidance actions 2 miles-10 degree or 1 miles-20 degree. 
3. 4.     The give-way vessel's RB have to change over 5 degree before 4 N.M. Range.
4. 5.     The final RB of give-way vessel's bow must be estimated by stand-on vessel.
5. 6.     The RB of give-way vessel increasing is more dangerous than the RB decreasing.
6. 7.     The correct timing for the stand-on vessel to alter course to port side is 2-3 SL to the collision point.