Sailed4seas的部落格

When, from any cause, the vessel required to keep her course and speed finds herself getting 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.

This usually referred to the action of a stand-on vessel at last stage. The distance appropriate for taking the best aid to avoid the collision is 3 n. Miles by MCA recommendation and 2 n. Miles by Dailan maritime college's study. The distance or range (the term used on the radar simulation course) here is the distance between two ships. At the same range with different meeting situation, the dist to POC (possible area of collision) is varied. For the overtaking case,, if two ship's speed have only 2 or 3 knots difference, this 3 or 2 miles range may have to take half hour or one hour time to reach the possible area of collision. In the end-on situation, for two vessels had 30 knots relative speed, this 3 or 2 miles range may only means 6 or 4 minutes to the collision. In the former case, the action time may look like too early. In the latter case, the action time is obvious too late. We can understand this 2 or 3 miles distance been recommended is mostly applied at crossing situation, still the proper action timing varied largely.

The range is the distance between two vessels. The distance to the POC is the distance own ship will advance to reach the POC which is the available distance for own ship to maneuver to avoid the collision. The range to other vessel has less significant meaning compare with the dist to POC in collision avoidance decision making process. The discussion below is why the distance to the POC (possible area of collision) or TCPA (time to closet point of approach) is more important to the OOW. See the drawing c-1 above. This distance to possible area of collision (DTC) is not two ship's range. However, through the understanding of the turning characteristics from the last chapter, the minimum DTC required can be assumed as 7 times of ship's length. This 7 SL can be used to activate effective transverse distance of one ship length from the original course line under the practical rudder angle been used.

The more dist to POC, the more time we have to avoid the collision.

                  Distance to POC and TCPA

For two end-on vessel, the minimum distance to POC before we take actions to clear the original course line should be 7 times of ship's lengths (fig.c-3).
For the crossing situation, the minimum dist to POC before we take actions to clear the original course line should be 7 times of ship's lengths (fig.c-1).
For overtaking case, the minimum dist to POC before we take actions to clear the original course line should be 7 times of ship's lengths (fig.c-2). 

It is obvious that the distance to POC can serve a very good indicator to guide us to take avoidance actions. Radar is original designed for combat use. So, navy use the maneuvering board to calculate the CPA of approaching target to see does it have threat to the live on board when ARPA had not yet in place. The CPA is the major safety concern to the military mariner. It gave the combat officer the target’s intention and the estimated missing range. As long as the CPA is not zero, the ship is safe from danger. Beside the CPA, the TCPA is also calculated to give a rough idea of the remaining time he has to take action to avoid the target. For a prudent combat officer, he must have had the necessary training to tell him what is the suitable action available for him in different TCPA to avoid the perish of his own vessel. For the merchant mariner, the threat comes from the physical contact of the target vessel and the means available to avoid the danger are own ship’s course and speed. The one hour notice to the engine room for stand by the engine is deeply rooted on each OOW mind setting. The usage of the engine to avoid the collision is always the last resort to avoid the engine damaged from the thermal stress when the engine revolution had to decrease. For the effective collision avoidance action by the rudder order along, mariner need 7 SL distance to react. This 7 SL distance need some time to accomplish it. The time is calculated from the distance to POC divided by the own ship’s speed or two ship’s range divided by relative speed. In either way, the time is the TCPA. What we merchant mariner need is the dist to POC (DTC) which is not indicated in the ARPA's data area, because this is the distance reflect the available sea room left for our avoidance action. This may leave to the radar maker to provide in the next generation ARPA data display.

We need a minimum DTC to maneuver own ship away from danger. As we learnt from the turning characteristics, 7 times of ship's length is the minimum requirement if we don't want very drastic rudder order to maneuver away from danger. Actually, it is advisable to take as early action as possible to avoid any rush action to adversely affect ship movement, sometimes may endanger the personal or cargo on board. But sometimes, we bind by the COLREG not to do any maneuvering for some other traffic may be around.   

The DTC divide by own ship's speed is the TCPA. Usually we get the TCPA from two ship's range divide by relative speed. Since the DTC is not available in ARPA data area, we have to take the TCPA to serve our purpose with some conversion been made.
If one vessel is 285 meters long, 7 times ship's length is 1955 meters.
If she have 20 knots speed, for 1955 meter advance, she have to sail for 3.17 minutes. This 3.17 minutes is the minimum TCPA if she use the 7 ship's length as minimum DTC. 

If she has only 10 knots speed, she needs 6.34 minutes to advance 7 ship's length distance. Hence the TCPA is doubled as she reduced to half original speed. If other vessel keep its original course and speed, we will get our vessel another 3.17 minutes to arrive at possible area of collision. The 3.17 minutes time delayed when own ship arrive the possible area of collision, the target already clear of the possible area of collision. This is the timing lapse concept from the original collision situation.
  

In the old days, captain note on the standing/night order book of his safety precaution. At what range the OOW should take proper actions to give way to stand-on vessel. Due to different meeting situation, it is hard to set a standard distance appropriate to each case.    The captain now can give more specific instruction to meet all cases. The criterion will be the DTC. This is a measurement of the available maneuvering space to the stand-on vessel.   The captain can specify how many DTC away he wants the OOW to take action to keep away from other vessel regardless of what kind of meeting situation is. The DTC data is not ready available in the radar or ARPA screen. The OOW should use the TCPA to replace the DTC master asked. TCPA = DTC/ship speed. The TCPA data is always available in the ARPA screen. The minimum TCPA (7 times ship's length) should also remember by heart. This is very important that this can be used as a indicator to show the deadline when the avoidance actions is not enough by using the rudder order along.

7 SL/ship speed=Min. TCPA. If the TCPA is less than this minimum TCPA, the avoidance action should use the engine speed as well.

CPA GAINING BY STAND-ON VESSEL’S ACTION

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.  .

For a 20 Kts vessel, 4 NM run is 12 minutes. The CPA gainig of 2 SL (600Meters) from her course altering. Convert to TCPA notation will be

12 Min before Collision (TCPA), the minimum course altering is 5 degrees.

9 Min before Collision (TCPA), the minimum course altering is 7 degrees.

6 Min before Collision (TCPA), the minimum course altering is 10 degrees.

3 Min before Collision (TCPA), the minimum course altering is 20 degrees.