Timing lapse for collision avoidance

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.

Using the rudder is for changing the course line to avoid the possible area of collision. If the DTC is less than 7 times of our ship's length, by using rudder along may lead to another close quarter situation. Only by using the engine speed reduction can buy us some more time to let other ship to pass the possible area of collision before own ship, i.e. Not to pass the possible area of collision at the same time. 

If alter course is not available at a confined waterway, the engine speed should reduce to bare steerage to make maximum speed reduction for maximum timing lapse. If the original speed is high and the TCPA time is not long enough to reduce the speed effectively, ship should perform the zigzag swing using full rudder order to drag her down. If the original ship speed is low( below 5 knots), the engine order "crash astern" should be used to get some more timing lapse. 

                                       ZIGZAG 

The zigzag maneuvering is recommended when a captain first sign on his new post. For the knowledge of this maneuvering of this particular ship under his commend will help a lot when the times come. Collision avoidance is one of these times. Whenever he got the chance arrived pilot station half hour early and no other traffic around, the ship captain should find a proper sea room and reduce his speed to full ahead. 

All position should be precisely defined, better be help with the GPS, for the new model have a function to memorize the present position by pushing a single button. The DGPS position can reduce the error within 15 meters. 

Master should stop the engine now and use the full rudder to swing the vessel. Full ruder sounds crazy and experienced master knows deeply in heart that the ship may lost control at some stage in this maneuvering. For a certain rudder order we used to swing the vessel, if the turning rate had not reduced by the midship rudder first, need almost double the original rudder order to stop the swing effectively. Even if we apply the double counter rudder immediately, the ship heading will still overshoot to some degree before the heading is steadied. 

The zigzag is not an ordinary maneuvering and the purpose is to bring the ship's speed down. Bearing this in mind, the first step is to stop the engine. Without the engine propulsion, the rudder effect is mostly reduced. And we apply full rudder to create largest effective rudder area in ship's fore-aft direction. But, the effective rudder area is not the major force to slow down the vessel. It is the drifting angle created by the turning momentum which made the whole ship's body act like a giant rudder to the water. The full rudder is needed to create the maximum drifting angle. 

The zigzag maneuvering is carried out under the engine stop condition. Without the engine revolution the ship will lose control at later stage. At what stage, ship will not response to the rudder should determine by sea trial. The zigzag needs to carry out many times to acquire the necessary knowledge of own ship’s response to different heading change. For the first time, the zigzag should have sheered 5 degree from the original course only. The hard over rudder should use to initial the turn and the counter rudder should use the hard over angle also. The heading may be over-shoot to 10 or 15 degree more before she can stop the turning. Keep the full rudder to swing the vessel to the opposite 5 degree from original heading and apply the counter full rudder again to stop the swing. Repeat this maneuvering till the ship lost control of her heading without the engine revolution to steady the vessel. 

If the vessel cannot check her first 5 degrees swing by the counter full rudder, you got a steerage unstable vessel. This kind of vessel have to use engine to kick ahead to stop the swing which is not likely to happen in most ship. But, it is likely to happen when you already apply the hard over rudder two or three times to zigzag the vessel since the ship's speed reduced drastically during the maneuvering which also reduced the rudder effect to stop the swing. If you don't use the engine to stop the swing, you will lose control of the vessel. 

This first time try is to know

1. 1.     how many times the vessel can swing before she become steerage unstable 
2. 2.     what residue speed by that time is 
3. 3.     how many SL has advanced. 

Another more handy option is stop the engine first. Then, give the quarter master ruder order to hard starboard and order the course 5 degree more than original course. When the quarter master had steadied on the new course, then order hard port again and steady on 5 degrees less from original course. Repeat the process few times again to reach the point of lost control and acquire the data above. This is more useful in the real situation when the traffic is heavy and precise control is needed. But, this handy maneuvering depends largely on the quarter master's steering habits and experiences. When approaching a pilot station with excess speed, it is very useful to stop the engine and zigzag it to reduce the speed to the pilot required boarding speed. This is the second usage of the zigzag.

The second time's try should swing the vessel 10 degree each side to the original heading. How much speed can be reduced with certain SL advance? After how many times full rudder order used, the vessel will lost her steerage?

If the vessel’s steerage stability is very good, the testing zigzag 15 degrees to each side may be conducted to acquire extra data of zigzag characteristic. 

After these maneuverings, master will notice another important feature of steerage. When the vessel does not response to her rudder effectively, the vessel still has some residue speed. How much residue speed is depends on the same factors as the rudder effect. As my own experience with twin variable pitch propeller single rudder vessel, the residue speed may as high as 5 knots. Master should determine his vessel’s residue speed by using these zigzag maneuvering.           

Another important characteristic of the zigzag is that the ship will not back to original course line i.e. she will stay in the side that the first full rudder been applied. This is the sideway displacement of the zigzag. When conducting the Williamson turn, vessel use hard over rudder to the man overboard side to kick off the stern. After the heading change 60 degrees, ship have to use the counter full rudder to turn to opposite direction and continue to the reciprocal heading, only by that time the ship can get back to original course line. The reciprocal heading is 180 degrees from original course, i.e. the vessel's heading have to swing 180+60=240 degrees before she can reach the original course line. In the zigzag maneuvering, the vessel had used the counter rudder to reach the opposite heading. As the rudder effect reduced due to the vessel speed had slow down, the sideway displacement also reduced accordingly. The reduced sideway displacement keeps the vessel out of the original course line.   

This character should bear in mind as part of spatial awareness concept when apply the first full rudder to avoid the danger. 

The above studies show that the vessel can virtually stop on the water within 6 or 7 SL by using the zigzag techniques. This seems useless for avoiding the collision if DTC/TCPA already less than 7 SL. But, the zigzag using here is to reduce the ship's speed to get some more time lapse to avoid the collision. Although the ship still will reach the possible area of collision, the arrival time had already prolonged. 

In the practical usage, how many degrees out of the original course line to swing depend on the available sea room. If starboard side had more sea room than port side, the port side swing need not to be in the same amplitude as the starboard side. It is up to what situation we have in the traffic. As long as the full rudder is used in the stern, it will serve to drag down the vessel’s speed. The efficiency of the speed reduction depends on the drifting angle of the ship’s bow we create in the zigzag maneuvering.  

It is also part of the emergency maneuvering when the main engine broke down and we have to stop the vessel to avoid some danger ahead and still had proper control of the ship’s last heading. This is the third usage of zigzag. The ship’s last heading when the main engine is dead is a very important factor to her drifting route thereafter. The experienced master will notice this effect when he drifts the vessel at the pilot station waiting the pilot boat.  

                              Crashing Astern

should be used when ship's speed is below 5 kts, or up to the speed specified in the engine maker’s manual. The rudder effect is poor in this speed range and the reversed engine revolution can be established immediately by the compressed air pressure reserved in air pressure tank. This maneuvering is exercised every time vessel approaching her anchorage or berth. The difference of the astern engine we use in anchorage and the one used in collision avoidance is the duration. In anchorage, we use the stern engine to stop the vessel and just a little bit of back speed to stretch the anchor chain. Generally speaking, the vessel is still under perfect control when anchoring. Just about she will lose control, the vessel already brought up to her anchor.

For collision avoidance, the astern engine may go way over than just stop the vessel. So, we should have some knowledge of the vessel's movement when going astern. 

The position of the PIVOT POINT 

When ship has ahead speed, the pivot point (PP) is located one fourth of the ship's length (SL) from the bow. The PP moves forward to one eighth of SL from the bow as the ship speed slows down to zero. For a right handed single fixed pitch propeller vessel, the PP moving ahead together with the side thrust of propeller to the port side make the ship's bow swing to starboard side when astern engine been used to reduce the ship's speed to zero. When the ship's speed almost reach zero, the side thrust of propeller (reverse revolution of a right-handed single screw vessel) is the only force act on stern (rudder effect is almost zero due to no speed at all), although the force of propeller's side thrust is no match to the side thrust from the rudder when ship still have some ahead speed. The dominant force of side trust from the propeller when vessel speed is almost zero and the PP move ahead make the turning torque powerful to see by eyes. Before the ship lost her heading speed, she can swing 10 to 20 degrees to starboard. When we use the rudder to turn the vessel, as we discussed in the first stage of the vessel’s turning characteristics, 10-20 degrees swing need two SL to accomplish it. But in crash astern, this swing can be finished less than half SL. This sudden swing serves as a very good indication of zero speed point in whole maneuvering. After this point, the ship will behave differently. 

This starboard swing may also be influenced by external force, since the leverage is largest when ship's speed almost reach zero. The upwind or upstream tendency is also prevailing at this stage. When the engine is put astern, the wind direction and force should be checked beforehand to estimate the possible pay off direction of the ship. 

As the ship's speed is reversed from ahead to astern, the PP shifted from ship bow to one eighth of SL from stern when she just has astern speed. The propeller's side thrust is still working. But, the leverage from the PP is reduced from 7/8 SL to 1/8 SL. Ship will not response vivid to the side thrust of propeller. Just a little astern speed more than zero, ship will swing to the side subject to the external prevailing force. Due to the starboard turning momentum carried from ahead speed, the ship may continue to swing or check by wind or current force. At this stage we have no means to control the heading unless the bow thruster is ready for use. 

            THE USAGE OF BOW THRUSTER IN CRASHING ASTERN

The bow thruster located almost 7/8 SL from the astern PP make it a powerful tool to control the heading. It is prudent to stand by the bow thruster for use every time captain tries to go stern engine. The bow thruster is the only means available in ship astern maneuvering to control the ship’s heading, the bow thruster is ready at this moment is important. 

Mariner used to use the ahead engine and rudder to control the ship's movement. Every OOW is also a qualified quarter master. This is the basic bridge training when we were only a cadet. But the use of astern engine and bow thruster is another story usually reserved for master only. Every ambitious ship's mate should watch closely of the usage of theses techniques by the pilot or master whenever he had the opportunity. Otherwise, he will only get the experience from ship's simulator. The effect of the bow thruster is, like the side thrust of propeller in ahead speed, most powerful when ship just starts astern speed. By the time, the astern PP is located far from bow thruster and can create the most efficient leverage for the bow thruster’s force. During crashing astern period (ship’s speed reduce from dead slow ahead to zero, then astern speed been established), bow thruster can be used to counter the unfavorable swing created by the external force (wind/current) and keep the heading in the desired direction (as to mitigate the collision impact). However, bow thruster's power in these regards has its limitation, depend on bow thruster’s output, ship’s type, wind/current force/direction…. It is prudent for the master to watch out the ship response to the bow thruster carefully through checking the compass reading of the heading. No general rule can be applied. The usage of bow thruster in crashing astern may have varied purposes as the traffic situation dictated. Taking the account of the short time period available for us to avoid collision, maybe we should stop the astern engine when vessel just had astern speed. The backward distance we can gain by using higher astern speed may not be justified for the unfavorable sheer caused by the wind/current, due to the astern PP shifting forward which reduce the leverage of bow thruster force.  

Actions To Take For Timing Lapse

Depend on the ship speed, we have three stages when consider the action to take for making some time lapse. As we say the minimum DTC is 7 SL, this is due to the ship need almost 6 SL to make necessary transverse distance to clear her original course line. With the different speed, the same DTC will have different TCPA. If A 285 meters vessel with 20 knots speed need 3.17 minutes to advance 7 SL, then she will need 6.34 minutes to advance 7 SL in 10 knots and 9.5 minutes in 5 knots. 

VESSEL IN FULL SEA SPEED

A 20 knots vessel has a collision risk in 7 SL DTC cannot reduce speed to zero within 7 SL advance (even the engine revolution going reverse will need 12-13 SL to stop the vessel), the option left for her is to alter course since the full rudder can complete a 90 degree turn within 4.5 SL (IMO resolution). In the open sea, using the rudder to avoid the collision is more efficient than reduce the engine. However, circumstances may dictate that using the full rudder (the first choice) to avoid collision is not a very good choice (other vessel traffic may be around her). It is time to call the captain. If the ship's original speed is high and has an ample time before the minimum 7 SL's DTC, stop the engine immediately (some main engine maker may recommend to use the dead slow ahead engine order in this stage to protect main engine) and steady on the same course is the second choice. Stopping the engine may sacrifice some rudder effect, but vessel can still has favorable steerage due to higher initial speed. If the speed is still high and the TCPA time is not long enough to reduce the speed effectively, ship should perform the zigzag swing using full rudder order to drag her down is the third choice. 

USING THE ASTERN ENGINE WHEN SHIP'S INITIAL SPEED IS HIGH 

The side thrust of reversed engine revolution is a dominate force only when the ahead speed almost reduced to zero. By that time speed reduces to almost zero, the PP locates on one eighth SL from the ship's bow. The side thrust of propeller can be ignored when ship speed is more than 5 knots. This feature stand true either the engine is going ahead or astern. If the astern revolution can be established in higher ahead speed (more than 5 knots), we should use it without hesitation. Why hesitated? "Ship swing swiftly to starboard side when engine goes astern" is deeply rooted in our impression. But, it is not true when she have more than 5 knots ahead speed. The PP is located not so far ahead as slower speed (one fourth ship's length from ship's bow) and the rudder is still more effective than the propeller's wheel effect(side thruster force). The ship’s heading can still be in perfect control with the engine revolution is reversed. This can be demonstrated on the drawing above that the ship is traveling along the original course line through 8 SL advance, although the engine is going crash astern. 

Using the reverse engine revolution to further reduce the speed is the option available in higher initial speed. As we had learnt from the chapter two, it is the speed difference from the initial speed to the reduced speed that makes the timing lapse method to avoid the collision feasible or not. For a fixed pitch propeller with diesel internal combustion engine vessel, the reverse revolution has to start by the compressed air reserve tank’s air pressure. It is very hard to establish the reverse revolution when the initial speed is high than maneuvering full ahead. This is why the zigzag maneuvering is an important knowledge to reduce the ship speed.  

 VESSEL IN MANEUVERING FULL SPEED

If the ship's engine is already stand by and has a collision risk when the speed is full ahead. Stop the engine and zigzag it should be used to gain some more speed difference to avoid the collision. This is the first available choice. The ship may lose control of her heading after 3 or 4 times rudder cycling, for the speed is reduced and no engine propulsion on the rudder. Losing steerage due to engine stopped is also known as Titanic Effect. The engine may ring the astern order to pull the vessel back when the speed had reduced to crash astern range, better with bow thruster stand by for use. This is the second choice. If the bow thruster is not ready yet, stop the undesired swing should use the full rudder and engine kick ahead.  

VESSEL IN CRASHING ASTERN RANGE

For a vessel navigate below 5 knots(dead slow ahead), rudder effect is poor. If a risk of collision is involved, the best choice is to reverse the engine (crashing astern) and stand by the bow thruster for the favorite heading control. As we learnt above example, she will have to take 9 minutes to reach the possible area of collision in 7 SL’s DTC. The engine will have enough time to establish the astern speed. Use the bow thruster to steady the heading to desired direction as the speed almost reduced to zero is important to avoid the wind or current force working on adverse effect. If the water depth is in one or times of ship’s draft, let go the anchor in short stay to drag along is also an effective way to slow down the vessel with fairly heading control. 

We can summarize the timing lapse method in three speed range: 

Full sea speed : stop the main engine, zigzag, crash astern  
maneuvering full speed : stop the engine, zigzag, crash astern, kick ahead engine to steady 
speed in crash astern range : go stern engine and use the bow thruster, drag the anchor