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?
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.
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.
