Radarplottex
Radarplottex
Radarplottex
Introduction
International Regulations for Preventing Collision at Sea
Rule 5 (Lookout) “Every vessel shall at all times maintain a proper look-out by
sight as well as by 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”.
The rules require a vessel fitted with radar to make use of its detection capability
for a full appraise by monitoring and radar plotting, especially in restricted
visibility.
1
The bright image of vessels and coast that radar “paints” on its PPI (the radar
screen) is valued by mariners at night, foul weather and in restricted visibility. Its
use is a requirement of Rule 5, (Lookout). However, there are potential dangers
for the unwary in interpreting the convincing map like image on the radar screen.
The image you “see” on the radar screen is derived from radio wave
echoes. A poorly tuned radar may detect only some of the picture or none
at all. You could falsely assume nothing is there.
To judge if you will pass at a safe distance or collide with a target cannot be
based on casual viewing, but requires the observation of change over a period of
time (an interval) to extract the real motion from the relative. A larger vessel’s
equipment, with automatic plotting aids (ARPA) can analyse developing collision
risks, but smaller vessels must correctly interpret the display by:
The radar’s EBL facility allows a bearing line to be generated on a screen that
can be pointed at any target displayed. By marking the targets with a pen, (either
on the screen or on a transparent overlay) and comparing their positions after an
interval of time, a prediction can be made of how they will continue to move.
The targets that cling to the EBL line (their relative angle remaining constant)
indicate high risks of collision, always assuming that the course and speed of
your own vessel and that of another vessel remain constant.
2
At 12:00 your own vessel steering 000º, (at the centre), detects targets 1, 2, 3
and 4 as dots. Each is marked on the screen and over the next 15 minutes the
EBL cursor is used to monitor the targets’ apparent movement.
At 12:15:
Anchored target 1 has diverged from the EBL and is painted as moving in the
opposite direction to your own vessels course. Its bearing draws aft from your
course line (passes astern). Its range increases.
Overtaking target 2 diverges from the EBL and is painted moving faster than
your vessel, to pass ahead. Its bearing draws forward. Its range increases.
Crossing target 3 diverges from the EBL and is painted moving slower than
your vessel and passes astern. Its bearing draws aft. Its range decreases.
Crossing target 4 bearing is steady on the EBL, range decreased. Collision will
occur without avoidance action. If the distance between 12:00 and 12:15 position
is compared with the distance between 12:15 and the screen centre we can
predict that collision will occur after 1.3 x the interval.
3
Proper use of radar
International Regulations for Preventing Collision at Sea specify radar usage:
Rules 6 (Safe Speed extract), “every vessel shall at all times proceed at a safe
speed so that she can take proper and effective action to avoid collision.”
Rule 7 (Risk of Collision extract) warns that “assumptions shall not be made on
the basis of…scanty radar information”
Rule 19 (Restricted Visibility extract) requires that “a vessel which detects by
radar alone the presence of another vessel shall determine if a close-quarters
situation is developing and/or risk of collision exists…she shall take avoiding
action in ample time”.
While our EBL watch alerts us to a close-quarters situation or risk of collision it
could provide insufficient detail to determine effective avoiding action. It is scanty
information and we must use the process of a Radar Plotting in ample time.
4
The radar display
There are two basic display systems, one with the heading marker fixed and
pointing to the top of the screen (Ship’s Head Up) and one with the heading
marker moving with the vessel’s course to align with the numbers marked on the
display’s bearing scale (North Head Up).
SHU NHU
Ship’s Head Up North Head Up
Target paint slews as the vessel The heading marker moves with vessel
yaws, creating bearing inaccuracy. yaw, greater bearing inaccuracy.
The principles are the same for both radar displays. Relative target bearings
should always be converted to true as the risk of collision is indicated by steady
compass bearing and decreasing range. The following pages show examples.
5
The NHU plot, step by step.
Our aim is to find:
the closest point of approach of the target (CPA) and its time (TCPA).
the course and speed of another vessel (WA).
the aspect (the vessel’s lights/profile from our view) and avoidance action.
The positions O and A are found from the first and final reports. A line is drawn
from O through A to pass C. The line CP is drawn by dropping a perpendicular
from the OA extension to C. The length of CP (in this case 2.2nm) is the CPA.
6
Step 3: Calculating the TCPA.
The line OA represents the relative movement of the target over 12 minutes (1/5
of an hour) and the extension AP is a prediction of its continuing movement (if
both vessels courses and speed remain constant). A glance will tell you that if
line OA represents 12 minutes and line AP is twice as long, then it represents
roughly 24 minutes. However, this is “scant information” and a more elegant
solution can be found in the formula:
7
Step 4: Resolving the plotting triangle.
The OA is the resultant of the two vectors (components of the plotting triangle):
8
Step 5: Finding the speed of another vessel.
The distance that the target covered in the interval is shown by the length of the
line WA. In this example we can measure this as 4 nm. Therefore, as it travelled
4 nm in 12’ (0.2 hrs) then in 1 hr it would travel:
1 x 4 = 20 kts
0.2
Here it can be seen how selection of time interval can simplify later calculations
3 mins 6 mins 12 mins 15 mins 20mins 30 mins
1/20th hour 1/10th hour 1/5th hour 1/4th hour 1/3th hour 1/2th hour
0.05 hour 0.1 hour 0.2 hour 0.25 hour 0.33 hour 0.5 hour
In our example it travelled 4 nm in 1/5th hour, so WA = 5 x 4 = 20 kts
9
Step 6: Finding the aspect of another vessel.
Aspect is the relative bearing of your own vessel taken from the target vessel's
fore and aft line. It is expressed red or green. Aspects derived from plots are
approximate, but tell you roughly the target’s profile from your viewpoint and what
navigation lights you should look for at the time of the final report.
10
Mathematically:
Aspect found by the reciprocal of the target's last bearing (its bearing of us, A to
C) and the targets true course (WA).
The plot shows that we are on the port side of the target vessel so the target's
aspect is R 58°.
While the vessels are not in sight of each other, to know aspect is not vital, but
when the other vessel becomes visible, then the rules for vessels in sight of each
other apply and suitable action will need to be taken depending on it being a
head on vessel, a crossing vessel or an overtaking vessel.
Time 10:12.
Target bearing 060°T drawing forward.
Target range 8.0 miles and closing.
CPA 2.2 miles in 23 mins at 10:35.
Target's course 298°T.
Target's speed 20 knots.
Aspect R58°.
11
The SHU plot, step by step.
Our aim is to find:
the closest point of approach of the target (CPA) and its time (TCPA).
the course and speed of another vessel (WA).
the aspect (the vessel’s lights/profile from our view) and avoidance action.
The positions O and A are found from the first and final reports. A line is drawn
from O through A to pass C. The line CP is drawn by dropping a perpendicular
from the OA extension to C. The length of CP (in this case 2.2nm) is the CPA.
12
Step 3: Calculating the TCPA.
The line OA represents the relative movement of the target over 12 minutes (1/5
of an hour) and the extension AP is a prediction of its continuing movement (if
both vessels courses and speed remain constant). A glance will tell you that if
line OA represents 12 minutes and line AP is twice as long, then it represents
roughly 24 minutes. However, this is “scant information” and a more elegant
solution can be found in the formula:
13
Step 4: Resolving the plotting triangle.
The OA is the resultant of the two vectors (components of the plotting triangle):
14
Step 5: Finding the speed of another vessel.
The distance that the target covered in the interval is shown by the length of the
line WA. In this example we can measure this as 4 nm. Therefore, as it travelled
4 nm in 12’ (0.2 hrs) then in 1 hr it would travel:
1 x 4 = 20 kts
0.2
Here it can be seen how selection of time interval can simplify later calculations
3 mins 6 mins 12 mins 15 mins 20mins 30 mins
1/20th hour 1/10th hour 1/5th hour 1/4th hour 1/3th hour 1/2th hour
0.05 hour 0.1 hour 0.2 hour 0.25 hour 0.33 hour 0.5 hour
In our example it travelled 4 nm in 1/5th hour, so WA = 5 x 4 = 20 kts
15
Step 6: Finding the aspect of another vessel.
Aspect is the relative bearing of your own vessel taken from the target vessel's
fore and aft line. It is expressed red or green. Aspects derived from plots are
approximate, but tell you roughly the target’s profile from your viewpoint and what
navigation lights you should look for at the time of the final report.
16
Mathematically:
Aspect found by the reciprocal of the target's last bearing (its bearing of us, A to
C) and the targets true course (WA).
The plot shows that we are on the port side of the target vessel so the target's
aspect is R 58°.
While the vessels are not in sight of each other, to know aspect is not vital, but
when the other vessel becomes visible, then the rules for vessels in sight of each
other apply and suitable action will need to be taken depending on it being a
head on vessel, a crossing vessel or an overtaking vessel.
Time 10:12.
Target bearing 052° Rel. drawing fwd. (008°T + 052°Rel = 060°T)
Target range 8.0 miles and closing.
CPA 2.2 miles in 23 mins at 10:35.
Target's course 290° Rel. (290° Rel + 008°T = 298°T)
Target's speed 20 knots.
Aspect R58°,
17
Action to avoid collision
In the previous example the CPA was just over 2 nm, a reasonable clearance for
small vessels, but less than the stopping distance of a very large vessel. Once
again, the prediction from the plot relies on the course and speed of both vessels
not changing.
However, there are situations of being in sight of one another and being in or
near restricted visibility where specific rules come into play.
“(i) An alteration of course to port for a vessel forward of the beam, other than for
a vessel being overtaken”
Rule 19 (e) “…every vessel …which cannot avoid a close-quarters situation with
another vessel forward of her beam, shall reduce her speed to be the minimum
at which she can be kept on her course….”
18
In deciding the proper action to take to avoid collision in our previous examples
of the SHU and NHU plot, we were steering 008° T, the target was bearing
around 060° T and we might consider the range decreasing and a predicted CPA
of 2.2 nm to be too close for comfort in restricted visibility.
Rule 19 (d) (i) states that you should avoid an alteration of course to port for a
vessel forward of the beam. This target is forward of your starboard beam.
19
Check your progress
In the following situations, complete a plot and final report. Check your answers
later.
20
Answers to check your progress
Plot 1. Our full report at 12:09 would be as follows:
Time 12:09.
21
Plot 2. Our full report at 09:12 would be as follows:
Time 09:12.
22
Plot 3. Our full report at 14:40 would be as follows:
Time 14:40.
23
Another vessel’s change in course or speed.
Detecting and interpreting change in target’s motion:
Targets detected at close quarters were plotted below to find their initial OAP
line, predicted CPA, course/speeds and aspects (all Red 75º). Following best
practice, the targets were subsequently monitored and deviations from the
predicted OAP lines were detected.
Caution is required to ensure target “wander” is not a changing relative view due
to our own vessel’s yaw. An additional consideration is that larger craft can take
ten minutes or more to settle on a new course and even longer to reach a higher
speed. In our decisions the Collision Regulations, Risk of Collision Rule 7c, must
be considered - Assumptions shall not be made on the basis of scanty
information, especially scanty radar information.
In this case however, at successive 3 minute intervals the targets consistently
lined up as new AP¹ lines, coincidentally originating from A as the moment of
target behaviour change. The OA line is the resultant of the vectors of course &
speed of own vessel (WO) and course & speed of another vessel (WA). As our
vessel maintained constant course & speed (no leeway/current were present)
this target behaviour must have been due to the other vessel’s changes in
course, or speed or both.
If our vessel slows from 20 knots to10 knots, or speeds up 30 knots, the W¹
24
Targets 1-3 initially being aspect Red 75º are crossing situations making us the
give way vessel (Rule 15). The obligation for avoidance action rests with us, but
as the other vessel took action Rule 8d Action to Avoid Collision comes into play
(action taken to avoid collision ... shall... result in passing at a safe distance. The
effectiveness ... checked until the other vessel is finally past and clear).
Only Target 1 positively identified itself by radio to notify its 25º to starboard
altered course, maintained speed and intention to pass ahead. A recommended
substantial course change (30º to 60º) would have increased their aspect to Red
111º or more, close to or becoming an overtaking give way vessel. Hence the
radio message to ensure we understood its actions. However, while Targets 2
and 3’s new CP¹A’s are displayed, the course/speed/aspect require re-plotting
with new O¹A¹W¹ vector triangles as shown below.
Aspects of all other vessels must be monitored and re-assessed in order to
visually identify them, allowing that they may not have detected us and their
changed behaviour may not be avoidance action but an operational manoeuvre.
If our vessel slows from 20 knots to10 knots, or speeds up 30 knots, the W¹
25
Note that a modest change in course was as effective as halving speed in
increasing CP¹A¹ to 1 mile but additionally delayed the TCP¹A¹ to 24”. Also,
Target Three’s remarkable ability to double its speed to 30 knots increased
CP¹A¹ admirably but also slashed the TCPA to just over 15”.
If our vessel slows from 20 knots to10 knots, or speeds up 30 knots, the W¹
26
Changing of course, slowing down or speeding up after a delay:
As with the previous examples of the O¹A¹W¹ plots can be constructed by
transferring the initial 6 minutes (9”- 3”) and constant OW vector from the point of
target motion change, called O¹. But in the case of this 3 minutes delay not at
position A, but from the point that the newly lined up target detections cross the
initial OA line extension. Position A¹ is found at the target detection point at the
repeat 6 minute interval. The direction and length of W¹A¹ enable the other’s
course, speed and aspect to be determined.
If our vessel slows from 20 knots to10 knots, or speeds up 30 knots, the W¹
Note that in practice it is very difficult to determine the precise moment that the
other vessel changes its motion. Caution is required to rule out target “wander”
caused by our own vessel’s yaw. Rule 7i Risk of Collision specifies Such risk
shall be deemed to exist if the compass bearing of an approaching vessel does
not appreciably change. An additional consideration is that larger craft can take
ten minutes or more to settle on a new course and even longer to reach a higher
speed. In our decisions Rule 7c, must also be considered - Assumptions shall not
be made on the basis of scanty information, especially scanty radar information.
In summary, some time is required for the target line to settle down. Note that
Target Six’s remarkable ability to double its speed to 30 knots increased CP¹A¹
but also slashed the TCPA to just over 15”. In fact, by the time we were able to
determine the new aspect it had already overtaken us and was finally past and
clear.
27
Own vessel’s change in speed.
To minimize collision risk by increasing CPA a vessel could stop, slow down or
(often less achievable) speed up. In seeking this improved CPA a new vector plot
is first drawn. In the examples below we altered speed at A, so this is effectively
also a new position O¹. The W¹A (our speed) is the vector that changes while the
W¹A¹ vector (another’s speed) stays constant.
28
Plotting own vessel’s change in speed.
Plot 1a. Own vessel’s slows its speed.
Our vessel while on a course of 030ºT at a speed of 20 kts plots a target
ahead with 0.5 miles predicted CPA requiring avoidance speed change.
29
Plot 1b. Own vessel’s change in speed after a delay.
Our vessel while on a course of 030ºT at a speed of 20 kts plots a target
ahead with 0.5 miles predicted CPA requiring avoidance speed change.
Finding speed change required to increase CP¹A¹ from 0.5 to 2 miles.
From the (current) target position after the delay, in this case 12:12 draw a line to
P¹ as the required 2 miles CP¹A. Transfer a line parallel to this extending through
A to cross the initial WO to find O¹. Measure the new WO¹ to find our vessel’s
travel over the initial between 12:30 -12:09 interval. Calculate the speed from the
multiples of this distance that would be covered in 60 minutes, in this case:
Measured 1 nm x 10 (6 mins) = 10 kts
30
Plotting own vessel’s change in course.
Plot 2a. Own vessel’s change in course.
Our vessel while on a course of 030ºT at a speed of 20 kts plots a target
ahead with 0.5 miles predicted CPA requiring avoidance course change.
Finding the CP¹A resulting from chosen 37º to Stb course change.
Transfer the avoidance course (030ºT + 37º Stb = 067ºT) from the outer bearing
scale and draw as a line from W extending in the avoidance course direction.
With dividers spanning WO, swing arc from O to cross the avoidance course line.
Call the crossing point O¹. Draw a line from O¹ back through A and extend past
the centre. A perpendicular from the centre crosses at P¹, the new CP¹A.
31
Plot 2b. Own vessel’s change in course after a delay.
Our vessel on 030ºT and speed 20 kts plots a target ahead with 0.5 mile
CPA requiring avoidance course change actioned after 3 minutes delay.
Finding the CP¹A¹ with delayed 54º to Stb course change:
Calculate the new avoidance course from the current course plus or minus the
turn away from it (in the example, 030ºT + 54º Stb = 084ºT). From initial W to O¹.
draw this directional vector the same length as initial WO. From O¹ draw a line
back through A and past the centre. Transfer a line parallel to this to pass
through A¹ (the 12:12 position) and past the centre. This gives the new CP¹A¹ of
2 miles.
Finding course required to increase 0.5 miles CPA to 2 miles after delay:
Draw a line from P¹ (the chosen new 2 miles CP¹A¹) to A¹ (the 12:12 position).
Draw a line parallel to this and extend through the initial A to well past the initial
WO line. With dividers spanning the initial WO line, sweep an arc from W to find
new position O¹ where the previously drawn line is crossed. Line WO¹ is the new
avoidance course, in this case 084ºT.
32
Geometry of plots
Explanation of Plot 1a from Page 29:
The underpinning geometry of the previous simplified plot can be seen below,
showing how the in both cases the solution essentially re-plotted a new O¹A¹W¹
vector triangle. For example:
33
Explanation of Plot 1b from Page 30.
The underpinning geometry of the previous simplified plot can be seen below,
showing how the in both cases the solution essentially re-plotted a new O¹A¹W¹
vector triangle. For example:
Finding speed change required to increase CP¹A¹ from 0.5 to 2 miles.
Find the O¹ on the initial OAP line representing the delay period, in this case 3
minutes after A. Draw a line from P¹O¹ (the chosen 2 miles CP¹A¹). From O¹
transfer a line parallel to the initial OW. Calculate the other vessel’s travel over
the delay period (1.3 x 3/6 mins = 0.65 miles) then transfer the initial WA vector
to P¹ and mark off the 0.65 miles. From that position a line parallel with P¹O¹ will
find W¹ and the length W¹O¹ provides the required speed, in this case:
Measured 0.5 nm x 3/60th hour (3 mins) = 10 kts
34
Explanation of Plot 2a. from Page 31.
The underpinning geometry of the previous simplified plot can be seen below,
showing how the in both cases the solution essentially re-plotted a new O¹A¹W¹
vector triangle. For example:
Finding the CP¹A resulting from chosen 37º to Stb course change.
A new WO vector in the avoidance course direction (030ºT + 37º Stb = 067ºT)
can be drawn from position A (at 12:09) or the last target position O¹ (as below).
Marking the length of our unchanging speed (the initial WO length) on this line
finds a point W¹. Transferring the initial and unchanged vector WA from W¹ finds
a point A¹. A line joining A to A¹and extended past the centre finds the new CP¹A.
35
Explanation of Plot 2b. from Page 32.
The underpinning geometry of the previous simplified plot can be seen below,
showing how the in both cases the solution essentially re-plotted a new O¹A¹W¹
vector triangle. For example:
Finding the CP¹A¹ with delayed 54º to Stb course change:
Extend the vector lines WO and AO lines by 3 minute to the points W¹ and A¹.
Transfer the avoidance course (030ºT + 54º Stb = 084ºT) from the outer bearing
scale and draw as a line from W¹ extending in the avoidance course direction.
With dividers spanning WO, swing arc from O to cross the avoidance course line.
Call the crossing point O¹. Draw a line from O¹ back through A and extend past
the centre. A perpendicular from the centre crosses this at P¹, the new CP¹A¹.
Finding course required to increase 0.5 miles CPA to 2 miles after delay:
Extend the vector lines WO and AO lines by 3 minute to the points W¹ and A¹.
Draw line from P¹ (new 2 miles CP¹A¹) through A (the 2:12 position) and extend.
With dividers spanning line WO, swing an arc from O to new position where the
extended P¹A line is crossed. Call this O¹. Draw line WO¹ and transfer it to the
outer bearing scale to read off the avoidance course, in this case 084ºT.
36
Extracts for radar from the International
Regulations for Preventing Collision at Sea
1972
Part A – General
Rule 2 Responsibility
(a) Nothing in these Rules shall exonerate any vessel, or the owner, master, or
crew thereof, from the consequences of any neglect to comply with these Rules
or of the neglect of any precaution which may be required by the ordinary
practice of seamen, or by the special circumstances of the case.
Rule 5 Look-out
Every vessel shall at all times maintain a proper look-out by sight as well as by
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.
37
(ii) Any constrains imposed by the radar range scale in use;
(iii) The effect on radar detection of the sea state, weather and other sources of
interference;
(iv) The possibility that small vessels, ice and other floating objects may not be
detected by radar at an adequate range;
(v) The number location and movement of vessels detected by radar;
(vi) The more exact assessment of the visibility that may be possible when radar
is used to determine the range of vessels or other objects in the vicinity.
38
the case, take early action to allow sufficient sea room for the safe passage of
the other vessel.
(ii) A vessel required not to impede the passage or safe passage of another
vessel is not relieved of this obligation if approaching the other vessel so as to
involve risk of collision and shall, when taking action, have full regard to the
action which may be required by the rules of this part.
(iii) A vessel the passage of which is not to be impeded remains fully obliged to
comply with the rules of this part when the two vessels are approaching one
another so as to involve risk of collision.
Rule 11 Application
Rules in this section apply to vessels in sight of one another.
Rule 13 Overtaking
(a) Notwithstanding anything contained in the Rules of Part B, Sections I and II,
any vessel overtaking any other shall keep out of the way of the vessel being
overtaken.
(b) A vessel shall be deemed to be overtaking when coming up with a another
vessel from a direction more than 22.5 degrees abaft her beam, that is, in such a
position with reference to the vessel she is overtaking, that at night she would be
able to see only the sternlight of that vessel but neither of her sidelights.
(c) When a vessel is in any doubt as to whether she is overtaking another, she
shall assume that this is the case and act accordingly.
(d) Any subsequent alteration of the bearing between the two vessels shall not
make the overtaking vessel a crossing vessel within the meaning of these Rules
or relieve her of the duty of keeping clear of the overtaken vessel until she is
finally past and clear.
39
Rule 16 Action by Give-way Vessel
Every vessel which is directed to keep out of the way of another vessel shall, so
far as possible, take early and substantial action to keep well clear.
40
she can be kept on her course. She shall if necessary take all her way off and in
any event navigate with extreme caution until danger of collision is over.
Rule 21 Definitions
(a)"Masthead light" means a white light placed over the fore and aft centreline of
the vessel showing an unbroken light over an arc of horizon of 225 degrees and
so fixed as to show the light from right ahead to 22.5 degrees abaft the beam on
either side of the vessel.
(b) "Sidelights" means a green light on the starboard side and a red light on the
port side each showing an unbroken light over an arc of horizon of 112.5 degrees
and so fixed as to show the light from right ahead to 22.5 degrees abaft the beam
on the respective side. In a vessel of less than 20 meters in length the sidelights
may be combined in one lantern carried on the fore and aft centreline of the
vessel.
(c) "Sternlight", means a white light placed as nearly as practicable at the stern
showing an unbroken light over an arc of horizon of 135 degrees and so fixed as
to show the light 67.5 degrees from right aft on each side of the vessel.
(d) "Towing light" means a yellow light having the same characteristics as the
"sternlight" defined in paragraph (c) of this Rule.
(e) "All round light" means a light showing an unbroken light over an arc of
horizon of 360 degrees.
(f) "Flashing light" means a light flashing at regular intervals at a frequency of 120
flashes or more per minute.
41
42