International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 1
Number1
March 2007
63
The Determination of a Minimum Critical
Distance for Avoiding Action by a Stand-on
Vessel as Permitted by Rule 17a) ii)
E.W. Rymarz
Gdynia Maritime University, Poland
ABSTRACT: In accordance with Rule 17a)ii) a stand-on vessel may 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 her way is not
taking appropriate action in compliance with the Steering and Sailing Rules. Such an action to avoid collision
must be taken in proper time. An OOW should know the minimum distance for taking avoiding action in
every particular case of approaching vessels. The safe distance mainly depends of the relative bearing of an
approaching vessel and her speed. In particular own vessel manoeuvrability should be taken into account. The
safe minimum distance could be calculated in advance with an Excel calculation sheet. This paper presents a
method for preparing a table of safe minimum distances to be used during a Navigational Watch.
1 INTRODUCTION
Rules of Section II Part B - Steering and Sailing
Rules – International Regulations for Preventing
Collision at Sea apply to vessels in sight of one
another. Vessels shall be deemed to be in sight of
one another only when one can be observed visually
from the other. Rules of Section II are based on the
general principle that when two vessels are
approaching one another in such a way as to involve
risk of collision, then one of them is required to keep
out of the way while the other is required to keep her
course and speed. The only exception to this
principle is Rule 14 (Head on situation). Risk of
collision shall be deemed to exist if the compass
bearing of an approaching vessel does not
appreciably change. The distance between vessels
should be also taken into account when risk of
collision is considered. Rules of Section II apply
only to vessels underway and while approaching one
another unintentionally.
So far as both vessels comply with the
COLREGS requirements and fulfill their obligations
in proper time and in a proper way then perfect
safety and successful prevention of collisions is
assured. Problems appear when one or both
approaching vessels fail to comply with Rules 16
and/or 17. Let us consider appropriate COLREGS
requirements. It would be especially useful to
highlight the interpretation of Rule 17, in particular
paragraph a) ii) and b) which are not easy in use. The
philosophy of this paragraph is easy to understand
and its intention is clear but practical compliance is
rather difficult in particular for young and
inexperienced OOWs.
2 TWO POWER DRIVEN VESSELS IN A
CROSSING SITUATION
The most frequent encounters at sea are power
driven vessels in crossing situations. When for
instance two power driven vessels are crossing (see
fig. 1) so as to involve risk of collision, one of them
(vessel A ) having the other on her own starboard
side is obliged by Rule 15 to keep out of the way
and, if the circumstances of the case admit, to avoid
crossing ahead of the other vessel. The other one –
64
the so called stand-on vessel (B) - is required to
maintain her course and speed when risk of collision
first begins to apply. How long then must she keep
her course and speed, waiting for avoiding action by
the so called give-way vessel? The distance between
vessels is still decreasing. The OOW of the stand-on
vessel would like to know why the give-way vessel
is not taking appropriate action. It could be several
reasons: 1) The OOW of the give–way vessel
doesn’t know that his obligation is to give way to
vessel B: 2) The OOW knows his obligation but
having very good ship`s manoeuvring characteristics
he considers that it is too early for avoiding action;
3) Finally, it could be possible that the OOW is not
present in the bridge. When the stand-on vessel is in
doubt whether sufficient action is being taken by the
give-way vessel to avoid collision she is obliged
immediately indicate such a doubt by giving at least
five short and rapid blasts on the whistle in
accordance with Rule 34 d). Much more effective is
to supplement such acoustic signals by a strong light
signal of at least five short and rapid flashes.
Even giving alternative signals (Rule 36) to
attract the attention of other vessel is acceptable. It
could be useful to call other vessel by VHF when the
approaching vessel is still far away. At the last
moment calling is useless and dangerous. If, on
giving such signals, there is no reaction from the
give-way vessel then the OOW of the stand-on
vessel can consider that it has become apparent to
him that the vessel required to keep out of the way is
not taking appropriate action in compliance with the
Steering and Sailing Rules. This is the exact moment
when the stand-on vessel is permitted to take action
to avoid collision by her manoeuvre alone. This is
the beginning of stage III. (fig.1)
I
II
III
IV
II – Keep course and speed – action required
III – Action permitted (vessel may take action)
IV – Action required (compulsory action)
A
B
Two power driven vessels are approaching on collision course.
B – Stand on vessel; A - Give – way vessel
Rule 17 a) i)
Rule 17 a) ii) and c)
Rule 17 b)
Fig. 1. Action by Stand-on Vessel
3 PERMITTED ACTION BY RULE 17A) II)
A stand-on vessel is not specifically required to take
action to avoid collision as soon as it becomes
apparent that the give-way vessel is not taking
appropriate action. She is only permitted to take such
an action. She is also permitted to keep her course
and speed until collision cannot be avoided by the
give-way vessel alone. This is the beginning of stage
IV (see fig.1) A stand-on vessel which fails to take
permissive action (Rule 17 a) ii)) in sufficient time
to avoid collision by her own manoeuvre is likely to
be held at fault if a collision should occur. Before
entering into stage III the Captain or OOW of stand–
on vessel should know the distance to the
approaching vessel which shall be necessary for an
effective action to avoid collision by the stand-on
vessel alone. The earliest moment for permitted
action will obviously be related to the range and the
rate of change of range which mainly depends on the
relative bearing and speed of other vessel. Not only
the earliest but also the very last moment should be
known. In particular what is the minimum critical
distance for taking avoiding action. This limit cannot
be crossed otherwise if no action is taken by the
give-way vessel, collision would be inevitable.
Normally such a critical distance is estimated by the
Captain or OOW and is based on their experience
and ordinary practice. The estimations, in particular
at night, could be inaccurate and approximate only.
Considerable errors could be possible. Precise
information regarding critical distances for particular
stand-on vessels can be calculated in advance.
Critical distances to approaching give-way vessels
are based on the known manoeuvring ability of the
own vessel and are dependent on the relative
bearings and speed of the other vessels. Critical
distances could be presented in a form of a table.
This can be done by using Excel calculating sheets.
4 USING AN EXCEL CALCULATION SHEET
The input data for an Excel calculation sheet can be
obtained from vessel`s sea trials documentation, in
particular information regarding turning circle and
crash stop manoeuvres. As an example given in this
paper all data information concerning the so called
“our vessel” are referred to a bulk carrier
PANAMAX vessel; DWT 62108 t. length 221m and
sea speed 16,2 kts. The calculation of the critical
distance for the PANAMAX vessel is based on the
assumption that a CPA 0,2 nm is considered as a
collision or “near-collision”. Therefore the critical
distance to the approaching give-way vessel is such a
distance that permits the stand-on vessel to avoid
collision by her own manoeuvre with passing
distance CPA 0,2 nm.(see table 2) The relative
bearing of the other vessel and her speed could be
measured with radar. This can be done when the
distance to the other vessel is still considerable. The
65
other necessary input data could be calculated from
the interrelations between a vessel`s motion
parameters and the trigonometric functions shown at
fig. 2. The main input elements of the stand-on
vessel are:
1 true course in degrees;
2 speed in meters per minute (m/min);
3 compass bearing in degrees;
4 relative bearing of approaching vessel;
5 Vy “shifting ahead” (Pc);
6 Vx Shifting aside” (Pb);
Input elements of the give-way vessel are:
1 initial true course;
2 speed in metres per minute (m/min)
The most essential step for the calculation of
critical distance is to prepare the appropriate
calculating program (Excel template). In this regard
a general knowledge for using the Excel calculation
sheet is necessary.
2
)(
cos
sin
wyx
y
x
VVVD
VoV
VoV
++=
⋅=
⋅=
α
α
Y
X
Tan
∆
∆
=
α
500
1000
1500
2000
0
500
-500
-1500
-1000
-Y
+X
+
Y
β
V
x
α
D
bearing
V
o
V
y
course
V
w
Fig. 2. Interrelations for data calculations
Fig. 3. PANAMAX Turning circle to stbrd. -full sea speed
ahead
CRASH STOP
PANAMAX
Czas
y x
00:00 0 0
00:20 160 0
00:40 320 0
01:00 460 0
01:20 590 0
01:40 720 0
02:00 860 0
02:20 975 0
02:40 1090 0
03:00 1200 0
03:20 1300 0
03:40 1400 5
04:00 1483 10
04:20 1560 15
04:40 1630 21
05:00 1692 30
05:20 1750 40
05:40 1805 52
06:00 1860 70
06:20 1910 90
06:40 1950 110
07:00 1981 127
07:20 2005 145
07:40 2017 154
08:00 2030 165
8:00
7:40
7:20
7:00
6:40
6:20
6:00
5:40
5:20
5:00
4:40
4:20
4:00
3:40
3:20
3:00
2:40
2:20
2:00
1:40
1:20
1:00
0:40
0:20
0:00
-250
0
250
500
750
1000
1250
1500
1750
2000
2250
-300 0 300
Fig. 4. PANAMAX - crash stop. From full ahead to stop the
vessel by full astern
66
Table 1 presents the first part of an Excel sheet
with all necessary data inserted. In column 2 there is
the time from the start of avoiding action (turning
circle or crash stop) at 0min.and 00 sec. to the end of
the turning circle or to stop the vessel in the case of
crash stop which takes 8 minutes. The full time of
the turning circle and the crash stop are divided into
25 parts, each of 20 sec. duration. Data in columns 3
and 4 (Pc and Pb) were calculated from the
interrelations presented at fig.2 where Pc=Vy and
Pb=Vx. See the triangle with the following sides:
Vo; Vx; and Vy.
Data in columns 5 and 6 are calculated
automatically after other data are inserted.
In the column 7(x) there is only 0 entered because
the course of our vessel is “N” 000° and there is no
east-west movement in the x axis. Data in column 8
(y) presents northerly movement of our vessel`s
position after every 20 sec.
Table 1. Critical distance calculation with Excel system for example PANAMAX vessel
Turning circle to starboard
Bulkcarrier Panamax DWT: 62108 t.; relative bearing port 30; speed: Vo=Vw
Data:
Name: bulkcarrier Panamax DWT:62108
Length : 221 m
Bearing of an approaching vessel 330°
Relative bearing : -30°
Closest Point of Approach 370 m
Speed of vessels : Vo=Vw
Result of analysis
Other vessel
Own vessel
No action Turning circle to strb.
1 2 3 4 5
6 7 8 9 10 11 12 13
No
Time
Pc(m)
Pbm)
x
y
x
y
bearing
distance
y
x
distance
1
00:00
0
0
-1133
1963
0
0
330
2267
0
0
2267
2
00:20
-83
144
- 989
1880
0
167
330
1978
165
2
1980
3
00:40
-167
289
-845
1796
0
333
330
1689
318
19
1712
4
01:00
-250
433
-700
1713
0
500
330
1401
442
48
1475
5
01:20
-333
577
-556
1630
0
667
330
1112
572
105
1247
6
01:40
-417
722
-412
1546
0
833
330
823
667
190
1065
7
02;00
-500
866
-267
1463
0
1000
330
534
728
268
909
8
02:20
-583
1010
-123
1380
0
1167
330
246
674
360
782
9
02:40
-667
1155
21
1296
0
1333
150
43
772
452
678
10
03:00
-750
1299
166
1213
0
1500
150
332
752
539
593
11
03:20
-833
1443
310
1130
0
1667
150
620
708
615
520
12
03:40
-917
1588
455
1046
0
1833
150
909
654
668
447
13
04:00
-1000
1732
599
963
0
2000
150
1198
587
702
390
14
04:20
-1083
1877
743
880
0
2167
150
1486
510
721
370
15
04:40
-1167
2021
888
796
0
2334
150
1775
436
716
399
16
05:00
-1250
2165
1032
713
0
2500
150
2064
371
690
484
17
05:20
-1333
2310
1176
630
0
2667
150
2352
317
656
607
18
05:40
-1417
2454
1321
546
0
2834
150
2641
269
605
767
19
06:00
-1500
2598
1465
463
0
3000
150
2930
235
546
947
20
06:20
-1583
2743
1605
380
0
3167
150
3219
214
480
1141
21
06:40
-1667
2887
1754
296
0
3334
150
3507
214
410
1346
22
07:00
-1750
3031
1898
213
0
3500
150
3796
230
340
1558
23
07:20
-1833
3176
2042
130
0
3667
150
4085
256
283
1764
24
07:40
-1917
3320
2187
46
0
3834
150
4373
295
238
1965
25
08:00
-2000
3464
2331
-37
0
4000
150
4662
344
198
2167
Own vessel
Other vessel
Course 000°
V 16,2 kts
500m/min
Vx 0 m/min
Vy 500 m/min
Starting point
x 0 m
y 0 m
Course 120,0°
V 16,2 kts
500m/min
Vx 433 m/min
Vy -250m/min
Starting point
x -1133 m
y 1963 m
Distance at the time 00:00 (m) : 2267
Nautical miles : 1,224
Length of vessel multiplicity : 10,3
67
Table 2. PANAMAX critical distances to approaching vessel for permitted avoiding action in accordance with Rule 17 a)ii)
Distance in metres (m), Nautocal miles (nm) and length of own vessel (L)
Relative
Bering of
approaching
vessel
Speed of approaching vessel
Vo = Vw - 5 knots
Vo = Vw
Vo = Vw + 5 knots
Turning circle
Crash stop
Turning circle
Crash
stop
Turning
circle
Crash stop
Port 10°
1823 m
1,0 nm
8,2 L
5427 m
3,0 nm
25 L
2269 m
1,2 nm
10,3 L
7022 m
3,8 nm
32 L
2721 m
1,5 nm
12,3 L
8607 m
4,7 nm
39 L
Port 20°
1715 m
0,9 nm
7,8 L
3836 m
2,1 nm
17,4 L
2204 m
1,2 nm
10 L
4954 m
2,7 nm
22,4 L
3286 m
1,8 nm
15 L
6031 m
3,3 nm
27,3 L
Port 30°
1478 m
0,8 nm
6,7 L
2685 m
1,5 nm
12,1 L
2267 m
1,2 nm
10,3 L
3626 m
2,0 nm
16,4 L
3030 m
1,6 nm
13,7 L
4462 m
2,4 nm
20,2 L
Port 40°
1021 m
0,6 nm
4,6 L
1816 m
1,0 nm
8,2 L
1766 m
1,0 nm
8 L
2872 m
1,6 nm
13 L
2439 m
1,3 nm
11 L
3658 m
2,0 nm
16,6 L
Port 50°
N/A
N/A
1350 m
0,7 nm
6,1 L
2256 m
1,2 nm
10,2 L
1956 m
1,0 nm
8,8 L
3051 m
1,7 nm
13,8 L
Port 60°
N/A
N/A
992 m
0,5 nm
4,5 L
1682 m
0,9nm
7,6 L
1568 m
0,9 nm
7,1 L
2532 m
1,4 nm
11,5 L
Port 70°
N/A
N/A
699 m
0,4 nm
3,2 L
1170 m
0,6 nm
5,3 L
1257 m
0,7 nm
5,7 L
2099 m
1,1 nm
9,5 L
Port 80°
N/A
N/A
484 m
0,3 nm
2,2 L
703 m
0,4 nm
3,2 L
1019 m
0,6 nm
4,6 L
1762 m
1,0 nm
8,0 L
Data at fig. 3 and 4 are presenting our
(PANAMAX) vessel`s positions for every 20
seconds on the x/y axes. These data are based on the
sea trials documentation. Data from fig.3 and 4
should be inserted into the Excel calculating sheet in
the column “Own ship” of table 1. When the critical
distance is calculated for permitted action “hard to
starboard” - turning circle - then data from fig.3 (x/y
positions) should be inserted in the columns 11(y)
and (12) - “Turning circle to starboard”. When the
critical distance is calculated for permitted action
“crash stop” then data from fig.4 should be inserted
in the same places as in the case of the “turning
circle” above.
Data in all other columns will be calculated
automatically. The most important information given
by Excel (in the inner table of table 1) is a distance
at the time 00:00 which means the critical distance.
Table 1 is an example only for calculation of one
particular case for the PANAMAX vessel, namely
for 30° relative bearing of the approaching vessel
and her speed which is the same as our speed: 16,2
kts.
A similar calculation must be carried out for
every 5 or 10 degrees of port side relative bearings
and for different speeds of the other vessel. When
preparing a table for presenting critical distances,
easy interpolation should be taken into account.
The table 2 presents critical distances from our
vessel (stand-on) to the other vessel (giving-way)
approaching on a collision course on unchanging
compass bearing. Distances in the table are given in
metres, nautical miles and length of own vessel. For
COLREGS purposes the most suitable are distances
in nautical miles. Fig.5 presents a diagram which is
easy in use, in particular for interpolation. Critical
distances could be prepared in different forms.
5 CONCLUSIONS
In the presented research work over one thousand
crossing vessel situations were examined and critical
distances for permitted actions were calculated. Two
types of permitted actions were considered: turning
circle to starboard and crash stop. Investigation
comprised different speeds of the give-way vessel
from11,2 to 21,2 kts. and relative bearings from port
10° to port 80°. It has been proved that in all cases
the most effective and safe permitted avoiding action
when the distance between vessels approached the
critical value, is turning hard to starboard. When a
give-way vessel is approaching on a collision course
on a relative bearing from 5 to 35 degrees, a crash
stop as a permitted avoiding action is nearly useless.
Results of conducting this research regarding critical
distances have been presented at table 2 and figure 5.
68
All calculated critical distances were verified and
confirmed by manoeuvring simulator. Participants
interested in the COLREGS are invited to see the
manoeuvring simulator demonstration.
6 PROCEDURE TO USE THE CRITICAL
DISTANCES TABLE
An example is given for a PANAMAX vessel (speed
16 kts). When for instance on the port side of the
vessel on relative bearing 30° a power driven vessel
appears, then use the following procedure:
1 Take the compass bearing and observe its
changing tendency;
2 If the bearing does not appreciably change and the
distance is decreasing– estimate the give-way
vessel`s speed by radar; (her speed is for instance
16 kts.)
3 Having relative bearing (port 30°) and speed 16
kts enter the table 2 and from the column
“Vo=Vw” -“Turning circle” and line “Port 30°”
take the critical distance. In this case the critical
distance is 1,2 nm;
This means that before the distance to the
approaching vessel has decreased to 1,2 nm you
have to give all appropriate signals to “wake-up” the
other vessel to undertake proper action. Seeing no
response you may consider that it has become
apparent to you that the vessel required to keep out
of the way is not taking appropriate action in
accordance with Rule 15 and 16. In this case you
may take action to avoid collision by your
manoeuvre alone before the time when the distance
reduces to 1.2 nm. Having advance information on
the critical distance for the current situation, the time
and form for taking avoiding action is rather clear.
REFERENCES
Cockcroft, A.N. & Lameijer I.N.F. 2004. A guide to the
Collision Avoidance Rules. Elsevier Butterworth-
Heinemann, Oxford.
Koszewski Z.& Gorazdowski S. 1965. Międzynarodowe prawo
drogi morskiej, Gdańsk, Wydawnictwo Morskie.
Curtis Frye 2003 Mocrosoft Office Excel 2003 step by step
(Polish translation by Kolczynski P.)
Rymarz W. 2004, Międzynarodowe prawo drogi morskiej w
zarysie (in Polish). Wydawnictwo Trademar, Gdynia.
Rymarz W. 2006. Międzynarodowe prawo drogi morskiej –
obliczenia za pomocą arkusza kalkulacyjnego Excel mane-
wrów zapobiegawczych zderzeniu zgodnie z prawidłem
17a)ii), Akademia Morska Gdynia (in Polish).