International Journal

on Marine Navigation

and Safety of Sea Transportation

Volume 3

Number 1

March 2009

Behaviour Patterns in Crossing Situations

J. Kemp

Professor Emeritus, Arundel, UK

1 INTRODUCTION

A recent paper (John Wilde Crosbie, 2009) con-

tained an examination of the development of the

COLREGS from the early 19

century to the present

day. This led to him to conclude that the current

form of the COLREGS is over-complicated and un-

suited to present day conditions. Also that they

should be replaced by a much simpler convention

based on proposals by Commander, later Vice-

Admiral, P.H. Colomb of the UK Royal Navy in the

late 1800s (Colomb, 1866, 1885). This is referred to

in the following sections as the Colomb/Crosbie

proposal, and some of its implications are consid-

ered in this paper.

A general rule to implement the Colomb/Crosbie

proposals might state that a vessel taking action to

avoid collision should not pass ahead of the other

vessel. Either vessel in an encounter would be per-

mitted to take appropriate action.

In a case where a vessel sees another crossing

from her own starboard bow, an alteration of course

to starboard to pass under the stern of the approach-

ing ship would be appropriate action and similar to

the usual action taken under the current rules. In the

case of a ship which sees another crossing from her

port bow, an alteration of course to port would be

appropriate action under the Colomb/Crosbie con-

vention but this would be quite different to the re-

quirement to maintain course and speed under the

current rules.

Clearly, there is a possibility in either of the

above situations that, if each ship attempts to pass

astern of the other, their actions might cancel and

there would be a renewed risk of collision. Howev-

er, for reasons which he explains in his paper, John

Wilde Crosbie (JWC) believes that such conflicting

actions would be rare and, if they should occur, they

could easily be resolved.

JWC bases his conclusions on an analysis of

COLREG developments and the views of commen-

tators, particularly in the UK during the 19

century,

when the COLREGS first became properly formal-

ised. He provides no experimental evidence to sup-

port his arguments, but the present author is remind-

ed of the results of some radar simulator trials he

ABSTRACT: The January 2009 issue of the Journal of Navigation included a paper (John Wilde Crosbie

2009) entitled, “Revisiting the lessons of the early steering and sailing rules for an e-navigation age.” Follow-

ing a description of the development of the COLREGS from the early 19th century, he concluded that the cur-

rent steering and sailing rules should be replaced by a single rule more suited to modern conditions. This

might take the form of rule stating that a vessel taking action to avoid collision should not pass ahead of the

other vessel.

Such a rule would require a radical change in the philosophy of collision avoidance at sea, and evidence is re-

quired that it would be both effective and acceptable by mariners. Radar simulator experiments, conducted by

the author some years ago, in another context, suggest that this might be the case. An analysis of the experi-

mental results and some conclusions are reported in this paper. The author believes that further trials, specifi-

cally designed to test the Crosbie proposals, would be desirable.

conducted some thirty years ago. These results have

never been published but, since they shed light on

the possible acceptability of JWC’s conclusions,

they are reported in the following sections of this

paper.

2 EXPERIMENTAL DESIGN

In this section, two radar simulator experiments are

described. In the first, each subject was presented

with a situation in which a “target vessel” was ap-

proaching from the port bow of the subject’s “own

ship” (see fig. 1). The target vessel’s course was at

right angles to the own ship’s course. The own

ship’s full speed was 15 knots, but it was initially set

at half speed of 10 knots. Collision would occur af-

ter 33 minutes if the subject took no action.

Figure 1. Experiment 1 Port bow approach

The second experiment presented each subject

with a similar situation except that the target ship

was approaching from the own ship’s starboard bow

(see fig. 2).

Figure 2. Exp. 2 Starboard bow approach

In both cases, the collision avoidance action taken

by the subjects could be allocated to one of three

categories: Alteration of course to starboard, altera-

tion of course to port, or change of speed only. The-

se categories are abbreviated to “Stbd”, “Port” and

“Speed” respectively in the tables of section 3.

Most of the subjects were mariners with at least

six years of watchkeeping experience. There was al-

so a smaller “control group” of naïve subjects with

no seagoing experience and no knowledge of the

COLREGS. Separate analyses were conducted for

the two experiments and for each of the experienced

and naïve groups of subjects. That is, four analyses

in all.

In every case, a χ

test was used to find whether

there was evidence that subjects had significant

preferences amongst the three categories of action

defined above. The null hypothesis was an equal

probability that the subjects would choose any of the

three categories of action. The alternative hypothe-

sis was that this would not be the case. A 0.05 level

of significance was considered sufficient to reject

the null hypothesis. The results are summarised in

section 3, below.

3 ANALYSES

3.1 Port bow approach, naïve subjects

Of the 15 naïve subjects who were presented with

the port bow situation, the numbers observed to take

each of the three categories of action appear in the

first row of table 1.

Table 1

___________________________________________________

Stbd. Port. Speed Total

___________________________________________________

Observed (O) 2 10 3 15

Expected (E) 5 5 5 15

(O – E)

9 25 4 38

___________________________________________________

= (O-E)

/E = 38/5 = 7.6

This is greater than χ

(2, 0.05) = 5.99. We there-

fore reject the null hypothesis and conclude there is

significant evidence that the subjects prefer some

manoeuvres rather than others. In this case, they

clearly prefer alterations of course to port rather than

alterations of course to starboard, and this provides

some justification for describing an alteration of

course to port as a “natural” manoeuvre in the given

situation.

3.2 Starboard bow approach, naïve subjects

Of the 14 naïve subjects who were presented with

the starboard bow situation, the numbers observed to

take each of the three categories of action appear in

the first row of table 2.

Table 2

___________________________________________________

Stbd. Port. Speed Total

___________________________________________________

Observed (O) 9 0 5 14

Expected (E) 4.7 4.7 4.7 14

(O – E)

18.5 22.1 0.1 40.7

___________________________________________________

= (O-E)

/E = 40.7/4.7 = 8.7.

This is greater than χ

(2, 0.05) = 5.99. We there-

fore reject the null hypothesis and conclude that

there is significant evidence that the subjects prefer

some manoeuvres rather than others. In this case, al-

terations of course to starboard are preferred to al-

terations of course to port. As before, this provides

some justification for describing alterations of

course to starboard as “natural” manoeuvres in the

given situation.

3.3 Port bow approach, experienced subjects

Of the 23 experienced subjects who were presented

with the port-bow situation, the numbers observed to

take each of the three categories of action appear in

the first row of table 3.

Table 3

___________________________________________________

Stbd. Port. Speed Total

___________________________________________________

Observed (O) 8 9 6 23

Expected (E) 7.7 7.7 7.7 23

(O – E)

0.1 1.7 2.9 4.7

___________________________________________________

= (O-E)

/E = 4.7/7.7 = 0.61

This is less than χ

(2, 0.05) = 5.99. We therefore

accept the null hypothesis and conclude that experi-

enced subjects do not have a preference among the

three categories of action. In particular, there is no

evidence to suggest that they prefer alterations of

course to starboard rather than alterations of course

to port in the given situation.

3.4 Starboard bow approach, experienced subjects

Of the 24 experienced subjects who were present-

ed with the starboard bow situation, the numbers ob-

served to take each of the three categories of action

appear in the first row of table 4.

Table 4

___________________________________________________

Stbd. Port. Speed Total

___________________________________________________

Observed (O) 23 0 1 24

Expected (E) 8 8 8 24

(O – E)

225 64 49 338

___________________________________________________

= (O-E)

/E = 338/8 = 42.2

This is greater than χ

(2, 0.05) = 5.99. We there-

fore reject the null hypothesis and conclude that

there is significant evidence to suggest that the expe-

rienced subjects have a preference among the three

categories of action in this type of encounter. It is

clearly a strong preference for alterations of course

to starboard.

4 INITIAL RESULTS

4.1 “Natural” manoeuvres

In sections 3.1 and 3.2, naïve subjects, with no

knowledge of the COLREGS, altered course to port

for a threat on their port bow and altered course to

starboard for a threat from the starboard bow. These

are consistent responses, since one situation is the

mirror image of the other. Also, since it was taken

by the majority of naïve subjects, we may consider

these to be “natural” actions rather than responses to

a set of rules.

4.2 COLREGS comparison, port bow threat.

In the case of a vessel approaching from a subject’s

starboard bow, the natural action is in accordance

with the COLREGS. In the case of a vessel ap-

proaching from a subject’s port bow, the natural ac-

tion is entirely different to that prescribed by the

COLREGS. Rule 17, somewhat illogically, requires

the subject’s vessel, (i) to keep her course and speed

and (ii) if she does take action, not to alter course to

port. Rule 19, which applies in restricted visibility,

states that an alteration of course to port should be

avoided for an approaching vessel forward of the

beam.

Experienced subjects, faced with a threat from the

port bow, were equally divided between alterations

of course to starboard (in accordance with the

COLREGS) and alterations of course to port (in ac-

cordance with natural action and with the Co-

lomb/Crosbie convention).

4.3 COLREGS Comparison, starboard bow

Moving on to the results for the situation where the

target vessel is approaching from a subject’s star-

board bow, we find that both naïve and experienced

subjects take similar action. That is, alterations of

course to starboard.

In this situation, an alteration of course to star-

board was the natural action of the naïve subjects

and it is also permitted under rule 15 of the

COLREGS and under rule 19 when, in restricted vis-

ibility, the approaching vessel is detected by radar.

With no conflicting considerations involved, all the

experienced subjects (except one who decreased

speed) altered course to starboard. In doing so, they

complied with both the COLREGS and the Co-

lomb/Crosbie recommendations.

5 THE EFFECT OF EXPERIENCE

5.1 Conflict with the COLREGS

The experiments described in section 3 above were

not, originally, designed to test the Colomb/Crosbie

proposals. However, as discussed in section 4, they

shed some light on how readily those proposals

might be accepted by mariners. The results show

that a considerable proportion of experienced mari-

ners appear willing to take action as implied by the

Colomb/Crosbie convention, even in cases where

such action is clearly opposed to action prescribed

by the COLREGS. Clearly, this proportion could be

expected to increase if the present COLREGS were

repealed.

5.2 Rapid versus slow disengagement

In many collision encounters, the navigator has a

choice between an action which resolves the situa-

tion quickly but which initially involves a more rap-

id approach to the other vessel, and an action which

gives a more prolonged disengagement and which

initially decreases the rate of approach to the other

vessel.

In the case of a threat from the starboard bow (as

in fig. 2) a manoeuvre for rapid disengagement cor-

responds with both the COLREGS and “natural” ac-

tion, - that is an alteration of course to starboard. As

the results reported in section 3.4 show, practically

all experienced mariners take this action.

In the case of a threat from the port bow (as in

fig. 1) a manoeuvre to achieve rapid disengagement,

- that is an alteration of course to port, is contrary to

the COLREGS. As the results reported in section

3.3 show, experienced mariners are equally divided

as to which way they alter course.

5.3 Rational for a hypothesis

In the port bow case of fig. 1, one choice is of a safe

but time consuming action of an alteration of course

to starboard or a reduction of speed. The other

choice is an apparently riskier, but more efficient, al-

teration of course to port or an increase in speed.

About half of the experienced subjects took the latter

choice although it was clearly not sanctioned by the

COLREGS so it is of interest to speculate as to why

this should be the case. The tendency of many expe-

rienced subjects to take risky action may be ex-

plained in terms of behaviour theory as developed

by B F Skinner

(Skinner, 1953). This suggests that

an alteration of course to port, although contrary to

the COLREGS, leads to a rapid disengagement and

relief from anxiety so it is reinforced on every occa-

sion that it is successful. An alteration of course to

starboard may break the initial collision situation but

it leads to a prolongation of the encounter and there-

fore a continued period of anxiety until disengage-

ment is finally achieved and the own ship can re-

sume its original course. If this suggestion is

correct, then alterations of course to port for a threat

on the port bow should be more common amongst

the more experienced mariners. This was taken as a

working hypothesis for a supplementary analysis.

6 A SUPPLEMENTARY ANALYSIS

6.1 Purpose

This analysis was conducted to test the above hy-

pothesis by investigating a possible relationship be-

tween the actions taken by mariners and their re-

spective lengths of experience. For this purpose,

their actions were divided into two groups according

to whether they were in conformity with the restrict-

ed visibility COLREGS (rule following) or in con-

flict with the COLREGS (rule averse).

Table 5. Experience v. Manoeuvre Class

___________________________________________________

Experience Manoeuvre

Years & months class

___________________________________________________

1 6–6 R

2 6–9 R

3 7–6 R

4 9–1 R

5 9–5 R

6 9–6 A

7 9–9 R

8 10–9 R

9 11–4 A

10 11–8 R

11 12–3 A

12 12–4 A

13 12–8 R

14 13–0 A

15 14–1 A

16 14–7 A

17 14–8 A

18 18–3 A

19 18–6 R

20 19–0 R

21 26–5 A

22 28–2 A

23 29-0 A

___________________________________________________

In the port bow approach, this corresponded to a

choice between a safe manoeuvre that would pro-

long the encounter (alteration of course to starboard

or reduce speed) and a riskier manoeuvre that would

resolve the encounter quickly (alteration of course to

port or an increase in speed). In table 5, the type of

manoeuvre chosen is tabulated against the length of

experience of 23 subjects. A Rule–following altera-

tion of course to starboard or reduction of speed is

coded as “R”. A rule-Averse alteration of course to

port or increase in speed is coded as “A”.

6.2 Analysis

To analyse table 5, we note that, because of wastage

amongst younger mariners, experience amongst a

random group is likely to be highly skewed rather

than normally distributed. Also the effect of experi-

ence on a mariner’s behaviour is unlikely to be line-

ar so that means and standard deviations, calculated

arithmetically, may not be reliable statistics in the

context of this analysis.

Of the available non-parametric methods of anal-

ysis, the Mann-Whitney U test seems appropriate

because the test depends upon ranking but not on an

interval scale and it does not assume a particular dis-

tribution

From table 5, we note that 11 subjects took ac-

tions of class “A” and 12 subjects took actions of

class “B”. The value of the Mann-Whitney U statis-

tic is calculated as 26. This is less than 28, the value

for a one tailed test at a 1% level of significance.

We therefore reject the null hypothesis and accept

the alternative hypothesis that, as experience in-

creases, mariners are more likely to choose actions

that resolve an encounter quickly. Typically, they

are more ready to alter course to port for a threat

from the port bow.

7 DISCUSSION

A full investigation of the Crosbie/Colomb proposal

would require consideration of many factors. This

paper simply describes two radar simulator experi-

ments which suggest that an investigation is worth

while.

In the case of a threat approaching from a broad

angle on the starboard bow (fig. 2) an alteration of

course to starboard was the favoured manoeuvre for

both experienced and naïve subjects. This was com-

patible with both the Colomb/Crosbie proposal and

the current COLREGS in both clear weather and re-

stricted visibility

In the case of a threat approaching from a broad

angle on the port bow (fig. 1) naïve subjects fa-

voured an alteration of course to port. Experienced

subjects were equally divided amongst an alteration

of course to port, an alteration of course to starboard

and an alteration of speed. This might be thought a

surprising result in that one would expect experi-

enced mariners to all comply with rule 19 or rule 17

of the COLREGS and avoid an alteration of course

to port. This result gives some support to the Co-

lomb/Crosbie proposal, which would allow such an

action.

It is also of interest that, in the same situation, a

manoeuvre, such as an alteration of course to port,

which leads to a rapid disengagement becomes more

acceptable as a mariner’s experience increases.

Returning to the above observation that some

experienced mariners chose to disregard Rule 17 or

19 we should not, perhaps, be too surprised since a

number of commentators have, over the years, noted

that mariners take a relaxed attitude to following the

COLREGS. For example, Syms (2003) analysed the

results of a Nautical Institute survey into mariners’

interpretations of Rule 19 in a hypothetical collision

situation and concluded that, Fewer than a quarter

picked the correct action for both vessels to alter

course to starboard. And, Salinas (2006) found that,

in relation to Rule 19d, ….. it has been clearly

proved there exists complete disagreement between

what the COLREGS state and what seafarers really

do.

8 CONCLUSIONS

At this stage, it should be made clear that the author

is not taking a position for or against the Co-

lomb/Crosbie proposal. He is simply presenting

some evidence that suggests that an action taken in

accordance with that proposal would be acceptable

to mariners in two particular situations.

The author does recommend that the Co-

lomb/Crosbie proposal is worth further investigation

and that further tests, using a simulator with a day-

light display, should be conducted with the specific

purpose of investigating the Colomb/Crosbie pro-

posal.

The author also notes that adoption of the Co-

lomb/Crosbie proposal would create such radical

changes in the Rules for Avoiding Collisions at Sea

that it might be impossible ever to achieve interna-

tional agreement. That might be shame.

REFERENCES

Colomb, P H & Brent, H W (1866) The Law of Port Helm, etc.

J D Potter, London

Colomb, P H (1885) The Dangers of the Modern Rule of the

Road at Sea. J D Potter, London

Crosbie, J W (2009) Revisiting the lessons of the early steering

and sailing rules for an e-navigation age. Journal of Navi-

gation, 62,109

Salinas, C F (2005) Restricted visibility: In search of a solu-

tion. Journal of Navigation, 59,349

Skinner, B F (1953) Science and Human Behaviour. Macmil-

lan, New York

Syms, R (2003) Nautical Institute Colregs Survey – Scenario 3.

Seaways, December 2003.