Effective action: Rule 8 and checking that it works
1The lesson in plain words
Act early, make a change big enough to see on radar, and keep plotting. Compare course and speed changes to find a safe passing distance.
Early and positive
Rule 8(a): if the circumstances admit, action to avoid collision is positive, made in ample time, and follows good seamanship. Rule 8(b): if the circumstances admit, any change of course or speed is large enough to be readily apparent to a vessel watching by eye or radar. Avoid a succession of small alterations: another vessel may not notice them.
How big is big?
The Rules set no minimum angle. Choose a substantial alteration that is readily apparent and produces a safe passing distance. With sea room, a course alteration alone may be the most effective action, if it is in good time, substantial, and does not create another close-quarters situation (Rule 8(c)).
Slow down or stop
Rule 8(e) requires you to slacken speed or take all way off when necessary to avoid collision or gain assessment time. For this comparison, assume an immediate manoeuvre and unchanged target course and speed. At 10:12, stopping predicts a CPA of about 3.3 M instead of 0.7 M; reducing to 3 knots predicts about 1.9 M. Real boats take time and distance to slow or turn. Check the actual result.
Compare the turns
With an immediate course change and unchanged target motion, turning to 030°, 30° to starboard, predicts a CPA of about 1.6 M. Turning to 090° predicts about 3.5 M, with her passing down your port side. Turning to port to 300° predicts about 0.1 M: almost a collision course. No predicted distance is guaranteed. Allow for the boat’s response time. Check sea room and other traffic before choosing an action.
Check until clear
Action must result in passing at a safe distance. Check its effectiveness until the other vessel is finally past and clear (Rule 8(d)). After you act, use new observations to reassess CPA and TCPA. Confirm a safe passing distance. If the margin remains unsafe, take further effective action. Either vessel’s manoeuvre can change the prediction. Resume your intended course only when safe. Full text: /colreg/rule-8.
2Key data
- Rule 8(a)
- Positive, in ample time, good seamanship
- Rule 8(b)
- Readily apparent if circumstances admit; avoid small steps
- Rule 8(e)
- Slacken speed, stop or go astern if necessary
- Idealised example: stop at 10:12
- Predicted CPA about 3.3 M, assuming an immediate stop
- Idealised example: turn to 090°
- Predicted CPA about 3.5 M, her passing down your port side
- Idealised example: turn to port, 300°
- Predicted CPA about 0.1 M: much worse
3Key points
- 1
Act early, and make one large change rather than several small ones.
- 2
No fixed angle guarantees safety; make the alteration readily apparent and check the resulting CPA.
- 3
Slowing or stopping can avoid collision or give you assessment time; check the resulting passing distance.
- 4
Keep plotting until she is past and clear.
4Frequent mistakes
Wrong: Alter 5° now and another 5° later if needed.
Right: Small steps are hard to see on radar. Make one large, early alteration.
Wrong: Once you have altered, the job is done.
Right: Rule 8(d): check the result until the other vessel is past and clear.
Wrong: Stopping is a sign of poor seamanship; always alter course instead.
Right: Rule 8(e) requires slowing or taking all way off when necessary. Compare the predicted passing distances; stopping is not always safest.
5Practice case & exam keys
At 10:12 in the worked example, your crew suggests a 10° turn to starboard now and another later if needed. Sea room is clear and no other traffic constrains you.
Show the answer
A 10° starboard turn predicts a CPA of about 0.9 M, below this exercise’s 1 M minimum. With clear sea room and other traffic checked, a substantial turn to 090° predicts about 3.5 M, or stopping predicts about 3.3 M. These assume immediate manoeuvres and unchanged target motion. Allow for the boat’s response time. Keep plotting until she is past and clear; resume your intended course only when safe.
Exam keys
- Rule 8(a)–(b): positive, ample time, large enough to be readily apparent; avoid a succession of small alterations.
- Rule 8(c): with sea room, a substantial alteration of course alone may be most effective.
- Rule 8(d): pass at a safe distance; check until past and clear.
- Rule 8(e): slacken speed, stop or reverse if necessary or to gain time.
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