Rule 6: a safe speed, at all times.

What does COLREGs Rule 6 say?

Every vessel must at all times proceed at a safe speed: one at which she can 'take proper and effective action to avoid collision' and can be stopped within a distance appropriate to the prevailing circumstances and conditions.

The rule then lists factors that must be among those taken into account when deciding what that speed is — six for every vessel in Rule 6(a), and six more in Rule 6(b) for vessels with operational radar.

Is there a safe speed in knots?

No. Rule 6 gives no figure. A safe speed is whatever lets you meet the two tests in its opening sentence — act effectively to avoid collision, and stop within an appropriate distance — in the conditions you are actually in. The same speed can be safe in open water on a clear day and unsafe in a crowded approach at night.

In our view the tests cut both ways. The factors include your own manoeuvrability in the prevailing conditions, so a speed too low to keep good control in strong wind or current can also leave you unable to take 'proper and effective action'.

Does Rule 6 only apply in fog?

No. It says 'at all times', and it sits in Part B, Section I — 'Conduct of vessels in any condition of visibility'. Clear weather, night and restricted visibility are all covered.

Restricted visibility adds to it. Rule 19(b) requires every vessel to proceed at a safe speed adapted to the prevailing circumstances and conditions of restricted visibility, and a power-driven vessel to have her engines ready for immediate manoeuvre. Rule 19(e) goes further: unless it has been determined that no risk of collision exists, a vessel that hears a fog signal apparently forward of her beam, or cannot avoid a close-quarters situation with a vessel forward of her beam, must reduce to the minimum speed at which she can be kept on her course, and if necessary take all her way off.

What factors does Rule 6(a) list for every vessel?

(i) The state of visibility. (ii) Traffic density, including concentrations of fishing vessels or any other vessels. (iii) The vessel's manoeuvrability, with special reference to 'stopping distance and turning ability' in the prevailing conditions. (iv) At night, background light — from shore lights, or back scatter from her own lights. (v) The state of wind, sea and current, and how close navigational hazards are. (vi) Her draught in relation to the available depth of water.

What extra factors does Rule 6(b) add for a vessel with radar?

Only for a vessel with operational radar, and in addition to 6(a): (i) the characteristics, efficiency and limitations of the radar equipment; (ii) any constraints imposed by the range scale in use; (iii) the effect of sea state, weather and other interference on detection; (iv) the possibility that 'small vessels, ice and other floating objects' may not be detected at an adequate range; (v) the number, location and movement of vessels detected; and (vi) the more exact assessment of visibility that radar may make possible, by ranging vessels or other objects nearby.

Are those the only factors?

No. Rule 6 says the listed factors 'shall be among those taken into account' — the list is a minimum, not a checklist to finish. Anything else that affects your ability to act in time or to stop belongs in the assessment too.

What does 'back scatter from her own lights' mean on a yacht?

Light from your own vessel thrown back at the bridge — deck lights, floodlights, a lit saloon or wheelhouse reflecting off spray, rain or your own structure. It costs you night vision and makes dim lights ahead harder to pick out. Rule 6(a)(iv) lists it alongside shore lights as a factor in choosing a safe speed at night, and it is one you can often fix by switching something off.

Does having radar mean I can go faster?

Not of itself. Radar appears in Rule 6(b) on both sides of the ledger. It can give you a 'more exact assessment' of the visibility and a picture of the traffic, which may support your choice of speed. It can also miss small vessels, ice and floating objects, and its performance depends on the set, the range scale and the sea and weather — all of which may argue for less speed, not more.

Rule 7(c) is the companion warning: assumptions must not be made on the basis of scanty information, 'especially scanty radar information'. A clear screen in fog is not, on its own, evidence of a clear sea.

How do I decide what speed is safe in practice?

Our suggestion: work through the 6(a) factors for the water you are in, then the 6(b) factors if you have radar, and ask the rule's two questions directly. If something appeared at the edge of what you can see or detect, could you take effective action in time? Could you stop in the room available? That means knowing your own vessel's stopping distance and turning circle, not guessing them.

How might Rule 6 come up in an MCA oral?

It may come up as a scenario that asks you to justify a speed. Examples of the kind of question to be ready for:

“What is a safe speed?”

“You're approaching a busy anchorage at night with the deck lights on. What affects your speed?”

“The visibility closes in to half a mile. What changes, and which rules now apply?”

“What does Rule 6 add for a vessel with operational radar?”

“Your radar shows nothing ahead in fog. Is it safe to keep your speed?”

Where do candidates go wrong on Rule 6?

The common slips: giving a figure in knots; treating Rule 6 as a fog-only rule; listing the radar factors and forgetting the ones for every vessel; and missing that the list is a minimum. Applying the factors to the scenario in front of you matters more than reciting them in order.