Collision Avoidance with Radar

Radar in the COLREG

In short: five rules make radar part of collision avoidance. Learn Rules 1–19 well enough to explain them, not just recite them.

• COLREG Rule 5 — look-out: every vessel, at all times, by sight and hearing (good 7×50 binoculars) and by all available means — radar included — to make a full appraisal of the situation and of the risk of collision.
• COLREG Rule 6 — safe speed: a list of factors for all vessels, plus extra factors for vessels with operational radar.
• COLREG Rule 7 — risk of collision: long-range scanning for early warning, and systematic plotting or equivalent observation of detected objects.
• COLREG Rule 8 — action to avoid collision: early, substantial, seamanlike.
• COLREG Rule 19 — conduct in restricted visibility.
• Know Rules 2 and 3 as well (responsibility; definitions).
• At management level you must judge both the integrity (how the data were processed) and the reliability (how accurate they are) of radar collision-avoidance information.

Rule 6 — the two mnemonics

On the bridge

• In fog the radar is often your first warning of another vessel — and Rule 6 then asks: is my speed safe for what my radar can and cannot show me?
• In the oral, describe an approaching vessel in order: type · length · aspect · special circumstances · underway · making way — then say: “I take a series of compass bearings to determine if risk of collision exists.”

Scanty radar information

In short: one or two echoes tell you nothing about a target’s course. Never decide on them.

• COLREG Rule 7(c): assumptions shall not be made on scanty information, especially scanty radar information.
• A single position shows where the target is, not where it is going. The next plots may show it is moving away — or straight at you.
• With three plots you can see whether the track is steady or changing.
• COLREG Rule 7(a): if there is any doubt, risk of collision is deemed to exist.
• A steady compass bearing with decreasing range = risk of collision. A changing bearing does not always clear it — with a large vessel, a tow or at close range, risk can still exist (COLREG Rule 7(d)).

Exam trick

• A Transport Malta examiner may show you one plotted point and ask what the target is doing.
• ✅ Answer: “Scanty information — I need more plots before I decide.”

Setting up the radar for collision avoidance

In short: collision avoidance is ship against ship, so the picture must be referenced to the water, not the ground.

• Sea-stabilised — inputs from gyro and log (speed through the water). The target’s true vector then shows its heading through the water, which gives you its aspect — and so which rule applies.
• Relative motion — the relative approach (OA) shows at once whether the target will pass close. When you alter, the effect shows immediately.
• North-up (matches ECDIS) or course-up (matches the view from the window). Head-up is unstabilised — the picture spins with every yaw — avoid it.
• Memo: navigation → true motion, ground-stabilised · anti-collision → relative motion, sea-stabilised.
• SOG for navigation, STW for collision avoidance. Ground-stabilised mode in a current distorts the target’s aspect — dangerous for anti-collision.
• ARPA can mix true and relative trails and vectors (for example relative vectors with true trails). Use the combination you can justify.

Operator technique — gain, pulse length and weak echoes

• Gain up to find small targets; gain down when measuring a target’s range and bearing.
• Pulse length follows the range scale: short pulse on short scales (about 0.5–3 NM) for frequent, detailed updates near the coast, long pulse on long ranges. It changes automatically; a manual override exists.
• A weak, small echo is not a false echo until proven so — when in doubt assume it is real and raise the gain to confirm (COLREG Rule 7(c): no assumptions on scanty radar information).
• Know the tuning/detuning dos and don’ts that help to show a radar SART.
• Head-up gives a relative bearing scale (on some sets 010, 020… / 350, 340…), and the radar drops to head-up by itself if the heading input is lost.
• On the bridge: a Master who finds every target tracked by AIS only and none by ARPA will not be pleased. ARPA is your own radar’s plot (sea-stabilised, relative); AIS is the other ship’s GPS (ground-stabilised COG/SOG). Compare both, rely on ARPA.
• Good picture quality is the base of every decision: the team leader keeps the bridge team working the radar to procedure.

What to switch off

• Remove the ENC overlay for collision avoidance: it adds clutter, and it may carry a GPS error, so a ship echo can be mistaken for land.
• In ground-stabilised true motion, the other ship’s vector seems unchanged when you alter — not wrong, just harder to read.

Where the target data come from

In short: radar does four jobs in collision avoidance, using four sources of target data. ARPA (target tracking) is the primary one.

The four jobs of radar

1. Detect vessels early → early warning of risk.
2. Get their motion data (plot, TT/ARPA, AIS, TT–AIS association) → assess the risk and act.
3. Check the other vessel’s actions and the effectiveness of both ships’ actions.
4. Decide when to resume course and speed, passing at a safe distance.

The four sources

• A plot or ARPA gives 7 items: CPA · TCPA · BCR · BCT · target course · target speed · aspect.
• ARPA shows first data after about 1 min; for reliable data allow about 3 min. AIS gives everything almost at once — but it is someone else’s data.
• Modern radars flag dangerous targets in red automatically, even before you acquire them.

Why ARPA, not AIS

• ARPA builds the triangle from your own sensors, under your control and maintenance.
• AIS repeats the other ship’s GPS and static data typed in by a person — errors are possible.
• A dead ship may have no AIS, but radar still shows it and you can plot it.
• AIS is still useful: fast data and the ship’s name if you need to call.

Is there a risk of collision? CPA and TCPA limits

In short: set a CPA limit and a TCPA limit, then sort every target into one of three boxes.

• The CPA limit is your guide: want 1 NM? Act on anything that will pass closer than 1 NM.
• Examples: limit 1.5 NM, target CPA 0.9 NM → a small alteration and done. Limit 1 NM, target CPA 0.1 NM → a major problem.
• Set a minimum BCR (bow crossing range) too — increase it for large ships, tows and at close range.
• Situations keep changing → keep identifying them: uninterrupted look-out, information from the targets, bridge team working together.

Taking avoiding action

In short: act per COLREG Part B (Rules 11–19), act early, and make it big enough to be seen.

• Actions must comply with COLREG Rules 11–19.
• Real life is rarely one against one: crossing, overtaking and head-on at once, usually at night or in poor visibility. Plan a series of actions, ranked by each target’s CPA and TCPA.
• COLREG Rule 8: the alteration must be readily apparent — not 1–2°, but no need for 90° or 180°. Show your intention clearly, especially as the give-way vessel.
• COLREG Rule 8(e): you may also slow down, stop or reverse.
• The cost of delay: plots at 08:00, 08:06 and 08:12 → acting at 08:18 instead of now needs a much bigger alteration. A few degrees in good time avoids 20–30° later.
• Stand-on vessel and the other does not act → COLREG Rule 17, and remember Rule 2.

Overtaking — COLREG Rule 13

• Overtaking = coming up from more than 22.5° abaft the beam.
• Any vessel can be the overtaking vessel — even a sailing vessel overtaking a power-driven one.
• In doubt, assume you are overtaking (COLREG Rule 13(c)) — err on the side of caution.

The trawler case

• A trawler (green over white) crossing, showing her starboard side, shooting nets astern.
• COLREG Rule 3(d): “engaged in fishing” = nets, lines or trawls that restrict manoeuvrability. Only trolling lines are excluded — so a trawler is a fishing vessel and you keep out of her way (COLREG Rule 18).
• Options: take a series of compass bearings · never cross close astern (nets of unknown length) · a round turn · alter to starboard, run parallel and pass astern at a good distance (or ahead if you are much faster and clear) · take all way off and let her pass.

The picture can change

• The other ship may lose steering or power, or have a fire → she becomes NUC, top of the COLREG Rule 18 list → keep monitoring.
• A vessel can run aground — her lights will change to show it.

What not to answer

• “I’ll call him on VHF” is not a collision-avoidance answer in the exam: it is not in COLREG, and MCA guidance warns of misunderstandings and lost attention (MGN 324 (M+F)). In practice many ships call — AIS gives the name — but early action makes it unnecessary.

Restricted visibility — COLREG Rule 19

In short: no visual contact = your most dangerous time. There is no stand-on vessel: both ships must act. Slow down and create distance.

• First actions when visibility drops: reduce speed, inform the Master, post an extra look-out (AB or deckhand), engines ready.
• A typical Master’s standing order: call the Master when visibility falls below 3 NM and put the engines on standby.
• COLREG Rule 19(b): engines ready for immediate manoeuvre — tell the engine room at once (a ship on heavy fuel may change over to diesel).
• COLREG Rule 19(c): apply Section I with due regard to the conditions. No visual compass bearings in fog → systematic radar observation.
• COLREG Rule 19(d) (target detected by radar alone): decide if a close-quarters situation or risk is developing, and if you act:
  • avoid altering to port for a vessel forward of the beam (except one being overtaken);
  • avoid altering towards a vessel abeam or abaft the beam.
• COLREG Rule 19(e): fog signal heard forward of the beam, or close quarters forward of the beam cannot be avoided → minimum steerage speed, take all way off if needed.
• Options: alter to starboard, reduce speed, stop or reverse (COLREG Rule 8(e)), or a mix. Always ask: “Where can I go to create distance?”

How slow is slow?

• Container ships doing 22 kn come down to 6–7 kn in North European fog banks.
• Better to arrive late than not to arrive — hold your ground with owners and charterers.
• In collision situations the safe exam answer always includes “reduce speed”.

Two readings of “abaft the beam”

• One view: under Rule 19 treat any target abaft the beam as the abaft-the-beam case.
• The other: use the Rule 13 overtaking arc (more than 22.5° abaft the beam).
• The point is debated — read COLREG Rule 19 itself and be ready to justify your answer.

Exam-style case

• Dense fog, narrow channel, container ship 2.5 NM ahead, CPA 0.8 NM, no reply on VHF.
• ✅ Reduce speed and prepare to take avoiding action.
• ❌ “Head-on” in the option is irrelevant (it only tells you the target is ahead) · maintain course and speed · increase speed and overtake to starboard (you are not overtaking).

Coordinated action and the trial manoeuvre

In short: both ships may act. Make sure the two actions add up, and test yours before you commit.

• Coordinated action = own and target changes produce the same change in the relative motion line → the CPA increases.
• If the two changes push the relative line opposite ways, they can cancel out — or make things worse.
• Verify with the trial manoeuvre and, if time allows, a manual plot.

Trial manoeuvre

• An ARPA function that simulates a proposed course and/or speed change and shows the resulting CPA and TCPA before you act.
• It ends when you stop it manually, or automatically after a set time.
• Use it twice: to choose the action, and later to check when it is safe to resume.

Checking effectiveness and resuming course

In short: an action is not done when you put the wheel over. Watch it work until the target is past and clear, then come back gradually.

• COLREG Rule 8(d): check the effectiveness of the action until the other vessel is finally past and clear.
• Three tests of an effective action:
  1. It complies with COLREG.
  2. It is visible to the other ship — by eye or by radar.
  3. The ships pass at a safe distance.
• Safe distance allows for interaction, yaw, a sudden sheer, and engine or rudder failure.

When to resume

• “Past and clear” = the CPA stays safe even after you restore course and speed.
• Resuming must not create another close-quarters situation (COLREG Rule 8).
• Check it with a trial manoeuvre, then return to the original course gradually.
• Keep monitoring until past and clear — overtaking, crossing or head-on.

The Master on the bridge

In short: at management level you supervise the radar picture and make the decision fast — so you must know the set without thinking.

• Called to the bridge, time is short: look out of the window first, then the radar (12 NM, 6 NM…). Ask the OOW what is happening only when there is no imminent danger. Your arrival should bring calm.
• Know every radar control well: no time wasted operating the set while deciding.
• Never rely on radar alone — integrate all sources: eyes, ears, radar, AIS.
• Officers sometimes delay calling the Master for fear of the Master’s reaction. Make them follow the standing orders (for example a clear rule on fishing vessels).
• A plot is only as good as the person doing it. The weakest officer is the one who does not know what they do not know — and a Master who cannot trust his officers never leaves the bridge.
• Advise OOWs not to give up manual plotting: it is the backup when ARPA fails.

In the simulator and the oral

• Report each encounter as source (radar / AIS / visual) + action taken.
• Oral scenarios start suddenly, with no build-up. Answers are often grey: explain your reasoning. With solid grounds the examiner cannot simply dismiss it.