Module 15 - Restricted Visibility

Yachtmaster Offshore Standard: Restricted-Visibility Command Board

What you will learn

Run a source-aware command board that keeps legal state, own-ship control, detection quality, every contact, sea room and crew action visible together.

Yachtmaster-level restricted-visibility judgement is not a remembered turn or a reassuring CPA number. It is a continuing command cycle that controls the yacht while testing whether the traffic and navigation picture is trustworthy enough for effective action.

The RYA describes a Yachtmaster Offshore as competent to skipper by day or night in a wide range of weather and assesses skippering, navigation, safety awareness and the collision regulations. Written revision can rehearse a defensible decision structure; it cannot demonstrate practical radar use, lookout, vessel handling, crew leadership or examination competence.

Open the command board by stating the legal and operational condition. Restricted visibility includes fog, mist, falling snow, heavy rainstorms and similar causes. When vessels are not in sight of one another in or near it, Rule 19 applies alongside Rules 4–10: lookout, safe speed, risk assessment, positive action and routeing duties remain active. There is no radar-only stand-on vessel and no automatic starboard-turn rule.

Keep own-ship control separate from target interpretation. Record speed and manoeuvring limits, engines ready for immediate manoeuvre, navigation lights, the correct Rule 35 signal, sight-and-hearing lookout and the next safe-water constraint. Safe speed is a continuous judgement, not a number chosen once when visibility first closes.

Audit the detection chain before trusting its output. Record radar range, tuning, rain or sea clutter, blind sectors, heading and speed inputs, motion or stabilisation mode and target-acquisition age. MCA guidance warns that incorrect heading or speed inputs can create serious ARPA errors and that a tracked solution can take up to about three minutes to settle. Keep AIS identity and ground-referenced data supplementary and do not use an uncertain VHF passing agreement to replace the Rules.

For every contact, record timed raw range and bearing observations, whether own course and speed changed, the maturity and input quality of any automatic vector, the applicable Rule 19 caution, the available sea room and the evidence that would show the action is working. There is no universal safe CPA or TCPA; the required margin depends on the complete circumstances.

Close the loop with a command brief: facts, uncertainty, immediate control action, helm and engine limits, crew roles, the next timed observation and the trigger for further action. If a fog signal is heard apparently forward and collision risk has not been determined not to exist, Rule 19 requires reducing to the minimum speed at which the vessel can be kept on course, taking all way off if necessary and navigating with extreme caution. Sound direction can be deceptive, so it is not a precise turn bearing.

Worked example

A fictional yacht holds steady heading and speed while observing radar contact A. At 0940 it is at relative bearing 020°, range 2.4 NM. At 0952 it is at relative bearing 018°, range 1.2 NM. The heading sensor now disagrees with the steering compass by 8°, and ARPA reacquired the target less than a minute ago.

1Raw range closure: 2.4 NM − 1.2 NM = 1.2 NM in 12 minutes.
2Convert time: 12 minutes ÷ 60 = 0.2 hours.
3Average relative closing speed over the interval: 1.2 NM ÷ 0.2 h = 6 kn.
4Nominal zero-range time if the relative motion did not change: 1.2 NM ÷ 6 kn = 0.2 hours = 12 minutes.
5Treat the nearly steady raw relative bearing and closing range as urgent evidence only while own heading and speed remained steady. Reject the fresh ARPA CPA and true vector until heading and speed inputs, acquisition age and repeated observations are satisfactory.

Sense check: The calculation is an urgency check, not a collision prediction, CPA or target true course. A two-degree relative-bearing change does not prove safe passing, and the comparison cannot be carried unchanged through an own-ship manoeuvre.

Yachtmaster Offshore Standard: Restricted-Visibility Command Board
Board field	Evidence to state	Decision boundary
Rule and visibility	Cause, visibility trend, visual-sight status and applicable Rules	Do not import stand-on language into a radar-only encounter
Own ship	Speed, manoeuvrability, engines, lights, signal, lookout and sea room	Safe speed and readiness continue as the picture changes
Detection chain	Range, clutter, blind sectors, heading, speed, mode and acquisition age	A precise output is not reliable when its input chain is not
Each contact	Timed raw observations, plot confidence, Rule 19 caution and action effect	No universal CPA/TCPA or automatic turn replaces the full situation
Crew loop	Decision, roles, next observation, call trigger and fallback	Keep checking until the contacts are finally past and clear

Build the command board before choosing a turn

Contact A closes forward, a second contact lies abaft the starboard beam, rain clutter hides small echoes and the heading input is suspect.

1. Legal state and own ship
State restricted visibility, no visual sight, Rules 4–10 and 19, current safe-speed judgement, engines, lights, signals, lookout and manoeuvring limits.
2. Detection quality
Name the heading-input conflict, acquisition ages, radar settings, clutter and blind sectors. Keep AIS separate unless association is supported.
3. Contacts and sea room
Keep one timed line per contact. Apply the forward-contact and abaft-contact alteration cautions while checking the complete navigation picture.
4. Command and verify
Give the immediate speed or course control, assign helm, radar, lookout and navigation, then set the next timed observation and effectiveness test.

Sense check: If the brief relies on one displayed CPA, loses the second contact or cannot say which input is suspect, the decision picture is not controlled.

Common mistake or limitation

Calling the vessel stand-on because an automatic plot assigns the target a familiar crossing aspect.
Accepting a digital CPA or true vector without checking heading input, speed input, stabilisation mode and acquisition age.
Turning starboard automatically for a forward contact without checking a contact abaft the starboard beam or the available sea room.
Negotiating by VHF while uncertain which radar or AIS target is answering, instead of controlling the yacht under the Rules.
Letting the navigator solve the electronics alone while safe speed, sound lookout, signals or helm readiness decay.

Recap

Keep legal state, own-ship control, detection quality, every contact, sea room and crew action on one command board.
Treat CPA, TCPA and true vectors as outputs of a fallible input and acquisition chain.
Use Rule 19's conditional cautions; do not invent a radar-only stand-on vessel or automatic turn.
Brief an immediate action and verify its effect until every contact is finally past and clear.

Optional quick check

Section 9 of 10

A sound signal is apparently forward, the source is not identified and a fresh ARPA vector depends on a suspect heading input. What is the strongest immediate command?

Sources and factual review

Reviewed 2 September 2026 - Compass Revision editorial review.

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