Plan in four stages, check all the way; pilotage is the same at close range, with every number ready before the entrance. The electronics serve both, if you know what each cannot see.
1 · The passage plan
Four stages and a way out
How do I work out the latest time I can leave before bad weather arrives?
Latest departure = the earlier forecast of bad weather − margin − passage time: with the front forecast for Sat 1200 and Sat 1800, a 6 h margin and a 15 h crossing, leave by Fri 1500.
The four stages
Appraisal: charts, pilot, tides, forecasts
Planning: tracks, waypoints, times, plan B
Execution: latest forecast, decide, brief
Monitoring: fix, compare with the plan, act
SOLAS V asks every vessel for a plan, and a radar reflector where practicable. Work timings backwards: latest departure = the earlier forecast of bad weather − margin − passage time; likewise back from a tidal gate. Landfall: raise a light before dawn, enter after it. Per stage: a port of refuge, safe in the forecast wind at any tide, and a port with a hospital. Fuel by dipstick: passage + diversion + reserve; bottled water for the passage plus spare days. Handing over: the plan's changes, the defects, the crew.
Latest departure
Crossing 120 M at 8 kn: 15 h
Front: model A Sat 1800, model B Sat 1200
Rule: in harbour 6 h before the earlier one
Sat 1200 − 6 h − 15 h = Fri 1500
Cross lanes at right angles at a steady heading and speed, clear of the precautionary area where ships turn, and keep waypoints off the point where they all alter. A platform's 500 m zone is closed to you; a wind farm at night hides small craft among its echoes.
2 · Watches and monitoring
Somebody is always checking
How often should I fix my position when approaching a danger?
Often enough that you cannot reach a danger between fixes: for example every 30 minutes offshore and every 6 minutes within 3 M of a danger.
Lanes and landfall where the crew is fresh and it is light; sleep before the night, not after. A lone watchkeeper under autopilot needs a watch alarm. Night orders never delay avoiding action: at close quarters act, then call. Fix more often as dangers close, so you cannot reach one between fixes; a fix well off the EP means stream, sensor or plot is wrong. A saved track is no log: no weather or decisions, and it dies with the power.
Night orders
Fix every 30 min offshore
Every 6 min within 3 M of a danger
Call me: CPA under 1 M, or any doubt
Close quarters: act first, then call
Watch alarm on; log every hour
3 · The pilotage plan
Ready before the entrance
How do I use a clearing bearing to stay off a danger?
Draw a clearing bearing from a charted mark past the danger and keep the mark's bearing on the safe side of it; at night and in a swell, lay it to a deeper contour.
In the plan
Tide = draught + ½ swell + margin − depth
Bridge: clearance + HAT − tide ≥ air draught
Clearing bearings; wheel-over points
Abort point: the last place to turn away
VTS: name, position, intention, ETA; listen
A clearing bearing on a charted mark fences off a danger; deeper water at night and in a swell.
Start a big turn early: wheel-over distance = turning radius × tan(half the turn). Leading lights hold a dredged cut's axis; a white sector may be wider than your water. Mark the abort point; where you cannot turn inside, reverse in. Without GNSS aim off to one side of the harbour; in an unlit bay fix by radar ranges off steep points and the sounder. Three vertical reds: no entry; the yellow beside them exempts only named craft. VTS traffic information is not control. In fog let the ferry clear first, and be able to stop within half the visibility.
4 · Radar
What paints
How far away can my radar pick up a coastline or a ship?
Out to the radar horizon, about 2.2 × (√h₁ + √h₂) miles with heights in metres: a scanner 9 m up sees a 64 m cliff at about 24 M.
Range = speed of radio waves × echo time ÷ 2; the picture ends at the horizon. Range to the near edge of an echo is sharp; bearing is smeared by the beam width: fix by ranges.
Radar horizon
≈ 2.2 × (√h₁ + √h₂) M, heights in metres
Scanner 9 m, cliff 64 m: 2.2 × 11 = 24 M
Ship's bridge 30 m: 2.2 × (3 + 5.5) ≈ 19 M
Her AIS from a high aerial reaches further
Setting up the picture
Tune, then gain up to a faint speckle
Sea clutter: leave a little showing
Rain control only to look into a squall
Long scale to warn, short for small craft
Long scale ≥ closing speed × warning time
Heavy rain hides what lies behind it. A guard zone misses an echo that first paints inside it; an arc at a close ship's range is side lobes. Yaw to clear a blind sector; an active enhancer high up makes you seen. A racon paints a Morse flash outward: range to its near end. A SART paints twelve dots, the casualty at the nearest. Parallel index: a line parallel to your ground track, on a stabilised north-up display. Clearing range: a VRM ring round a cliff that echoes, at the hidden danger's reach plus margin. No true trail: stopped. Overlay offset evenly all round: doubt the position.
5 · Radar plotting and ARPA
Her CPA, her true course
How do I find a ship's CPA from radar plots?
Plot the echo twice and extend the line through the plots: that is her relative motion, and it passes you at the CPA. Distance left along it ÷ relative speed gives the TCPA.
Plot an echo twice. The line through the plots, extended, is her relative motion: it passes you at the CPA; distance left along it ÷ relative speed = TCPA. Steady bearing, closing range: collision. Two targets: the earlier CPA first.
Relative motion + your own course and speed = her true course and speed. After you alter or slow, redraw from the last plot: her true vector minus your new one. A bend in the plot means she has altered: start again from the bend.
ARPA draws this, with limits. Read the CPA, not where the true vector points. Vectors take three minutes to settle. A heading error skews true vectors, not CPA and TCPA. Close-passing echoes can swap. Ground-stabilised vectors show track over the ground; in a cross-stream her lights disagree, so judge aspect sea-stabilised.
6 · AIS
What she says of herself
Can I rely on AIS to show every vessel around me?
No: many fishing boats, yachts, warships and tows have no AIS, and it shows only what a ship's GNSS and crew report. Plot an echo with no AIS like any other.
AIS says what a ship's GNSS and crew report, over VHF. Many fishing boats, yachts, warships and tows have none, and the status is typed by hand. Plot an echo with no AIS like any other; two echoes in step under one name are tug and tow: never pass between. Class B sends 2 W every 30 seconds and may be filtered off a busy display. A virtual aid exists only on the screen: plot the danger on paper.
MMSI
Device
970
AIS-SART, no radar dots. ACTIVE: go, alert the MRCC; TEST: log it
972
MOB beacon: a person in the water
974
EPIRB with AIS
7 · GNSS and sensors
One source is none
How can I tell if my GPS is being jammed or spoofed?
Jamming shows as jumps, wild SOG, a wrong clock or no fix; spoofing gives a smooth, confident, false position. Radar ranges and depths are independent: believe them, and report it.
High HDOP: poor satellite geometry, worse fix. SBAS sharpens the fix and flags faults, but jams too. Jamming: jumps, wild SOG, wrong clock, no fix. Spoofing: a smooth, confident, false position. Radar ranges and depths are independent: believe them, report it.
GNSS lost
Write down the last fix, its time and log
Start a DR; fix by radar ranges and depth
Gone: AIS and DSC positions, track steering
Still good: radar, compass, log, sounder
Backup: handheld GNSS, VHF, own batteries
One antenna for everything: a single point of failure. Wind mode follows the wind, not the compass; off-course alarm in gusts: reef; drive fault: hand steering, slow down. Below-keel sounder reading + draught = the depth to compare with the chart; offshore it shows fish as shoals. Log factor = true ÷ logged distance, at slack water. GNSS SOG and COG already hold the stream.
Dutchman's log
Marks on deck 18 m apart
The chip takes 5.0 s: 3.6 m/s
3.6 × 3,600 ÷ 1,852 = 7.0 kn
Through the water, not over the ground
Practice questions
Questions from this topic, as the app asks them. Open one to see the answer.
Passage: 140 M crossing
Read: pilot book, tidal atlas
Read: forecasts, Notices, crew list
Drawn on the chart: nothing yet
You have spent the evening on the pilot book, the tidal atlas, the forecasts and the crew list for a 140 M crossing. What is the next stage of the plan?
Appraisal — reading the sources once again
Planning — tracks, waypoints, timings and plan B
Execution — the final go and the crew briefing
Monitoring — checking progress against the plan
Show the answer
B. Planning — tracks, waypoints, timings and plan B
Appraisal (gather and weigh), planning (draw it all), execution (decide and brief), monitoring (check it on the way).
All 100 questions of this topic are in the app: practice by topic with the reason after every answer, your mistakes brought back, and a timed mock exam. Free, no sign-up, in the browser or in Telegram.
