Offshore the chart is small in scale and nobody checks the plot but you. Know what the projection does to distance, what each publication gives, and how far to trust the plotter.
How do I measure distance correctly on a Mercator chart?
On the latitude scale level with the leg: Mercator scale grows towards the pole as the secant of the latitude, so the scale in the chart's title holds only at the latitude it names.
On a Mercator chart a straight line is a rhumb line: one true course throughout. The price is scale, which grows towards the pole as the secant of the latitude. Measure distance on the latitude scale level with the leg; the scale in the title holds only at the latitude it names.
A great circle is a straight line on a gnomonic chart, but that chart keeps no angles. Take points off the line every 5° of longitude, plot them on the Mercator and sail rhumb lines between them. Over a few hundred miles it saves almost nothing.
How do I calculate course and distance between two positions without plotting them?
By Mercator sailing: tan course = d.long ÷ the difference of meridional parts, and distance = d.lat ÷ cos course. To work a position forward, use mid-latitude sailing, with departure = d.long × cos latitude.
A minute of latitude is a mile; a minute of longitude is a mile only on the equator. East–west miles are departure: d.long × cos latitude. Decimals of a degree are not minutes: 0.25° is 15.0'. On a chart too small to plot on, work the position by mid-latitude sailing, or a long leg by Mercator sailing.
Which chart should I plot on when two charts of the same area disagree?
On the largest-scale chart that covers the leg: where two charts differ, believe the larger scale. Offshore, carry the plot onto the next chart by latitude and longitude.
Plot on the largest-scale chart that covers the leg; where two charts differ, believe the larger scale. The folio holds, on paper, the passage chart and the approaches and plans of the destination, every port of refuge and any pilotage on the way. Offshore, carry the plot onto the next chart by latitude and longitude.
A horizontal datum shift of 0.08' is 150 m: invisible on the passage chart, 6 mm on a 1:25,000 approach. An old plan sounded to MLWS shows more water than one reduced to LAT. Read the units: 7₃ on a fathoms chart is 7 fathoms 3 feet, 13.7 m.
How do I convert a true course to magnetic?
Add westerly variation, subtract easterly: 205°T with 6°W variation is 211°M. Set the variation on a Breton plotter and it is applied for you.
A Breton plotter needs only a meridian or parallel under its grid, never the table's edge; its arrow points the way you go, or you read the reciprocal. Creeping dividers and a bowed plotter falsify every leg: 3° over 60 M is 3 M. Plot lightly in soft pencil. Take hand bearings where a transit has proved the compass.
True to magnetic: add westerly variation, subtract easterly. Set on the plotter, it is applied for you. Light sectors are true, from seaward: convert your bearing first.
Minutes are sixtieths, not decimals. A knot is 1.852 km/h.
How close can a yacht go to an oil platform or rig?
No closer than its 500 m safety zone, which rigs and single buoy moorings have too. In fog a platform sounds Mo(U), heard about 2 M off.
| On the chart | For a yacht |
|---|---|
| DW route | Deep-draught ships: keep out, or cross at right angles |
| Precautionary area | Ships converge and turn: extra care |
| Pecked arrow | A recommended flow, no lane rule |
| Area to be avoided | Binds the ships its note names |
| Platform, rig, SBM | 500 m safety zone |
| Cable, wellhead, dumping ground | Never anchor |
| Submarine exercise area | Open: get the broadcast, keep a lookout |
In fog a platform sounds Mo(U), heard about 2 M off. A wind farm has national rules in its chart note: who may transit, and when. Check the blade-tip height against your mast; never anchor among its cables. A fouled cable is slipped, buoyed and reported. An ODAS data buoy is a yellow special mark, Fl(5) Y 20s. A light buoy replacing a lightvessel has a fraction of its range.
How far away will I see a lighthouse at night?
At the lesser of two ranges: the geographical range, 2.08 × (√H + √h) miles for heights in metres, and the luminous range, the nominal range cut down by tonight's visibility.
| Publication | What it gives |
|---|---|
| Sailing Directions | Coast from seaward, local rules; passage notes: sets, steep seas, fish farms |
| List of Lights | Each light in full, corrected weekly |
| List of Radio Signals | NAVTEX and weather schedules, VTS rules |
| Routeing chart | A month's winds, currents, gales; the rose's centre figure: % of calms |
| Tidal stream atlas | Hourly pages; 08,15 is 0.8 kn at neaps, 1.5 at springs |
| Almanac | Apply the supplements; check each waypoint on the chart |
You see a light at the lesser of two ranges: the geographical, 2.08 × (√H + √h) miles for heights in metres, and the luminous, the nominal range cut down by tonight's visibility. Beyond both there is only the loom.
Weekly Notices to Mariners correct the charts and, in their later sections, the publications. Navigational warnings stand until cancelled: read the lists for each NAVAREA you cross, plot them in pencil, convert their UTC times.
How do I set the safety contour on my chart plotter?
Safety contour = draught + squat + half the swell + margin; the plotter then bolds the next deeper contour in the cell. Only an ENC can sound the alarm, not an RNC, which is a picture of the paper chart.
An RNC is a picture of the paper chart; an ENC is data, so only an ENC can sound an alarm, about what you set and what its cells hold.
| Setting | Rule |
|---|---|
| Safety contour | Draught + squat + half the swell + margin; the next deeper contour in the cell is bolded |
| Safety depth | Soundings equal or shallower show bold |
| Look-ahead | Speed × warning time |
| Route check | Corridor as wide as your likely set |
A contour at the bare draught misses a shoal the troughs uncover; a factory 30 m alarm cries wolf. A leg with no cell is unchecked; the 'Basic' display hides soundings. Overscale: the cell's error, about 0.5 mm at its compilation scale, grows with the zoom: buy the approach cells. At speed, zoom out to see minutes ahead. A plot copied from the plotter checks nothing: keep a DR, cross-check by bearings, depth or radar. No alarm replaces the lookout.
Questions from this topic, as the app asks them. Open one to see the answer.
Your mate measured tonight's leg on the small-scale passage chart, reading the dividers on the latitude scale at the bottom edge. What is the leg's true length?
A. 60.0 M
Mercator minutes of latitude grow as sec lat: 66.0 × cos 50° ÷ cos 45° = 60.0 M. Measure beside the leg.
You check the plotter's figures for tonight's Channel crossing against the chart. What course and distance do you lay off?
C. 190°T, 102.5 M
d.lat 101.0' S; d.long 27.0' W × cos 49.5° = 17.5 M W. √(101.0² + 17.5²) = 102.5 M; S 9.9° W = 190°T.
Your passage chart and the older approach chart of your landfall both note that their datum differs from WGS84. Where does it matter on tomorrow's 130 M passage?
C. At the landfall, on the approach chart
0.08' ≈ 150 m: 0.2 mm on the 1:750,000, below plotting accuracy; 6 mm on the 1:25,000 — enough to put you on the rocks.
Your mate lays the Breton plotter along the leg on the passage chart and calls out 058°. You glance at the chart. What true course goes in the plan?
A. 238°T
The arrow must point the way you go: reversed, it reads the reciprocal, 058° + 180° = 238°T — and the leg runs SW.
Your planned track runs down the lane marked on the chart. What do you do?
D. Replan to keep out — it is for deep ships
A deep-water route is kept for deep-draught ships; vessels that don't need it should avoid it.
Tomorrow's route crosses the area in this coastal warning. What do you do?
C. Plot the area and route round it while firing
A navigational warning defines its area by positions: plot it on the chart and keep out of it at the times given.
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.