A chart, a compass and a clock are enough to know where you are. Read the chart, convert a course, fix your position and allow for the tide: then the plotter is a check, not a crutch.
Which scale on the chart do I measure distance in nautical miles on?
Use the latitude scale at the sides of the chart, level with your position: one minute of latitude is one nautical mile (1,852 m). Never measure distance on the longitude scale at top and bottom.
A position is a latitude, read on the side scales, and a longitude, read at top and bottom, in degrees and minutes: 37.75° is 37°45.0'.
One minute of latitude is one nautical mile (1,852 m): measure distance with dividers on the latitude scale level with your position, never on the longitude scale. A course read against a meridian is true.
A knot is one mile an hour: 18 miles at 5 knots take 3.6 hours, or 3 h 36 min. For pilotage use the largest-scale chart aboard.
What does an underlined depth figure on a nautical chart mean?
It is a drying height: the spot stands that far above chart datum and uncovers. Ordinary depths are metres below chart datum, about the lowest level the tide reaches, so there is usually more water than charted.
Depths are metres below chart datum, about the lowest level the tide reaches, so there is usually more water than charted. An underlined figure is a drying height: it stands above datum and uncovers.
| On the chart | Meaning |
|---|---|
| 2 with a small 5 | 2.5 m |
| S · M · Sh · R | Seabed: sand, mud, shells, rock |
| (4) beside a rock | An islet 4 m high, never covers |
| Cross in a dotted circle | A dangerous rock |
| 25 Wk | A wreck, 25 m over it |
| PA | Position approximate: give it room |
| Foul · cable | Never anchor here |
The echo sounder reads from its transducer: add its depth, then subtract your draft. Your air draft includes the aerials. Check the edition date and corrections in the chart's margin: old surveys miss rocks, and harbours change.
How do I convert a true course into a compass course with variation and deviation?
Apply variation, then deviation: true to compass, subtract east and add west; compass to true, add east and subtract west. So 120°T with 3°E variation and 2°W deviation is 119°C.
The chart is drawn to true north; the compass points to magnetic north. Variation is the angle between them. It changes with place and year, and the compass rose gives both: 3°10'E 2015 (8'E) is 4°38'E eleven years later.
Deviation is the boat's own magnetism. It changes with the heading, is listed on the steering compass's deviation card, and grows if you stow metal nearby. A hand-bearing compass held clear of metal needs variation only.
120°T − 3° (east) = 117°M; 117 + 2° (west) = 119°C.
True to compass: subtract east, add west. Compass to true: add east, subtract west.
How do I plot a fix from compass bearings of landmarks?
Each bearing of a charted object is a position line, drawn from the object on the reciprocal bearing. Two lines crossing give a fix, a third checks it; choose objects close by, 60° to 120° apart.
One bearing of a charted object gives a position line. Plot it from the object on the reciprocal: a light bearing 050°T is drawn from the light on 230°T. A second line gives a fix, a third checks it.
Small triangle: take the corner nearest danger. Large one: take the bearings again.
What is the difference between a DR position and an estimated position?
A dead reckoning (DR) position is plotted from the last fix with the course steered and the distance by log. Add tidal stream and leeway and you have an estimated position (EP), drawn as a triangle.
From the last fix, plot the course steered and the distance by log: that is the dead reckoning (DR) position. Add tidal stream and leeway for an estimated position (EP). A fix is drawn as a circled dot, an EP as a triangle. Log position, course and log reading hourly, in case the electronics fail.
Leeway is the slip downwind, seen as the angle between wake and centreline. To hold a track across a stream, aim up-tide: by the 1-in-60 rule a 2-knot stream at 6 knots needs about 20°.
The plotter's COG and SOG are course and speed over the ground; compass and log work through the water. The difference is the stream, unless it is the same on every heading: then the log is wrong. A growing cross-track error means you are being set: steer back towards the track.
How do I work out how much water there is now over a charted depth?
Add the height of tide to the charted depth, since both are measured from chart datum. The rule of twelfths gives the rise: 1, 2, 3, 3, 2, 1 twelfths of the range in each of the six hours.
The height of tide is measured up from chart datum: depth now = charted depth + height of tide. The range is high water minus low water. Springs, just after new and full moon, bring the highest highs, lowest lows and strongest streams.
The tide rises by the rule of twelfths: 1, 2, 3, 3, 2, 1 twelfths of the range in the six hours.
A tidal stream is named by where it runs to. A feathered arrow on the chart is the flood, a plain one the ebb; a tidal diamond gives its direction and spring and neap rates hour by hour. Mediterranean tides are small, but narrow straits like the Evripos run at several knots.
How do I use leading marks and a clearing bearing to get into harbour safely?
Keep two charted objects in line: a transit is an exact position line with no compass, and if the front mark slides off, steer towards it. A clearing bearing on a charted object fences off a danger.
Inshore you navigate by eye, to a plan made beforehand: marks, bearings to hold, where to turn. Two charted objects in line make a transit, an exact position line with no compass; if the front mark slides off, steer towards it. A clearing bearing fences off a danger.
GPS is accurate; the chart under it may not be. Plotters hide rocks when zoomed out, small bays can be drawn offset, and older paper charts need a datum shift for WGS84 positions. Zoom in along every leg, put waypoints in safe water off the entrance, and if the depth or the view disagrees with the screen, believe them. A passage plan also names ports of refuge.
Questions from this topic, as the app asks them. Open one to see the answer.
Your plotted position lies three-quarters of the way up from the lower parallel to the upper one. What is your latitude?
A. 37°47.5'N
The parallels are 10' apart; three-quarters up is 7.5' north of 37°40'.
Which of the depths on this chart will dry at low water?
C. The underlined 0.8 — a drying height above chart datum
Underlined figures are heights above datum that uncover.
The chart gives this variation; your compass has no deviation. Your plotted true course is 090°. What do you steer?
B. 094°M
True to magnetic: add west ("west is best").
You take bearings of the lighthouse, the tower and the headland and plot them. They form a small triangle. Where are you?
A. Inside the cocked hat — take the corner nearest danger as your position
A three-bearing fix; the triangle shows the error.
You leave the 10:00 fix as shown. Where is your 11:00 estimated position?
C. 6 M east and 1.5 M south of the fix
Plot the water track 6 M along 090°, then the tide 1.5 M towards 180°.
Your draft is 1.9 m. Can you cross the shoal on the track at the time shown?
B. Yes — 1.5 m charted + 1.2 m tide = 2.7 m, about 0.8 m clearance
Depth now = charted depth + height of tide.
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.