The lead crew gives you the route, but you take your own boat along it. For that you read depths off the chart, turn miles into hours, steer a compass course, hold a line by eye and keep the plotter honest.
What do the depth numbers on a nautical chart mean, and why are some underlined?
They are soundings: depths in metres below chart datum, a level close to the lowest the sea normally reaches. An underlined figure is a drying height: a rock or bank that stands that much above datum.
The figures scattered over the water are soundings: depths in metres below chart datum, a level close to the lowest the sea normally reaches. An underlined figure is the opposite, a drying height: a rock or bank that stands that much above datum. Keep well clear of it.
Mediterranean tides are tiny, a few tens of centimetres, so the charted depth is about the water you get. Compare it with your draft and keep a margin under the keel, at anchor and alongside.
Scale decides the detail. 1:10 000 is a larger scale than 1:50 000: a smaller area, drawn bigger. Pick your way into harbour on the largest-scale plan you have, usually the one in the pilot book.
How do I measure a distance in nautical miles on a chart?
On the latitude scale at the side of the chart, level with the leg: one minute of latitude is one nautical mile. Not the longitude scale along the top, whose minutes shrink as you go north.
One minute of latitude is one nautical mile. So you span a leg with dividers and read it off the latitude scale at the side of the chart, level with the leg. The longitude scale along the top will not do: its minutes shrink as you go north. Two bays due north and south of each other, 8' of latitude apart, are 8 miles apart.
| You want | Work out | Example |
|---|---|---|
| Time | distance ÷ speed | 15 M ÷ 5 kn = 3 h |
| Speed | distance ÷ time | 10 M ÷ 2.5 h = 4 kn |
| Distance | speed × time | 6 kn × 1.5 h = 9 M |
Dead reckoning keeps your position without the plotter: the course steered and the distance run from the last known position. Note the time you leave, and speed × time gives the miles to mark off along the course.
How do I turn a true course from the chart into a compass course?
Subtract easterly variation going from the chart to the compass, and add it coming back; westerly works the other way. A true course of 090° with 5°E variation is steered as 085°.
The chart is drawn to true north; the compass points to magnetic north. The angle between them is variation, printed in the compass rose on the chart. In Greece, Croatia and Turkey it is a few degrees east.
With easterly variation, subtract it going from the chart to the compass and add it coming back. A true course of 090° with 5°E is steered as 085°. A hand-bearing compass bearing of 220° is drawn on the chart as 225° true. Westerly variation works the other way.
Deviation is the error the boat herself adds, from metal and electronics near the compass. Check it on a line you know: on a leading line of 045° true, with 5°E variation, the steering compass should read 040°.
Error east, compass least. Error west, compass best.
The two marks of a transit have opened: which way do I steer?
Towards the front mark: if the rear mark is to the right of the front one, you are right of the transit, so come to port until they close in line again.
Two marks in line, one behind the other, make a transit: a line drawn on the ground itself. While they stay in line you are on it, whatever wind and current do. A compass course cannot show that you are being set sideways; a transit shows it at once.
If the marks open, steer towards the front one: the rear mark to the right of the front means you are right of the line, so come to port until they close.
A clearing bearing fences off a danger with one compass bearing of a mark. The pilot book may say: keep the lighthouse bearing less than 070°. While your hand-bearing compass reads less, you are outside the rocks; if the bearing grows past 070°, turn offshore until it drops back.
Why does my chartplotter show the boat on land when I'm in deep water?
The electronic chart is offset, not the GPS: GPS is good to a few metres, but electronic charts can be out by tens or hundreds of metres. Trust your eyes and the echo sounder.
GPS gives your position to a few metres. The chart drawn under it is another matter: electronic charts can be offset by tens or hundreds of metres. If the plotter draws your boat on the land while you are afloat in deep water, the chart is wrong: trust your eyes and the echo sounder.
In a narrow pass use the plotter, and check it all the time against what is real: marks, transits, bearings and the depth. Treat Go to with care. It draws a straight line to the waypoint, through any headland, rock or shoal in the way. Follow that line across the chart before you follow it on the water.
How do I check the day's route on the chart before I leave?
Draw each leg and follow it along its whole length: redraw any leg crossing a rock, a point of land or a sounding under your draft plus a margin. Then add up leg times and stops, with time in hand.
Before you leave, draw the day's legs on the chart and follow each one along its whole length. A sounding shallower than your draft plus a margin, a rock or a point of land on the line means you redraw the leg to pass clear.
Then add up the times, leg by leg, with the stops, and compare the result with the time the lead crew wants you in. Keep something in hand: under sail the speed is never quite what you planned.
| Step | Sum | Clock |
|---|---|---|
| Leave | — | 10:00 |
| Leg 1: 10 M at 5 kn | 10 ÷ 5 = 2 h | 12:00 |
| Lunch and swim | 1 h 30 | 13:30 |
| Leg 2: 8 M at 4 kn | 8 ÷ 4 = 2 h | 15:30 |
Questions from this topic, as the app asks them. Open one to see the answer.
You are planning a short cut across the bay. One figure on the chart is underlined. What does it tell you?
A. A rock or bank dries 1.2 m above datum
An underlined sounding is a drying height: it stands that much above chart datum. Keep well clear.
You want a swim stop before the afternoon leg. What is the latest time you can leave the swim stop?
A. 14:00
12 M at 6 kn takes 2 hours; with an hour in hand you arrive at 16:00, so leave by 14:00.
You have drawn the course to the next headland on the chart. What do you steer by the compass?
C. 136°
True to magnetic: subtract easterly variation — 140° − 4° = 136°.
You are motoring along this coast toward the headland. The pilot book gives the clearing bearing shown. What do you do?
A. Turn offshore until the bearing drops below 070°
078° is on the wrong side of the clearing line, toward the rocks: turn away until it is below 070°.
You are motoring along the coast in clear weather. What do you make of this?
D. The chart on the plotter is offset — trust your eyes
Electronic charts can be offset by tens or hundreds of metres; what you see and the sounder come first.
You check the route you drew for this afternoon against the chart. What do you do?
C. Redraw the leg to pass clear of the 1.8 m patch
The track crosses a sounding shallower than your draft; plan every leg clear of it with a margin.
All 20 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.