NAVIGATION · Lesson 5 of 6
Distance, Speed and Depth
25 min · Beginner friendly
You’ll learn toMeasure distance on a chart using the latitude scale · Solve time, speed and distance problems quickly · Interpret log and echo sounder readings, including offsets
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About 9 min
Navigation runs on three quantities: how far, how fast, and how much water is under you. Get comfortable with the units — nautical miles, knots and metres — and with the simple arithmetic connecting them, and half of practical chartwork becomes sums you can do at the helm.
Take a guess
Decide before you read: you are 11 miles from the marina, making a steady 5 knots, and the lock gate closes in 2 hours 20 minutes. Do you make it?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
The Sea's Own Units
- Nautical mile (nm)
- 1,852 metres; one minute of . The standard unit of distance at sea.
- Cable
- One tenth of a , about 185 m. Handy for short distances in pilotage.
- Knot (kn)
- One nautical mile per hour. The standard unit of speed at sea — never say knots per hour.
The nautical mile survives because it is defined as one minute of latitude — your distance unit is printed up the side of every chart. To measure, span the distance with and read the span against the latitude scale level with your working area; for longer runs, the dividers to a convenient round figure and walk them along the course line, counting steps, then measure the remainder.
Knowledge check
Roughly how long is one cable?
The Log and the Tide's Tell
The measures speed through the water — traditionally a tiny impeller spinning in the flow under the hull, named for the wooden log once thrown over the side on a knotted line. It also keeps a running total of distance sailed, like a car's odometer; navigators write the reading down at every because the difference between two readings is the distance run between them. Your reports something different: ().
Knowledge check
The log reads a steady 5.0 knots; the GNSS shows SOG 6.5 knots. What is the most likely explanation?
Time, Speed and Distance
One formula rules them all: distance = speed × time, with time in decimal hours. The only trap is the time format: minutes must become decimal hours before they touch the formula — 30 minutes is 0.5 h, 36 minutes is 0.6 h, 90 minutes is 1.5 h. Divide minutes by 60, always.
Worked example
Distance run
You motor-sail at a steady 6 knots for 1 hour 30 minutes. How far have you travelled through the water?
- Convert time to decimal hours: 1 h 30 min = 1.5 h.
- Apply the formula: distance = speed × time = 6 × 1.5.
Distance = 9.0 nm.
Worked example
Passage time and ETA
Salt Island lies 14 nm ahead and you expect to make 4 knots. You depart at 0900. When do you arrive?
- Time = distance ÷ speed = 14 ÷ 4 = 3.5 h.
- Convert to hours and minutes: 3.5 h = 3 h 30 min.
- Add to departure time: 0900 + 3 h 30 min = 1230.
ETA 1230 — about three and a half hours of sailing.
Knowledge check
You have 12 nm to run and your speed is 5 knots. How long will it take?
Work it out
You pass the Westhaven fairway buoy at 1015 with the log reading 23.4. At 1145 the log reads 32.4. Find the distance run and your average speed through the water.
Within 0.1 nm is fine.
Within 0.1 kn is fine.
Depth and the Echo Sounder
The echo sounder times a pulse of sound from a transducer in the hull to the seabed and back — sound covers roughly 1,500 metres per second in seawater, so the display updates continuously. It is wonderfully reliable, with one critical subtlety: depth below what, exactly? The transducer sits below the waterline but above the bottom of the keel, and most sounders apply an offset so the display reads from somewhere more useful.
Knowledge check
Two identical yachts lie anchored side by side in the same pool. One sounder reads 5.4 m, the other 3.6 m. Neither instrument is faulty. What explains it?
| Setting | Display shows | Watch out for |
|---|---|---|
| Below waterline (surface) | Total water depth, comparable with + | You must mentally subtract draught to know |
| Below keel | Actual clearance under the deepest part of the boat | Reads alarmingly small numbers; zero means aground |
| Below transducer (no offset) | Raw measurement | Neither of the useful figures — know your geometry |
Worked example
Checking the sounder against the chart
Anchored off Marram Bank: charted depth 3.0 m, calculated height of tide 2.4 m. Your yacht draws 1.8 m and the sounder is set to read depth below the keel. What should it display?
- Expected total depth below the surface = charted depth + height of tide = 3.0 + 2.4 = 5.4 m.
- The sounder shows depth below the keel, so subtract the draught: 5.4 − 1.8 = 3.6 m.
- Compare with the display. Agreement within a few tenths confirms chart, tide calculation and instrument all at once.
The sounder should read about 3.6 m below the keel.
Key points
- 1 nm = 1,852 m = one minute of latitude; a is a tenth of that (~185 m).
- A is one nautical mile per hour — speed, distance and the chart scale all share the same DNA.
- The log gives speed and distance through the water; GNSS gives speed over the ground; the difference is the tide talking.
- Distance = speed × time, with time in decimal hours; the triangle rearranges it.
- Six-minute rule: distance in 6 minutes = speed ÷ 10.
- Know your echo sounder offset — below waterline, below keel, or below transducer — before you rely on it.
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