Lesson 15 of 40 · Module 2 — The compass and fixing your position

Keeping a navigational log

What to record, when, and why it can save the day when electronics fail.

Day Skipper10 minVideo · Notes · Interactive · Exercises

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0:00 / 4:15Chapter 1 of 9: A record of the passage

Transcript

A navigational log is the written record of a passage, kept as you go. Every hour, and whenever anything changes, the navigator writes down the time, the log reading, the course and the conditions. It is the boat’s memory: if the electronics fail, the log holds everything you need to work out where you are. This lesson completes Module 2, by putting the compass, fixes and estimated positions to work together.
Each entry starts with the time, on the twenty-four hour clock, and the reading of the log, the instrument that measures distance through the water. Next comes the course actually steered, by the steering compass. Then the conditions: wind direction and force, the barometer, the weather, the visibility and the sea state. Last comes your position and how you found it, with engine hours and any events in the remarks.
Make an entry at least once an hour, even when nothing seems to change. Make one too at every change of course, because each course in the log runs from its entry to the next. And log every event: a fix, a reef, the engine starting or stopping, a change of watch, or a fault. Close to danger, or when the visibility closes in, write more often.
Here is the yacht Kittiwake, setting off from Port Kerran on a June morning, bound west-about round the island. At zero nine forty she passes close to the Kerran Approach buoy, and the navigator logs that as a fix. At ten hundred comes an hourly entry, with a GPS position. At ten thirty she alters course to three four five degrees compass. At eleven hundred the watch changes, and the GPS position goes in again.
At eleven twenty the plotter screen goes blank, and the GPS position with it. There is no need to panic. The navigator logs the time, the log reading and the fault, and the helmsman keeps to the plan. At eleven thirty, as planned, Kittiwake alters course to zero three zero degrees compass, and that goes in too.
To plot from the log, first turn each course steered into a true course. For three four five degrees compass, the deviation card gives about three degrees west, so the magnetic course is three four two magnetic. Variation in 2027 is two degrees forty-four minutes west, rounded to three degrees west, so the true course is three three nine. In the same way, zero three zero compass has no deviation, and becomes zero two seven true.
Distances come straight from the log: subtract one reading from the next. From eleven hundred to eleven thirty the log went from twelve forty-six point zero to twelve forty-eight point five, so Kittiwake sailed 2.5 miles through the water. From eleven thirty to twelve hundred she sailed another 2.6 miles. The passage plan gives the tidal stream for the hour as zero six five degrees true at one knot, so in that hour it carries her one mile.
Now plot it. From the eleven hundred GPS position, lay off three three nine degrees true for 2.5 miles, then zero two seven for 2.6 miles. That gives the twelve hundred dead reckoning position. Add the tide, zero six five degrees true for one mile, and the end of that line is your estimated position: a triangle, labelled with the time. Write it in the log as an EP, and fix your position as soon as you can, because an EP is only an estimate.
Good habits make a log you can trust. Use the twenty-four hour clock, and write the time zone at the top of each page, because almanac tide tables give times in Universal Time, and British Summer Time is an hour ahead. Write clearly, cross out a mistake with a single line instead of rubbing it out, and sign the log when you hand over a watch. That completes Module 2: next come the buoys, lights and rules of the road.

In this lesson you will learn

Explain what a navigational log is for, and why it matters when the electronics fail
Know what goes in each entry, and when to make one
Keep a log to a good standard: 24-hour clock, time zone, legible and signed
Work out distance run and speed from log readings, and convert logged courses from °C to °T
Rebuild an estimated position on the chart from the log

What the log is for

In Lesson 14 you worked out a dead reckoning position and an estimated position from the course steered, the distance run and the tidal stream. Every one of those numbers has to come from somewhere, and at sea that somewhere is the navigational log (or logbook): a written record of the passage, kept as you go.

A good log does two jobs:

It lets you work out where you are. If the plotter dies, the last logged position, the log readings and the courses steered since are everything you need to plot an EP. You will do exactly that later in this lesson.
It is the record of the passage: what you did, what the conditions were and why you made your decisions. It keeps the next watch in the picture, and if anything ever goes wrong it is a clear account written at the time.

Test your log with one question: if the plotter died now, could you work out where you are from what is written here?

A chart plotter keeps its own track, but a track you can’t see is no help when the screen is blank. A paper log needs no power and lives next to the chart. Some navigators keep the log in an app or a notebook instead of a printed logbook; what matters is that it is written as you go, and that you can still read it when the electronics are off.

What goes in each entry

Printed yacht logbooks vary, but the columns are much the same. Each row is one entry:

Column	What to write	Why it matters
Time	24-hour clock, four figures: 1030	Every DR, EP and ETA is worked from time.
Log	The log reading, to a tenth of a mile: 1243.5	Subtract one reading from the next to get the distance run through the water.
Course	The course actually steered, by the steering compass: 345°C	The direction for your DR and EP. Convert it to °T to plot it.
Wind	The direction it blows from, and its force on the Beaufort scale: SW 4	It shapes your sail plan and speed and, from Module 5, your leeway.
Barometer	Pressure in hectopascals (hPa, the same as millibars): 1015	A steadily falling barometer is an early warning of worsening weather.
Weather, visibility, sea	Fair · good · slight	The conditions now, and whether they are changing.
Position	Latitude and longitude, or a bearing and distance from a charted object, and how it was found: fix, GPS or EP	The starting point for working forward. “How” tells you how far to trust it.
Remarks	Events: engine on or off with engine hours, sail changes, marks passed, watch changes, forecasts, faults	The story of the passage. Engine hours help with fuel and servicing.
Initials	Who made the entry	Shows who was on watch, and who to ask.

Two columns need a little more care.

Course steered. Write the course the helmsman has actually been steering, averaged over the period (ask them), not just the course you asked for. When you alter course, make an entry with the time, the log reading at that moment and the new course. Each course in the log then applies from its entry until the next.
Log reading. Write the reading itself rather than the distance since the last entry. Then a missed or muddled entry doesn’t matter: the difference between any two readings is still the distance run between them. If the log stops working, say so, and estimate your speed instead.

Check your understanding: at 1030 Kittiwake alters course from 280°C to 345°C. What must that entry include?

When to write

At least every hour, on the hour, even when nothing seems to change.
At every change of course, and when your speed changes, for example when the engine goes on.
At every event: a fix, passing a buoy or headland, reefing or changing sails, the engine on or off, a change of watch, a forecast, a fault, anything unusual.
More often when it matters: close to danger, in pilotage waters, or when the visibility closes in.

If you are in doubt, write it down. A few seconds with a pencil is cheap; a gap in the log, when you need it, is not.

InteractiveLog it, or not?

Ten things happen on a morning passage. For each one, decide whether it goes in the log. Each answer comes with the reason.

1.It is 1400. Nothing has changed since the 1300 entry.
2.You alter course from 280°C to 345°C.
3.The crew make a round of tea.
4.You tuck a reef in the mainsail.
5.You start the engine to motor-sail.
6.Someone changes the music on the stereo.
7.Sam hands the watch over to Alex.
8.You take a three-point fix.
9.Drizzle sets in and the visibility drops to under a mile.
10.You dim the plotter’s screen for night use.

0 of 10 sorted

Keeping a good log

24-hour clock and a time zone. Write which zone you are using at the top of each page, for example “Times: BST (UT+1)”. The tide tables in UK almanacs give times in UT (so does the Port Kerran table in Module 4), while some local tables and apps have already added the hour for BST. Check which you have: a log that mixes the two is a trap.
Write it at the time, not from memory later.
Make it legible, so someone else can navigate from it. The next watch may have to.
Cross out, don’t rub out. Put a single line through a mistake and write the correction beside it, so the record stays clear and honest.
Sign or initial each entry, and both of you sign at a change of watch.
Say how each position was found: “GPS”, “3-point fix”, “transit”, “EP”.

Rebuilding your position from the log

This is the moment the log earns its keep. Kittiwake is sailing west-about round the Isle of Kerran. At 1120, just as she passes the West Ledge buoy, the plotter screen goes blank and takes the GPS position with it. The navigator logs the fault, the helmsman keeps to the plan, and at 1200 she needs to know where she is. The method works for any passage:

Find the last reliable position in the log, a fix or a GPS position, with its time and log reading.
Split the time since then into legs at each course change. For each leg, the distance run is the difference between the log readings.
Convert each course steered to true: deviation from the card for that heading, then variation (CADET: Compass ADd East → True).
From the last position, lay off each leg in turn on its true course and distance (one arrow: water track). Where the last leg ends is your DR.
Add the tidal stream for the whole time since the last position (three arrows: set and drift). The end is your EP. Mark it with a triangle and the time, and write it in the log.

Here is the yacht’s deviation card, and the variation for 2027 from the compass rose:

Deviation card: deviation for each ship’s head by compass
Head °CShip’s head °C	000	045	090	135	180	225	270	315
DevDeviation	2°W	1°E	4°E	5°E	3°E	0°	3°W	4°W

Interpolate between entries for other headings. Variation in 2027: 2°44'W, rounded to 3°W for chart work.

Worked example

Kittiwake’s 1200 EP, worked from her log

1Last reliable position: the 1100 GPS position, 49°45.0'N 004°19.6'W, log 1246.0.
2Leg 1, 1100–1130 on 345°C: distance 1248.5 − 1246.0 = 2.5 M. 345°C lies between 315° (4°W) and 000° (2°W) on the card, which gives 2.7°W — call it 3°W. So 345°C − 3°W deviation = 342°M; 342°M − 3°W variation = 339°T.
3Leg 2, 1130–1200 on 030°C: distance 1251.1 − 1248.5 = 2.6 M. 030°C lies between 000° (2°W) and 045° (1°E) on the card, which gives 0°. So 030°C with no deviation = 030°M; 030°M − 3°W variation = 027°T.
4Tide, 1100–1200, from the passage plan: 065°T at 1.0 kn for 1 hour, a drift of 1.0 M.
5Plot: from the 1100 position, 339°T 2.5 M, then 027°T 2.6 M, gives the 1200 DR at 49°49.7'N 004°19.2'W. From the DR, lay off 065°T 1.0 M.

1200 EP 49°50.1'N 004°17.8'W, about 2.9 M north of Kerran Head light and well clear of Head Ledge.

InteractiveKittiwake’s log: rebuild the EP

Read the log, then press the button to make the plotter fail. Work out each leg’s distance and true course and the tide’s drift: each line is drawn on the chart as you type. Then check your working.

Log of Kittiwake · Port Kerran → Kerran Roads

Sunday 6 June 2027 · Times BST (UT+1)

Kittiwake’s log for the morning of Sunday 6 June 2027
Time	Log	Co °C	Wind	Baro	Wx · vis · sea	Position (how found)	Remarks	Init
0900	1236.4	180	SW 3	1016	Fair · Good · Slight	Port Kerran	Slipped. Engine on (412.6 h)	SM
0940	1239.5	280	SW 4	1016	Fair · Good · Slight	Fix: Kerran Approach buoy close abeam	Sails set. Engine off (413.3 h)	SM
1000	1241.1	280	SW 4	1016	Fair · Good · Slight	GPS 49°42.4'N 004°14.1'W		SM
1030	1243.5	345	SW 4	1015	Fair · Good · Slight	—	A/c for W of West Ledge buoy	SM
1100	1246.0	345	SW 4	1015	Cloudy · Good · Moderate	GPS 49°45.0'N 004°19.6'W	Watch change: Alex on watch	SM/AJ

Chart plotter · 1120 · GPS 49°46.7'N 004°20.0'W

Module 2 in one logbook

This lesson closes Module 2, and the log is where all of its skills meet:

Lesson	What it gives the log
9 · Variation	Converting between true and magnetic with the variation from the compass rose, brought up to date: 2°44'W in 2027, rounded to 3°W.
10 · Deviation	The steering compass’s own error, read from the deviation card for the ship’s head. With the T V M D C ladder and CADET, a logged course in °C becomes °T.
11 · Position lines	Hand-bearing compass bearings of charted landmarks, converted to true with variation alone and drawn as position lines labelled with the time.
12 · Three-point fix	A fix (⊙) to log with its time and log reading, and a cocked hat that shows how far to trust it.
13 · Transits and clearing bearings	Position lines with no compass error, a check on the steering compass, and NMT/NLT clearing bearings to keep you clear of danger.
14 · DR and EP	Working forward from the last fix with the course steered, the distance run from the log and the tidal stream: the log supplies every number.

Next comes Module 3: the buoys, lights and rules of the road that you will meet on every passage, and that will fill the remarks column of your log.

Key terms

Navigational log (logbook): A written record of the passage, kept as you go: time, log reading, course, conditions, position and events.
Log (instrument): Measures speed and distance through the water. Its running total of distance is the log reading.
Distance run: Distance through the water between two entries: the later log reading minus the earlier one.
Course steered (°C): The course actually steered by the steering compass, averaged over the period.
Fix: A position found from observations, such as bearings, a transit or passing close to a charted mark.
Dead reckoning (DR): A position worked from course steered and distance run alone.
Estimated position (EP): The DR moved by the tidal stream: your best estimate without a fix. Plotted as a triangle.
Set and drift: The direction a tidal stream flows towards (°T), and how far it carries you in a given time.
UT and BST: Universal Time, used in almanac tide tables, and British Summer Time, an hour ahead (UT + 1). Note on the log which one you use.
Engine hours: Running time from the engine’s hour meter, logged when the engine starts and stops.

Exercises

Check your understanding. You get a second try at each question, then the worked answer. Score 70% or more to complete the lesson — your progress is saved in this browser.

What is the main navigational reason for keeping a log as you go?

Which is the best rule for when to make entries in the log?

From Kittiwake’s log: how far did she sail through the water between the 0940 fix and the 1100 entry?

Time	Log	Co °C	Position	Remarks
0940	1239.5	280	Fix: Kerran Approach buoy close abeam	Sails set. Engine off (413.3 h)
1000	1241.1	280	GPS 49°42.4'N 004°14.1'W
1030	1243.5	345	—	A/c for W of West Ledge buoy
1100	1246.0	345	GPS 49°45.0'N 004°19.6'W	Watch change: Alex on watch

What was Kittiwake’s average speed through the water between 0940 and 1030?

Time	Log	Co °C	Position	Remarks
0940	1239.5	280	Fix: Kerran Approach buoy close abeam	Sails set. Engine off (413.3 h)
1000	1241.1	280	GPS 49°42.4'N 004°14.1'W
1030	1243.5	345	—	A/c for W of West Ledge buoy

At 0940 Kittiwake settles on 280°C. Using the deviation card and the 2027 variation (2°44'W, rounded to 3°W), what is the course in °T?

Deviation card: deviation for each ship’s head by compass
Head °CShip’s head °C	000	045	090	135	180	225	270	315
DevDeviation	2°W	1°E	4°E	5°E	3°E	0°	3°W	4°W

Interpolate between entries for other headings. Variation in 2027: 2°44'W, rounded to 3°W for chart work.

On the way home, Kittiwake’s log shows a course steered of 120°C. What is that in °T?

Deviation card: deviation for each ship’s head by compass
Head °CShip’s head °C	000	045	090	135	180	225	270	315
DevDeviation	2°W	1°E	4°E	5°E	3°E	0°	3°W	4°W

Interpolate between entries for other headings. Variation in 2027: 2°44'W, rounded to 3°W for chart work.

Next morning Kittiwake leaves Kerran Roads east-about, and the plotter fails again. The passage plan gives the tidal stream for 1000–1100 as 050°T 0.8 kn. Work out each leg (deviation card below, variation 3°W), lay the lines off on the plotting chart, then tap the 1100 EP on the second chart.

Time	Log	Co °C	Position	Remarks
1000	1258.6	090	GPS 49°51.0'N 004°10.0'W	1½ M E of Kerran Roads
1040	1262.2	150	—	A/c
1045	1262.7	150	—	Plotter blank, no GPS
1100	1264.0	150	EP ?

Deviation card: deviation for each ship’s head by compass
Head °CShip’s head °C	000	045	090	135	180	225	270	315
DevDeviation	2°W	1°E	4°E	5°E	3°E	0°	3°W	4°W

Lay off each leg (water track), then the tide, starting from the plotted position.

Kittiwake’s plotter fails at 1120. Which logged position should the EP be worked from?

You asked the helmsman for 345°C, but they say the average over the last hour was nearer 350°C. What goes in the log?

Kittiwake’s log is kept in BST. The Port Kerran tide table, which is in UT, gives high water on 6 June at 1712. What time is that in the log?

0 of 10 answered · 0 correct

Complete the log

A fresh log extract each time: find the distance run and the speed, convert a course to true, plot the EP, and one question on keeping the log.