Between fixes, heading, log and clock carry your position forward. Add leeway and stream for an estimated position; turn the triangle round for the course to steer.
How do I convert a true course to a compass course?
Subtract east, add west: first the variation, then the deviation for the heading you are on; compass to true is the reverse. So 058°T with 4°E variation and 2°W deviation is 054°M, then 056°C.
Variation comes from the chart's rose; deviation from the steering compass's card for the heading you are on: read it again for each new heading.
True to compass: subtract east, add west. Compass to true: the reverse.
By the 1 in 60 rule one degree of error is a mile off track after 60 miles.
How do I work out a DR position?
From the last fix, lay off the true course steered and the distance run through the water. Distance is speed × time in decimal hours: 1 h 12 min at 5 knots is 1.2 × 5 = 6 miles.
A DR position is the true course steered and the distance run through the water, laid off from the last fix. Distance is speed × time, with time in decimal hours: 1 h 12 min is 1.2 h.
Mark a DR with a line across the track, an EP with a triangle, a fix with a circle, each with its time. Legs on different headings go end to end.
Check the log: at slack water it should agree with the SOG; a fouled paddlewheel under-reads. Over a measured distance take the mean speed of two opposite runs. A log showing 2.2 M for 2.0 over-reads: divide by 1.1. If it fails, take speed from the engine revs, not the SOG.
What should I write in the log book on passage, and how often?
Time, log reading and the heading really steered, every hour and at each alteration, with a position on paper. From that the plot can be rebuilt, so a dead plotter costs an hour of DR, not the night.
A plot can be rebuilt only from what was written: time, log reading and the heading really steered, each hour and at each alteration, with a position on paper. Then a dead plotter costs an hour of DR, not the night.
| Time | Log | Steered | Note |
|---|---|---|---|
| 0100 | 41.2 | 255°C | Fix |
| 0200 | 46.0 | 255°C | DR: 4.8 M, 252°T |
In fog, slow down and work each stretch at its own speed. Closing a coast blind, aim to one side of the harbour by more than your likely error: then you know which way to turn. Near dangers plot every six minutes: a tenth of the speed.
How do I allow for leeway and tidal stream to get an estimated position?
Apply leeway to the heading first, away from the wind, to get the water track; then lay off the stream's set and rate × time from its end. That point is the EP.
Leeway pushes the boat away from the wind: wind on the port side, add it to the heading; on the starboard side, subtract. Greatest close-hauled, nil on a run, it shows as the angle between wake and stern. The result is the water track.
From the end of the water track lay off the tidal stream: its set, the direction it flows towards, and rate × time. That point is the EP; the line from the fix to it is the ground track, the one to check against dangers. Arrowheads: water track one, ground track two, stream three.
Take the stream from a tidal diamond or the atlas; each hourly value serves half an hour either side. Interpolate the rate by the day's range, even beyond springs. On your track the diamond beats the atlas.
How do I work out a course to steer across a tidal stream?
Draw the track to the destination, lay off the stream for the period, and swing an arc of boat speed from its end to cut the track: that line is the course to steer. Never join the stream to the destination.
Across a stream you aim up-tide. Find the angle by drawing:
Never join the stream to the destination. If the arc cuts beyond it, rework with the shorter time. A slower boat needs a bigger angle; a stream across the track faster than the boat leaves no answer. Then steer into the wind by the leeway and convert to compass.
For a long crossing lay off every hour's stream end to end and swing one arc for the whole time. On that one course the ground track bows down-tide and returns as the stream turns: the shortest way through the water. Do not chase the cross-track error or point at the waypoint; where the curve crosses a danger, work each hour's course.
How do I work out my ETA against a foul tide?
Divide the distance to go by the speed made good over the ground: log speed less the foul stream. 14 miles at 6 knots against 1.5 knots is 14 ÷ 4.5 = 3 h 07 min.
The ETA comes from the speed made good over the ground: the SOG from the triangle, or log speed plus a fair stream, less a foul one. Mind the decimals: 0.4 h is 24 minutes.
The plotter's ETA assumes the present SOG lasts: if the stream will turn, work each stretch yourself. Beating, tacks 45° off the bearing make good 0.7 of your speed. Against a foul stream each extra knot counts for more: weigh the time saved against the fuel.
Should I add the tidal stream to my plotter's COG and SOG?
No: COG and SOG are over the ground, with stream and leeway already included, so adding the stream would count it twice. Heading and log are through the water.
Heading and log are through the water; COG and SOG are over the ground, stream and leeway included. The COG line, not the bow, shows where you are going. Never add the stream to them: it would count twice.
The plotter's own course to steer is just the bearing back to the route, stream ignored. At low speed in a swell COG and SOG jump: use compass and log.
An EP is an area that grows every hour since the fix. At the next fix, read the gap: along the track it is the log or the stream's rate; across it, heading, leeway or set. From a plain DR to the fix is the stream you had: 2.4 M in two hours is 1.2 kn. Restart from the fix.
Questions from this topic, as the app asks them. Open one to see the answer.
You have laid off the course to the next waypoint on the chart. The boat has no deviation on this heading. What course do you give the helmsman?
D. 250°M
True to magnetic: add westerly variation — 247° + 3° = 250°M.
You are motoring out of Dartmouth on a calm morning with the log speed steady. How far through the water have you run by now?
A. 10.5 M
0915 to 1100 is 1 h 45 min = 1.75 h; 6.0 × 1.75 = 10.5 M. Not 1.45 h.
On your night watch the plotter dies. Which entries in the log let you carry on the plot?
A. Time, log reading and heading steered
DR is rebuilt from time, distance run by the log and the heading; plotter COG/SOG are gone with it.
You are close-hauled on port tack with the wind as shown. Which water track do you plot?
B. 092°T
Leeway pushes you away from the wind: wind on the port side, so 085° + 7° = 092°T.
You cross with a stream that runs east for the first two hours and west for the last two, equally. What does your ground track look like?
D. A curve bowing east, back on the line at the end
Steering one course, you are set east, then set back west by the same amount; you meet the line again at the end.
You are running along the north coast of Brittany at a steady speed over the ground. When do you reach the fairway buoy?
C. 1214
13.2 ÷ 5.5 = 2.4 h = 2 h 24 min; 0950 + 2 h 24 min = 1214. 0.4 h is 24 min, not 40.
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.