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The plotter shows where you are. Not why
Electronic navigation on the plotter is good, and it fails. A flat battery, a wet unit, an update at the wrong moment. If you also know where you are, you notice within the minute. If you do not, you notice later.
Chart, course, dead reckoning
Course over ground out of heading, tidal stream and leeway. Dead reckoning is not a museum piece but the fallback when the screen goes dark: heading, speed and time give you a position good enough for the next harbour.
Bearings and fixes
Two bearings on fixed objects cross at your position, three make a small triangle and show how carefully you worked. The same bearing on two lights every half hour at night tells you whether the anchor is holding.
Buoyage
In IALA region A, red is to port when entering. Cardinal marks run clockwise, north, east, south, west, and the cones are the memory aid: two points up is north.
The collision regulations
Rules of the road, lights, sound signals. The three questions are always the same: can I see him, can he see me, and who gives way to whom. A boat under engine is not a sailing vessel, even with the sails up.
Tides, stream and weather
Tide tables before letting go, not after. Read the barometer through the day, because it is not the reading that counts but the change. Cloud and the Beaufort scale give you the order of magnitude before the phone signal confirms it.
The log book
Position, course, weather, engine hours, anything out of the ordinary. Variation and deviation are worked in, otherwise the series does not add up. A log book is the one record on board that nobody can reconstruct afterwards.
Navigation for sailors: plan, track, cross check
Most good navigation on a sailing boat happens before you leave the berth. At the chart table you choose the route, mark shallows, restricted areas and channels, note the tide times and pick harbours of refuge in case the weather turns. Once under way you are no longer planning, only checking.
That checking has two halves. One is the running calculation: heading, speed and time give a position you put on the chart. The other is comparing it with the world outside, with buoys, landmarks, the echo sounder and the satellite fix. When the two agree, all is well. When they drift apart, you have a warning long before an error becomes dangerous.
How to read a nautical chart: depths, symbols and scale
The figures on a chart are depths in metres below chart datum, a low water level that the sea seldom falls below. In tidal waters you add the current height of tide to the charted depth. Heights of lights and bridge clearances refer to a different level, stated in the chart title block. Read that block properly at least once for every chart you use.
Always measure distance on the latitude scale at the side, never on the longitude scale along the top or bottom. One minute of latitude is one nautical mile. The compass rose gives true north and the magnetic variation for a stated year with its annual change, which you bring up to date yourself.
A chart is only as good as its corrections. Buoys are moved, wrecks reported, channels shift. Corrections are published in the Notices to Mariners and written onto paper charts by hand. Electronic charts are kept current by updates, but only if someone actually installs them.
Plotting a course: from the chart to the course to steer
Plotting a course turns a line on the chart into a number the helm can steer. First read off the true course between departure and destination. Then check whether a tidal stream will set you off it. Draw a tidal triangle: from the start, lay off one hour of stream; from the end of that vector, swing an arc equal to your speed through the water until it cuts the track. The line joining them is the course to steer.
After that come leeway, variation and deviation. Estimate leeway and steer that much further to windward. Apply variation and deviation with the correct sign: going from true to compass, subtract easterly values and add westerly ones. Write the sequence out once, neatly, and by the tenth time you will stop making sign errors.
- Draw the track on the chart and read off the true course.
- Take the stream from the tidal atlas or the chart and draw the tidal triangle.
- Estimate leeway and allow for it to windward.
- Apply variation, updated to the current year.
- Apply deviation for that heading from the deviation card.
- Log the course to steer, the time and the log reading.
Dead reckoning: a position from heading, speed and time
Dead reckoning is the sum that always works, power or no power. From your last reliable position, lay off the heading you steered and along it the distance run, taken from the log or from speed multiplied by time. That gives the dead reckoning position. Allow for stream and leeway as well and you have an estimated position, which sits much closer to the truth.
For mental arithmetic under way, the six minute rule helps: in six minutes a boat covers one tenth of her speed in knots, in nautical miles. At six knots that is six tenths of a mile. Plot your position regularly, roughly every hour offshore and more often near the coast. A bearing on one landmark taken twice, with the first line moved along by the distance run, turns an estimate into a proper running fix.
When the plotter fails: what to keep on board
Failures rarely give notice. Keep ready a paper chart of the area at a usable scale, a plotter or protractor, dividers, a pencil and a rubber, a hand bearing compass and a pilot book. A second device with its own battery, such as a tablet with charts, is a sensible addition, but no substitute for a chart you can read without power.
Habit matters more than kit. If you note position, heading and log reading every hour, you have a starting point the moment the screen goes dark. If you relied on the display alone, you first have to work out where you were. So now and then, in fair weather, sail a leg with chart and compass only and compare with the satellite position at the end.
Frequently asked questions
What is dead reckoning?
Working out your position from the heading steered, speed and time elapsed, starting from the last known position. Add stream and leeway and you get an estimated position.
How do I measure distance on a chart?
With dividers on the latitude scale at the side, close to the latitude of the leg. One minute of latitude equals one nautical mile.
What do the numbers on a nautical chart mean?
They are depths in metres below chart datum. In tidal waters you add the current height of tide.
In what order do I work out a course to steer?
True course from the chart, then stream via the tidal triangle, then leeway, then variation and finally deviation. The result is the compass course.
What is the six minute rule?
A rule of thumb for mental arithmetic: in six minutes a boat covers a tenth of her speed in nautical miles. At five knots that is half a mile.
Do I still need paper charts on board?
As a fallback, yes. A paper chart works without power, software or signal, and every calculation on it can be checked.
What this has to do with insurance
After a loss the log book is the best evidence you have: it shows weather, position and time, and it shows them in a sequence that was written before the loss. With us there is more to it: our blue water cover requires the digital NAMMERT log book once the boat moves more than 300 nautical miles from its reported location. Keeping it meets a contractual duty and at the same time produces the document that counts in a claim.
