Course Plotting - a Primer

Course plotting or fixing your course is on of the most basic and important of all skills for a cruising sailor. What follows is an outline of the essential elements of fixes and some special types.

DEAD RECKONING

The term 'dead reckoning' takes its name from 'deduced reckoning' (abbreviated DR). In other words, you are deducing your position from several pieces of information and plotting it on a chart.

So, the simplest method to find your way from A to B is to:

  • mark the positions of A and B on the chart draw a line from A to B
  • measure the direction of this line using a parallel ruler or Breton-style Plotter and the compass rose
  • apply the corrections to get true compass direction (variation and deviation)
  • steer in that direction using the ship’s magnetic compass.


The ship’s log or knot meter will tell you the distance in nautical miles that you have travelled through the water. Take a note of the log reading when you are at A, or set it to zero: and the log reading will then tell you how far you have travelled from A towards B. Because the Great Lakes are freshwater bodies, we can discount the effects of tides and tidal streams.

However, we can account for LEEWAY as shown below. After an appropriate period of time (say 30 minutes if you are making a steady x knots), measure off the logged distance on the side of the chart with your dividers. Then, mark your position on the line from A to B. This process is known as Dead Reckoning.

Your DR position is shown on a chart as a semi-circle with a dot on the course line, marked with the initials DR and the time in 24 hour format.

LEEWAY

Leeway is defined as the difference in degrees between the heading of the boat and the reciprocal of the wake. Simply put, in a strong breeze, in addition to forward movement, there is a sideways movement of the boat in the water as the wind pushes it.

Leeway can be easily determined by comparing the boats heading with the reciprocal measurement of the wake. Note the boats heading in magnetic degrees. Using the hand-bearing compass, take the bearing of the wake. Convert that to its reciprocal (or opposite) by adding or subtracting 180o. Compare the two. The difference is your leeway.For example:

  • boat heading = 100°
  • wake bearing = 270°
  • wake reciprocal = 90°
  • leeway = 100° - 90° = 10°

Account for leeway by adjusting your DR course line. Make sure you adjust in the correct direction! Some navigators find it useful to draw a wind arrow on their chart to ensure that its clear which way the boat is being blown off course.

FIXES

The first type of fix is a visual fix. This is taken with a hand-bearing compass. Take a sight on a charted landmark such as a water tower or a spire. Note the bearing. Convert the compass bearing to a true bearing. Using a parallel ruler, trace that bearing on a compass rose on the chart. Then 'walk' or 'transfer' the bearing to the charted landmark..

Now, take another sighting on another landmark or charted object. Try to have a good angle of intersection – narrow or tight angles are not as accurate. Repeat the process of converting the compass bearing to true and then drawing it on your chart. Your position should be the intersection of those 2 headings. (I say.'should be' because accuracy is important. Double-check all of your calculations.)

To enhance the accuracy of your fix, take a third sighting, repeating the process. Don't be surprised if the lines don't intersect exactly – they likely won't. Instead, they will likely create whats called a 'cocked hat' and you are somewhere inside that triangle. Prudence would dictate that you mark your position within the cocked hat on the side closest to any hazard.

A fix is marked on a chart with a dot surrounded by a circle. The time of the fix (in 24 hour notation) and log reading are noted adjacent to the fix.

RUNNING FIX

If you cannot identify two charted landmarks, it is possible to located your position by taking 2 fixes on the same landmark. Plot your course line (with True bearing noted above the line and speed noted below). Take your first bearing (again, in True degrees) and label with bearing and speed. Maintain your course and heading for a length of time. Watch the landmark as the angle to it 'opens' up. When there is a significant change in angle, take another fix.

Determine the DR of the 2nd fix and plot it. Label it with a dot inside a half circle with the time and the notation “DR” Now, using parallel rules, advance the first bearing line forward to the DR position you plotted. The running fix (marked as RFIX) is the point where advanced first bearing intersects second bearing.

BEARINGS & CONTOURS

An alternative when fixes on three suitable landmarks is not possible is to use a combination of a bearing line and a contour. Here, you would use your depth sounder. Make sure you know whether your depth sounder is set to measure from keel or waterline. By plotting a bearing and seeing where it intersects a known contour line, you can fix your position

TRANSIT LINES

When 2 objects line up, they are described as being 'in transit'. A line connecting them is referred to as a transit. The object in the foreground will be lower than the one behind. Often a transit is printed on the chart.

For example, entering into Port Colborne ON harbour, there is a charted transit line of 015.5 degrees. In other circumstances, you create your transit line when there are two objects that you can line up – a church spire and a marker buoy for example.Transit lines can be combined with a contour line to determine a fix.

DISTANCE OFF

If you can determine your distance off, i.e. the distance between your vessel and a landmark which is marked on the chart, you can draw an arc of a circle using the landmark as the centre - this is a position line. Take a bearing on the landmark – chimney, lighthouse, tower – anything with a marked height noted. Draw the bearing line. Next, using the landmark as the centre point, use your compass to draw a semi-circle with a radius that matches the distance from your boat to the landmark.

A nautical almanac will have a table for distance off when rising or dipping. (There is one on the RESOURCES page) You need to estimate the height of your own eye above sea level and the height of the light which will be marked on the chart.

This can work very well with lighthouses at night because lighthouses are always charted, their light characteristics are noted and the rising and dipping (called the 'loom') of a light can be seen on clear nights. At night, if you observe carefully, you will be able to see the light just as it rises above the horizon and/or see the loom of the light as it dips below the horizon.

SECTORED LIGHTS

Sectored lights are lights that display more than one colour. They are arranged so that a different colour is seen depending on where in the sector you are situated. The sectors are marked on the chart. When you see the light as one colour you know you are in that sector and when the light changes to another colour, you have moved into the sector for that light. An example would be the RWG (Red White Green) sectored light marking the entrance to Edgewater Basin at the west end of Cleveland harbor.

CLEARING LINES

Sometimes when navigating, you don’t need to know exactly where you are you are. It may be enough to know approximately where you are, and to be sure that you are clear of dangers. If you can draw a single position line, you can tell at a glance which side you are on to keep out of danger. This line is known as a clearing line. You can use transits, bearings on landmarks or Aids To Navigation to construct bearing lines.

One observation can tell you if you are safe or not, which can be far quicker than plotting your position. As a memory aid, you can note a clearing line bearing and put a green '+' sign on the side you want to stay on so you know at a glance if you are on the right side of it.

You can also use depth contours to either avoid shallow water or to follow a route for example in heavy fog.

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