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Plotting Equipment

This article was first published on SailNet in April of 2000.

World-renowned racer Ellen MacArthur knows the importance of taking proper care of her nav station and its attendant tools.
Nothing reflects a navigator's ability more than keeping neat, clean, accurate charts and logs. An untidy chart, worn and smudged in places, often causes one to conclude that the navigation performed was careless. Charts are very inexpensive relative to the value of your boat and the safety of everyone on board. For this reason, always replace worn or outdated charts with the latest editions and use the new ones with care. The same attitude goes for all navigation equipment. Having good plotting equipment, properly used, can only help to enhance the work of a professional navigator.

I suggest that you have several good pairs of dividers and plotters on board. Place the older ones in your emergency navigation kit, and use the newer ones for your daily plotting work. Several fine-tipped, soft-lead pencils are imperative. Use a No. 2 or a No. 3 pencil, since a No. 4 lead is too hard for use on a chart. A pencil sharpener is very handy to keep the pencil points sharp.

Using Dividers    You should learn to manipulate the dividers with one hand while leaving the other hand free to work with the plotter, the pencil, or the chart. Most good navigation dividers have a tension screw that can be adjusted to prevent them from becoming either too loose or too stiff for easy use. Cheap dividers lack a tension screw and will soon become too loose for proper use. Adjust the points of the dividers to approximately equal length. A small screwdriver is required for these adjustments and should be a part of your navigation equipment. If you drop your dividers, the odds are good that the points will bend or break. Therefore, it is best to have replacement points on board.

Simple yet effective—good dividers are one of the navigator's most important tools.

Try to find a pair of dividers with long arms. The more distance you can cover with the dividers half open, the easier it will be to measure distances on the chart. For long distances between waypoints, you will need to pre-measure a convenient distance based on the scale of the chart (i.e. 1 nm, 2 nm, 5 nm, etc.) and step the dividers along the course line until you are close to the end waypoint. Count the number of times you step the dividers, and multiply the number of steps by the preset distance. Then add the remaining distance to the waypoint from the last measurement to obtain the total distance.

Another type of divider you should have is one with a pencil lead on one end in place of the normal steel point. This divider will allow you to draw arc distances from lights, plot danger circles around areas to avoid, and plot radar ranges. A fix formed by drawing in the arcs of multiple radar ranges provides a more accurate fix than a single range and bearing.

Plotting Tools    You may wish to consider having several plotters on board that can be used together, since this will enable you to plot long legs on the chart. The plotter used by most navigators is a semicircular protractor with a long straight edge attached to it. A small hole at the base of these protractors indicates the center of the arc of the angular scale. Two complete scales, both graduated in degrees, cover the outer edge of the protractor. The angle is measured by placing the straight edge along the course line and reading the scale where the protractor crosses a meridian on the chart. The outer scale is used to read all angles between north through east to south, while the inner scale is used to read all angles between south through west to north. An abbreviated inner scale measures the angle from the vertical, and is usually used with the latitude lines to measure courses close to due south or due north when a longitude meridian is not readily available.

This navigator is taking care to ensure that he uses his plotter  accurately, as all navigators should.
There are many other types of plotters, of course, and the next most popular one is the parallel rule. It is used with the compass rose on the chart to measure the various course and bearing angles. It takes some practice to use parallel rules properly, and a large, smooth surface is needed to use this type of plotter effectively. Often, you will need to step the ruler some distance to get to a compass rose. Consequently, the parallel ruler is not easy to use on smaller vessels.

The US Power Squadron uses a square plastic plotter that is very easy to use. If you are a member of the USPS, or have taken any of its courses, you most likely have one of these plotters. Another plotter that I like has attached rollers that makes rolling the plotter to and from a nearby compass rose very easy. This plotter is much easier to use than the parallel rules on a smaller navigation table, but care must be taken that the angle doesn't slip while it is being rolled. The standard drafting triangles of 45 / 45 degrees, and of 30 / 60 degrees, are also very handy tools to have available at your navigation station.

Plotting Tips    There are several tricks of the trade to use when drawing course lines. The most important one is to estimate the course direction before measuring it. By having a good idea of what the course should be before you place the plotter on the chart, you automatically avoid the "wrong way Corrigan" effect of reciprocal courses. With some practice you will soon be estimating your course lines to within 10 degrees. Do this by mentally dividing the chart into four quadrants with a north-south and an east-west line. Most charts are oriented with true north at the top, and the other cardinal directions at the bottom, the left, and the right. As an example of use, any course line proceeding toward the northeast quadrant will be between 0 and 90 degrees, and you can then divide this quadrant in half to estimate if the course line is between 0 and 45 degrees, or between 45 and 90 degrees. Dividing this area in half again will allow you to estimate the direction between 0 and 22 degrees, or 23 and 45 degrees. Do the same for courses in the other quadrants and if the course line is near a compass rose, use it to help mentally estimate the course.

Even if you have all the right tools, it never hurts to have another navigator check your chart work.

There are a few tricks to drawing long course lines between waypoints when your plotter is too short and you lack a second one. You can fold the chart so that the crease falls between the waypoints and then draw the course line along the crease. If you object to such creases in the chart, you can roll an edge of the chart up to the waypoints and pencil a dashed course line along the temporary plot line. Then place the protractor or plotter along the dashed line in several steps to complete drawing the course line.

Once you have measured the course and distance between waypoints, record the true course on top of the course line near the departure waypoint, and place the distance directly below the course line underneath the course. Use a capital C for the course and a D for the distance as shown below. The course is always assumed to be a true direction unless otherwise annotated. Thus, the notation:

C 090

D 8.0

would indicate a course of 90 degrees true and a distance of 8.0 nm.

If you desire to use magnetic courses on your chart, apply the variation to the true course, and place a capital M after the course to indicate that it is a magnetic course, i.e. C 235M. If you also apply your deviation to the magnetic course, then you will have a compass course and should place a C after the course, i.e. C 237C.

The reason for this standardization is so that any navigator can take over your duties on a moment's notice and understand your work. The effort to standardize your navigation work is well worth your time and will mark you as a professional.

 Suggested Reading:

Navigation Basics by Jim Sexton

General Navigation Techniques by Jim Sexton

Choosing Charts by Paul and Sheryl Shard


Buying Guide: Chartplotters

Jim Sexton is offline  
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