Mylar as an Aid to Navigators

John Ellsworth with ways to use this interesting material.
First appeared in SAIL magazine • All rights reserved • © 2006 John Ellsworth

If you practice the art and science of coastal piloting, consider obtaining a sheet of flexible, transparent woven polyester plastic, or film, to use as an overlay on your nautical charts.

Mylar made by Dupont (or Dura-Lar Matte made my Grafix Plastics) are both excellent choices. Either is quite suitable for tracing in either ink or pencil and the surfaces are erasable. Just don't use acetate--it won't hold ink or pencil marks.

Using the film sheet (henceforth, "Mylar") as an overlay, you can determine distances, courses, bearings, and tracks. The transparent sheet can also help you obtain a fix using horizontal sextant angles, find the course steered if no simultaneous sightings are available, or determine a line of position (LOP) using depth soundings.

Determine distance. A conscientious pilot is is continually measuring distance; gauging the dividers, walking them off between points, and juggling in the mind which place? How far? How much time? Of course, it's fun, but it could be easier.

On the dull side of a sheet of Mylar (or on either side of the Dura-Lar Matte), start from the center and draw a series of radiating concentric circles. Space them by increasing each radius by one, five, or ten scaled miles (depending on the chart's scale). Place the transparent sheet on the chart with the center at your location or point of departure and you have it-the distance to every port in your range.

Determine course or bearing. You can also make your own direction device with one of these sheets by ruling a series of parallel line sets all the way across a foot-square, or larger, sheet. Draw a set of four or five lines 1/8" apart, then another set 1/4" apart. Draw the sets alternately until the sheet is filled. It is best to draw them in indelible ink.

This lined sheet will give you a compass direction fast and quite accurately. Simply place the transparent sheet on the chart so that one line is over your planned course or bearing line. One of the other lines will fall across the center of the compass rose and give you the correct true or magnetic direction.

This device is especially helpful in a wet cockpit or on a rolling or pitching deck, where obtaining a reading with parallel rulers, triangles or a protractor could cause fumbling and loss of time.

Plot a fix using horizontal sextant angles. A film sheet can also help you plot a fix by measuring the two horizontal sextant angles obtained from three fixed identifiable objects. To get a fix, first measure the angle between the center object (a tank in Figure 1) and the object to the left. We'll call this angle A. Then quickly measure the angle between the same tank and the object to the right; here it is the radio beacon, or angle B.

Draw a straight line on the Mylar. From one end of the line, plot angle A on one side, B on the other. Place the Mylar on the chart and maneuver the tri-legged diagram about until all three lines pass through the centers of the sighted objects. Your position at the time of the sextant reading is at the common junction of the lines.

This method is reliable because the sextant is not affected by compass error, but it should be noted that care must be taken in selecting the objects. A circle can be formed through three objects that are not in a straight line and the closer you are to this circumference, the less accurate the fix. If your sailboat happens to be ON the circle, a fix will be indeterminable. So select objects that are nearly in a straight line, or try to have the center one closer to you than the other two.

Determine course made good. Perhaps you can't get a reliable two or three point fix. As you know, when simultaneous observations are not available a running fix from a single navigational aid can provide a slightly more reliable position. But advancing earlier bearings ahead to the time of the latest observation can't always account for an unknown current. And if the current is strong, a running fix can be quite unreliable.

Whatever the current though, the direction of the course made good can be determined; and this can be most helpful when plotting a running fix. Again Mylar is very useful in this situation. Plot your bearing lines on the chart. (See figure 2.) Then take a piece of Mylar and draw a straight line on it. Use any assumed speed and, using the time difference, mark the distances run between bearings.

Here I have used six knots, and therefore the boat would sail 0.2 of a mile between the times of the first two bearings and 0.3 of a mile from the second to the third.

Now maneuver the Mylar with these distances around on the chart until all the distance marks fall on the bearing lines. (See figure 3.) The direction of the line will be the course being made good: it is not an LOP, and of course any other assumed speed would have resulted in a parallel line a proportional distance away.

Determine track. If a reliable fix was obtained within a reasonable time before the first bearing of the running fix, a track can be determined by drawing a line from the fix in the direction of the course made good. (See figure 4.) An actual position would be wherever the track intersects a bearing line.

Determine LOP via soundings. Mylar can also help you establish an LOP via a chain of soundings. And with this method, you'll also get a rough estimate of the boat's position along an LOP.

First write down a series of soundings made at short intervals; the length of the series would be dictated by the chart scale and the existing situation.

For example, the eight soundings in figure 5 were taken at five-minute intervals starting at 1300 (64 feet). AS You can see, longer intervals would have been necessary if the soundings were farther apart, or if the boat was moving slower.

Draw a straight line on a piece of Mylar. Superimpose the line over the latitude or chart distance scale and mark the line at the respective distances (according to your speed) at which the soundings were made. At each mark, record the time and corresponding depth; correct the soundings for the prevailing height of tide.

Now check the chart for a fathom curve that is the same depth as one of your readings (in figure 5 the 10 fathom curve and a 60' recording match). Place the Mylar on the chart a reasonable distance north of your assumed position and align the two equal depths, Keep the Mylar line parallel to the course, the two matched readings aligned and slide the Mylar south until the Mylar readings match the actual chart soundings.

It is possible that an unknown current may have affected the course a bit. So when searching for the match, it may be necessary to seesaw the mylar slightly, using the fathom curve and its matched reading as the fulcrum.

Because it may be possible to fit the line of soundings to several places on the chart, this method should be used with great caution. Keep in mind that exact concurrence won't occur at all times for sounding inaccuracies, incomplete chart data, etc. may affect the match. But by seeking a general agreement, you should get a fairly adequate LOP.

I've covered several uses for Mylar in piloting and I hope you'll practice their application. Don't forget that Mylar is also very good as an overlay on your chart to preserve it.

There are many more uses for Mylar in piloting and no doubt you'll discover many of them yourself as you become more proficient in its use.

Notes: You can buy Mylar or Dura-Lar Matte in rolls, pads, or large sheets. On the internet, an 11x14-inch pad of Matte Dura-Lar goes for about $14.00. Pads also come in 9x12, 14x17, and 19x24-inches. You may be able to do better by going to a local art or drafting supply store. A 25x40 sheet of Dura-Lar Matte should not cost more than $5.00. On the internet, I saw such a sheet for $3.69, but you had to place a minimum order of 10 sheets.