Inside ranges

A 344
by Bill Brogdon

Ranges are the easiest to use, most popular, most reliable, and most expensive minor aids to navigation for marking channels. A pair of range marks defines a line of position, usually with great accuracy.

Unlike other LOPs, a range doesn't depend on shipboard equipment for accuracy. It is easy to see that the ship is on the range line when the two dayboards or lights are lined up "in range." If the ship is to the right of the range line, the rear mark appears to the right of the front one; if the rear mark is to the left, the ship is to the left of the range line.

It is possible to learn a great deal about a range by examining the chart. If the front and rear ranges are a long distance apart, moving a small distance across the channel produces a large angular change. Range marks that are close together show little angular change as the ship moves from side to side in the channel. It is easy to find the horizontal angle between the front and rear range lights by trigonometry. The horizontal angle is also the difference between the angles from the range line to the ship, measured from the front and the rear ranges.

A rear range mark that is well back from the front range shows angular changes well, but the farther back it is, the higher it must be. And higher towers cost considerably more than low towers. If the front range mark is near the end of the channel reach, the horizontal angle changes very quickly when a ship is at the near end of the channel. If it is set back, the changes are slower. There are a variety of non-navigational factors that influence the placement of range towers. Sometimes the water is too deep at the "best" place to put the front or the rear range, or the land rises too steeply or isn't available.

The vertical angle between the rear light and the front light also involves factors that are not apparent at first glance. In designing range marks for a channel, it is necessary to allow for the observer's height of eye above the water; an increased height of eye gives a larger angle, while a lower height gives a smaller angle. Of course, the height of eye is something of an educated guess, based on the ships that are normally expected to use the channel. It is also necessary to consider the range of tide.

Remember that heights of lights on the chart and in the Light List are above the mean high water. When the tide is lower than mean high water, which it is most of the time, the height of eye is lower with respect to range marks. In designing ranges, it is necessary to deduct the range of tide from the height of eye in order to avoid building a range with too-small vertical angles. The designed height of eye for a range becomes apparent when, for example, running a small boat along the range of a major ship channel. In that case, the front range may appear higher than the rear range at the near end of the channel. On a ship, with a large height of eye, the ranges appear normal: rear above the front.

It is important to consider the curvature of the earth and refraction when calculating the vertical angles of a range. The angles are small enough and the distances are long enough for these effects to be important. In addition, in the case of ranges, one's line of sight is nearly in a horizontal plane where refraction is greatest. It is also important to have the rear range daymark, not just the light, appear above the front range daymark at the far end of the range, or it won't be useful in the daytime.

A range designer attempts to balance these factors to give reasonable vertical angles, and horizontal angles sufficient to allow a navigator to detect side-to-side position easily. The angles involved are small. People with good eyesight can see two lights one minute of arc apart, and most people can see two lights two minutes apart. At the far end of the range, it is commonplace to have angles of a few minutes. Range designers attempt to have minimum angles of 4.5 minutes at the far end, but sometimes cannot achieve this goal.Sensitivity

A range is said to be "sensitive" when a small side-to-side motion is readily apparent. A range covering a short reach of a channel can be far more sensitive than a range covering a reach several miles long. At the far end of a range covering a long stretch of channel, it may be necessary to move many yards to the side to see any change. This is low sensitivity. In addition to using subjective terms for sensitivity, engineers have devised a formula to define a coefficient of sensitivity.

In plain language, the coefficient of sensitivity is twice the horizontal angle divided by the vertical angle, when the ship is at the extreme edge of a channel. Suppose the vertical angle is equal to the horizontal angle; the two lights would form a 45 degree angle to the vertical. This would translate into a sensitivity of approximately two, since ranges built to that standard show sufficient change as the ship moves to the side of the channel. Sensitivity varies along the channel reach and usually gets much higher closer to the range marks.

The sensitivity coefficient isn't the whole story. Suppose two ranges have a sensitivity of exactly two, but one has a horizontal and vertical angles of three minutes at the channel edge and the other eight minutes. It is easier to see the changes in the larger angles. So a designer has to take into account the angles themselves, as well as the sensitivity.


The designer also has to calculate the lighting intensities appropriate to the distances and the visibility common to the area. The designer tries to avoid a front range mark that is a great deal brighter than the rear mark. This is nearly impossible to avoid if the front range mark must be close to the channel.

In the U.S., the front range light is usually quick flashing (one flash per second) and the rear range light is isophase or equal interval (a three second flash followed by a three second eclipse). Usually the two lights are the same color, white being necessary for long distances. There are many variations, however, and numerous ranges in harbors have fixed (steady) lights of different colors that burn day and night. This is usually done where there isn't room to build dayboards. In the U.S., the dayboards are twice as high as they are wide, and have central stripe 1/4 the width of the dayboard. Not every range is lighted; there are a few ranges which have only dayboards. Unlighted ranges usually mark short lengths of a channel along isolated stretches of a waterway.Using ranges

By presenting some of the factors important to range design, we have been working towards actually using ranges. It also should show some of the reasons for the tremendous variations among ranges. Channel lengths and widths and the available sites for ranges vary widely, and so do ranges to mark them. It is relatively easy to gain some appreciation for the sensitivity of a range by looking at its length, and at the distances between the front and rear range marks. A long reach of channel marked by ranges close together cannot have high sensitivity.

The most practical way to test the sensitivity of a range is by moving from side to side slightly while looking at the range. The apparent change (or lack of change) in the horizontal angle is a good indication of the range sensitivity.

A range marks the line between the two lights (or dayboards) with high accuracy, independent of a compass. The Coast Guard Light List shows the direction from the front range light to the rear light to the nearest minute of arc for ranges whose positions have been surveyed to high accuracy. Others with positions somewhat less accurate are shown to the nearest degree.

When on the range line, it should be almost second nature to check the compass. However, it is more accurate to steer along the range line for several minutes, checking the compass frequently, than to simply take one reading. Avoid checking the compass immediately after lining up on the range; the compass needs a short time to settle down.

Going along a coast, it is often hard to see a range marking an entrance channel until one is nearly abeam of it. Then the lights are easy to see. This is due to the nature of the range lanterns, which are made like small searchlights, pointed along the channel. They show very little light off to the sides. This is especially true if the range designer has succumbed to the temptation to use lanterns with very narrow beams. They give high intensity along the range line, but very low intensity slightly off to the side. Because of these factors, navigators have learned that it is better to slant in at an angle than to approach at right angles to a range.

Approaching a range at a relatively shallow angle makes the lights appear in plenty of time to make the turn. This is important for ships, which turn slowly. It also makes life easier for boat navigators; the range appears well before it is necessary to turn. It is also good to find some prominent object near the rear range and know that the range will appear when that object is near the bearing up the channel.

Some range lights use lanterns that show all around, but with a higher intensity along the channel. This is a common practice on rivers and for ranges which mark relatively short reaches of channels. Range lights which show all-around are especially valuable when approaching to turn into a channel reach at the end nearest the range. Searchlight type lanterns are hard to see from the side, especially if there are lights in the background.

In most rivers, it is necessary to steer at a small angle to the range in order to compensate for current, and to have that angle change as the ship goes along the channel. The natural river channel and flow of current is often at an angle to the dredged channel, causing the set. Watching the range closely gives immediate clues to a cross-channel set, and allows a pilot to make quick corrections. The ability of a range to show this motion is superior to any other aid to navigation. Of course, the angle changes with changes in tidal current, and with changes in ship speed.

In addition to being excellent for staying in the center of a channel, ranges are the best guide for turning into the next channel reach. Notice how much the range for the next channel reach is open when starting a turn, and whether the ship winds up on the range, or to the left or right. Next time around, it's easy to adjust the turn by starting with the range a little more open or closed. The appearance of the range is a better guide than a mark abeam, since it automatically compensates for left-to-right position in the channel.

As a ship proceeds through a turn, the range is of continuing help. It is easy to take off rudder as the range closes, to slide right into alignment with it as the ship comes to the new course. Well into the turn, if the range isn't closing rapidly enough, one can ease the rudder. If it's closing too quickly, use more rudder or a shot of more power. It is also easy to steer a few degrees left or right of the normal course after completing the turn, to move into the correct part of the channel.

Many times the range isn't ahead, but astern. It might seem confusing at first to use an "over the shoulder" range, but it soon becomes as easy as using one ahead. It often is necessary to switch to the other bridge wing before a turn, when the next reach is marked by an over the shoulder range. Get on the same side as the range to keep it from being blocked from view. (It's a lot better to get in place before the turn than during it.)Off the range line

We have been speaking of riding the range line, which is necessary for large ships and handy when the channel is clear. However, if there is traffic, a navigator likes to stay in that comfortable place part way between the channel centerline and the starboard edge. Ranges are highly useful for this, but using them isn't nearly as straightforward as staying on the range line. Staying in a particular part of the channel, say, halfway from the range centerline to the starboard edge, the horizontal angle gets larger and larger as the ship gets closer to the range marks. If the horizontal angle is kept the same, a ship would get closer to the range centerline as it nears the range marks.

It's generally easiest to learn a range by keeping one's vessel in the desired part of the channel and making a point to notice the changes at specific points along the channel. If the channel reach has dredging daymarks, use them to judge the side distance, and look carefully at the range to see how it will appear at night. Otherwise, use the perspective of the buoys to judge position. At the far end, the range will be "open" slightly. As the ship gets closer to the range structures, the angle between them opens more and more, with the ship the same distance off-center. Learning how these angles change along the channel allows a pilot to take full advantage of the range.

Most of my shipboard experience has been aboard small ships—less than 400 feet long, with drafts of 20 feet or less. When meeting a large ship that must stay near mid-channel, I like to give as much room as I can. Learning the appearance of a range at various points along the channel, near the buoy line, allows me to use that information to run safely near the channel edge. While buoys can go off station, a range never does.

An accompanying diagram shows the vertical and horizontal angles on the Snow Marsh Channel in the Cape Fear River. It is a typical range in a river well-marked by ranges and by complementary dredging daymarks. In the accompanying graph, the right-hand edge is farthest from the range marks. Notice how the horizontal angle at the edge of the channel increases, moving up the channel to the range marks (to the right), The points are 1/10 of the channel length apart, about 1.4 of a mile.

The horizontal angle changes slowly in the first 2/3 of the way in from the far end, then increases rapidly. This is a typical pattern for a range with the front light relatively close to the end of the channel. Watching the range with binoculars gives an immediate clue to a cross-channel set. Usually, the horizontal angle changes gradually, with the ship staying the same distance off the range line.

As the ship approaches the end of the channel nearest the range, the angular change is quick, and can be misleading. The ship remains the same distance off-center, but the range opens more and more. A navigator who doesn't recognize how quickly the angle should change is drawn ever closer to the range line. In the absence of traffic, this causes no problem. But it leads a ship ever closer to the centerline approaching the turn, not a good tactic when meeting another ship. It's a good idea to learn both how the ranges appear when meeting another ship, and how the ranges appear when near the channel edge; then it is easy to use most of the channel width without risking going aground.

This effect only occurs at the end of the channel closest to range marks. At the other end, the angle changes very slowly as the ship moves along the channel.Single-point ranges

Ranges are expensive since they must have two structures, usually large ones. Engineers have pursued the idea of a "single-station" range like King Arthur's knights sought the Holy Grail: with great effort and dedication, but little success. Researchers have used tri-color lanterns, narrow sector lights, lighted panels that change appearance from different angles, lasers, and so on. Some of these ideas work well at short distances, but none are as simple, reliable, or give as much information at long distances as a two-station range.

Ranges fail utterly when the visibility drops. Gone is that instant check on position in the channel, side set, and heading that a range provides in clear weather. Radar is vital for navigation in fog, but can't show changes as accurately or as quickly as a range. This is one of the reasons why running a channel in fog is so hard: there are no visual clues from ranges or from the perspective of a line of buoys.

I have been able to "use" a range occasionally by radar. Both structures were built in shallow water, and showed up well on the radar screen. I turned the electronic bearing line (EBL) to go through the front range target; if it went through the rear range target as well, the ship was on the range line. Off to the right, the EBL goes to the left of the rear range target. It isn't as accurate as visual information, but is quite useful along the near end of a range.

In spite of not doing everything, ranges perform the task that they are designed to do extremely well. It is possible, with some study, to get far more from them than would first appear. It isn't necessary to work arcane calculations, but it is necessary to experiment a bit and observe each range carefully, to learn its capabilities and peculiarities.

Contributing editor Bill Brogdon, former head of the Coast Guard's office of navigation, lives in North Carolina.

By Ocean Navigator