Reading Ranges - SailNet Community
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Reading Ranges

Keeping your eyes out is particularly important while underway in narrow channels, especially if current is a factor.
One of the best sailing rules that I was brought up with was "Get your eyes out of the bilge"—meaning, "Look around!" That was easy to do in the days when the only electronic device that tempted a wandering eye was an analog knotmeter of dubious accuracy and sensitivity. But today, when speedos, wind instruments, GPS displays, and course plotters accurate to with 0.01 knot, 1 degree, and a couple of inches (or so we are solemnly assured), that dictum may seem to be irrelevant. Who needs to look around when everything you need to know is right there on a panel of neat flashing lights?

Well, we all do. The gadget may be wrong—in fact, it should be presumed to be inaccurate—and even when it's correct the "sea" or "lake" that it's sailing on is an abstraction. This water has no waves, no tankers, no gusty wind, no tide rips, no anything that makes going out on the water such a concrete, nondigital, and challenging experience.

Looking around means just that. One way to tell inexperienced sailors from old salts is to follow their eyes as they come up from the cabin below. The new guy finds a target and stares at it, but the experienced guy's eyes never settle down. They immediately look forward toward the destination, alongside at other boats, and up at the sails. Good sailors' eyes are restless eyes. The eyes of a good sailor who is steering are constantly flickering between the compass, sails, wind indicators, waves, and instruments. Those eyes are searching for facts, problems, anomalies, and context.

What those eyes (and the brain to which they're attached) are  looking for is an overall picture of the situation using frames of reference. Among the most valuable of these frames of reference are fixed objects, whether on land or on the water. We use them to indicate relationships with nearby boats in the phenomenon of "making land." And we also use them to gauge the effect of tidal or wind-driven currents using ranges or transits to allow us to determine whether we are "gaining on" or "losing on" an aid to navigation.

The two illustrations by Mark Smith from The Annapolis Book of Seamanship illustrate these two excellent, simple visual guides.

Figure 1—The skipper to starboard knows that he or she is "making land" on the other boat because the land appears to be moving forward. 
It's commonly known that when two objects alongside each other are going at the same speeds on a collision course, the bearings (compass directions or relative angles) between them do not change from moment to moment. We learned this as kids when, while walking to school, we either carefully avoided colliding with the class bully or carefully tried to collide with the class beauty. On boats underway, good sailors are constantly taking bearings on nearby vessels to gauge the chances for collision. If the bearing does not change, a collision will occur. So slow down, speed up, or alter course.

Less well known is the fact that land can also be used as a gauge. Figure 1 shows a typical situation near shore. If the land doesn't appear to be moving, the two boats are going the same speed. If it seems to be moving forward, you're gaining on the other vessel and will pass ahead of her. This is called making land on the other boat. If the land seems to fall back, you're losing land and the other boat is going faster. It's that simple—no compass bearings needed. You needn't do anything else except look at the movement of the land as the other boat's bow passes by. (Of course, the land is not really moving. It only appears to be because the other boat is falling back or pulling up.)

Figure 2—To pass the upcoming buoy to starbord, this skipper is steering "uptide" to compensate for the set and drift of the current. 
Somewhat similarly, ranges provide a helpful frame of reference when you're sailing in tidal waters or in waters with seiches or other wind-driven currents. When two fixed objects are in the same line of sight (one in front of the other), they form a range, or what our British cousins call a transit. The objects can be lighthouses, trees, church steeples, even buoys (unless they are drifting about a lot in light or heavy winds). Whatever they are, they must be either ahead of or behind the boat. If the two objects are aligned, you're steering a straight course. If the relationship changes, your course is wavering.

In Figure 2, a boat is sailing in a side current pushing from port to starboard. The steerer is heading the boat at a nun buoy, which she intends to leave to starboard (or pass on its left side). The steerer compensates for the set and drift (the current's direction and speed) by steering uptide (to port). The question is, How far should she steer uptide?

There are ways to answer that question mathematically using the Rule of 60 (a topic for another column) if the current set and drift are known exactly. It's far easier to gauge the heading using a range formed by lining up the nun buoy with a fixed object behind it, in this case a church steeple. As the crew watches the range, the steerer experiments with different headings until the nun stays lined up in front of the steeple.

By using fixed objects like a bridge tower and a building behind it as ranges, you can judge whether you're gaining or losing bearing while underway.

Sailors will speak of "gaining" or "losing" on a range or on a buoy as the range shifts. In this example, if the steeple "moves" to port of the nun the boat is gaining on the buoy and will safely pass on its port side in order to leave it to starboard. But if the steeple "moves" to starboard, the boat is losing on the buoy and will pass on its starboard side.

Each of these tricks takes a little practice to master, but each will be a loyal friend to any vessel—assuming that the crew keeps their eyes out of the bilge.

John Rousmaniere is offline  
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