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Using Ranges

July 1, 2013

An often misunderstood (and consequently misused or ignored) aid to navigation is a range. Whether it is an official range of two lighted markers shown on a chart or the two sticks in the sand that Dave aligned to help me get the dinghy through a narrow cut in the reef, ranges are an important tool for mariners to use.

A range is based on the geometric fact that it takes two points to determine a line. It works best if the furthest point (marked by a stick, tree, church steeple, etc.) is the tallest and the closer point lower. When the two points overlap you are on the line they define. Ranges mark deep water, an entrance through a reef or a safe passage through hazards to navigation.


Preparing ranges to use to get Eureka off the beach in Union

There are many tricks to help mariners remember how to line up a range; onboard Eurisko our mantra is "Chase the lower one." This means that if the closer (usually the lower) marker is to the left of the other, turn to port. If it is to the right of the further one, turn to starboard.

Ranges can be used behind you, as well. When leaving a harbor with a range, simply turn around occasionally to verify that the markers are aligned. If not, it is easier to use compass headings than "left or right." If the closer marker is to the south of the further, turn to the south.

A few years ago, while anchored in Isla Mujeres, Mexico through a three-day gale, we used a range of our own. Frequently we have heard boaters say they use a building on the port side and a tree on the starboard, for example, to verify that they have not dragged their anchor. This is a good tool for estimating your location, but with this sort of range you can often drag 100' without noticing it. With our method, we were aware of literally 10' of change in our position.


Finding ranges on shore was easy in Isla Mujeres

First, note the boat's most frequent heading. Next, find two objects on shore, both on the same side, that line up when the boat is at that heading. We have used trees, rocks, radio towers, the summit of hills; any stationary object will work. In Isla Mujeres we chose a street light and the windows of a house. When possible, we prefer lighted objects for a range so that we can use them at night. If, when the boat is at the same heading, the closer range marker moves AFT of the further, you have pulled up on your anchor; perhaps because of current or a decrease in the wind velocity. If the closer marker moves FORWARD of the further, you may be straightening out a chain rode, stretching a nylon one, or dragging anchor. Since the wind was consistently strong enough to maintain a steady tension on the three nylon rodes and keep our chain rode pulled up, when our range moved we knew we had dragged one or more of our four anchors. By walking forward on deck until the ranges were again aligned, we knew our anchors had let us shift about 10 feet.

Many boaters use ranges without realizing it. When requesting local knowledge about a tricky body of water, you may hear advice such as, "When you can see the sugar mill through the cut, you can turn." Understanding ranges and being able to create your own will make you a better navigator as well as give you more confidence when anchored.

Previously published in Sail Magazine.

MONDAY we'll continue our trip up the East Coast, with some interesting anchorages that only Walkure could let us enjoy as much as we did.

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