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Eyeball Navigation

April 29, 2013

Sailing through the tea colored waters of the ICW, it's hard to imagine ever being able to read the water depth by color. If you've never experienced being able to see individual coral heads in 80 feet of water, such stories seem like fairy tales. But it's true. In the Bahamas and much of the Caribbean, even along parts of the US East Coast where more ocean water than river water is present (especially in the Keys), it is more difficult to judge water depth simply because you can see so clearly so far down that your mind doesn't allow you to believe that the water is truly as deep as your chart says it is.

In these places, particularly if this is your first experience with water of any color other than shades of brown, there are rhymes that can help you remember what the colors mean:
Brown, brown, run aground.
White, white, you just might.
Green, green, in between.
Blue, blue, sail on through.


A shoal almost on the opposite shore

These cute little phrases are great if you have kids and they stick with you long after you no longer require them in order to navigate through shallow water. But they don't help you any if you're still in the "shades of brown" water.

Before we get too far into some tips for eyeball navigation in even chocolate milk-colored water, let's talk a bit about why it is so important. Some sailors mistakenly think, "I don't need to know how to read the water depth. I have a depth sounder." As true as that may be, what your depth sounder does not tell you, is how deep the water is way over there. And sometimes that is more important that the depth under your keel.


The light line in the distance is a shoal.

Coming into Albuquerque Cays late one afternoon, we could not find the deep water channel that our chart assured us had been there in the 1800's when this area was last surveyed. A lot can change in that amount of time and GPS accuracy was low, so running in using waypoints was not wise. Instead, we stood as high as we could (there was a kid on the bow rail) and looked for deep water. Since the depth goes from "enough water to float two Euriskos" to "shallow enough to hole your boat on a coral head" in a matter of feet, our depth sounder was useless. We followed the blue line of water, avoiding the dark shapes that were likely coral heads, and sniffed our way into the anchorage. We never would have been successful if we could not read water depth.


An obvious sandy spot

Closer to home, we were looking for a place to ride out a cold front in south Florida last winter. We were near Peck Lake and greatly enjoy that anchorage, especially now that we draw 13 inches and can sneak into the south cove all by ourselves. But that anchorage is too exposed for the predicted winds, so instead, we decided to see if we could get into North Jupiter Narrows, a shallow creek just north of Peck Lake. The chart showed a shoal at the mouth, but deep water (in a 13 inches of draft relative kind of way) to starboard of this shoal. The shoal was clearly marked with a stake, but what our eyes were telling us did not agree with the chart. The water to starboard of the stake was a much darker brown than to port. We could see the gradual change in shades of brown to port as the water got deeper. But to starboard the water was a stubborn monochromatic brown. This told us there was no change in depth from the shoal to the edge of the creek. Because our eyes and the chart didn't agree, we anchored Walküre at the mouth and hopped in the dinghy with the lead line. We sounded the entrance and verified that our eyes were correct. The previous channel of deep water to starboard had silted over and now the deepest entrance (still only 3 feet deep) was to port. We sounded our way into the creek in the dinghy until we were sure that our eyes could find water deep enough for us to float until we go in behind the lee of a small island. Without eyeball navigation skills, we would have run aground trying to follow the chart—which is really embarrassing when you only draw a little over a foot!


You can see how far the shoal sticks out from this island

As a general rule, in water that is only shades of brown instead of blue, white, green and brown, it's best to remember that shallow water looks lighter. What you are seeing is light reflecting off the bottom since shoals are usually caused by sand. Deeper brown water does not reflect sunlight, it absorbs it, leaving a dark deep-water trail for you to follow.

Don't wait until you can see the bottom to start practicing your eyeball navigation skills. Dave and I point out shoals as we see them so that the other one can practice, too, even if they are well out of the channel. Another good way to practice is to try to find shallow spots that are shown on your chart as you sail by them, even if they are nowhere near your rhumb line. If you see, for example, that there is a straight line of 3's on your chart (as in a spoils area along the ICW), look for it. Try to identify where it begins and where it ends. Practice visually trying to find the channel in areas where you know there is a great change in depth right outside the channel. By honing your eyeball navigation skills, you are increasing the chances that you can safely sail into an uncharted area. And those are the best anchorages.


Birds don't stand on water

Shallow water is easier to identify when the sun is behind you, preferable over your shoulder. It is also possible to spot shoals by using waves or wakes. Waves break when the water becomes too shallow to support them, so if you see a spot where waves are breaking before anywhere else, it is shallower there. We have used this technique to get into shallow anchorages along the ICW when the sun is not right. We wait for a powerboat to go by and watch where his wake breaks. That's where the shoal is.

One last tidbit, this one from Dave's days as a mate on a sportfish, long before we started cruising. The owner of the boat told Dave one day, "See those birds standing over there?"
"Yeah."
"Birds don't stand on water. It's a shoal."
Ah yes, birds don't stand on water. We have reminded each other of this many times when we see a bird "standing" on the water. Even if there is no shoal there, he's standing on something you don't want to sail over—usually a stake or old piling just under the surface of the water.

Whether your water is turquoise, green, blue or shades of brown, you can use eyeball navigation to avoid shoals. And watch out for those birds standing on water.

MONDAY we'll answer the question every wanna-be cruiser thinks but often doesn't ask. "What do you do about the holidays?"

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