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Learn to Read the Water - Sail Magazine

Learn to Read the Water

Reading the water is an important skill you should practice often to ensure there’s always water under your keel
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Reading the water is an important skill you should practice often to ensure there’s always water under your keel

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Sailing through the tea-colored water of the ICW, it’s hard to imagine ever being able to read water depth visually. If you’ve never actually experienced being able to spot coral heads on the bottom, 80 feet down, such stories seem like fairy tales.

But it’s true. In fact, in the Bahamas and much of the Caribbean, even along parts of the U.S. East Coast (especially in the Florida Keys), it can sometimes be difficult to accurately judge the water depth simply because you can see so far down—to a point where it’s hard to believe the water is truly as deep as your chart and depthsounder say it is. 

In places like these, particularly if this is your first experience sailing in water any color other than brown, there is a simple rhyme 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 

The line between the shallows and navigable water couldn’t be clearer than on this sunny day in the Caribbean

The line between the shallows and navigable water couldn’t be clearer than on this sunny day in the Caribbean

In clear water, when you see a brown patch it usually means there is something close to the surface, like a rock or coral head, and that you very well may run aground on them.

White is usually sunlight reflecting off a sandy bottom. Depending on the clarity of the water and the intensity of the reflection, the sand may be deep enough for you to sail over it. But then again, it may not. 

Green water, on the other hand, can mean a number of different things. Sometimes it means there is deep water but also grass on the bottom, and the color is being reflected back to you. Other times green can be an indication of shallows. Your best bet, in this case, is to proceed with caution because the water is “in between.” 

Water that is a beautiful shade of blue is usually deep enough to sail over: as a general rule, the deeper the shade of blue, the deeper the water. As the water gets shallower, the color of the bottom will be reflected through it, washing out the blue as you begin seeing the white sand, green growth or brown coral.

Although the light is far from optimal it’s still possible to see the shoal stretching being this point

Although the light is far from optimal it’s still possible to see the shoal stretching being this point

Some sailors mistakenly think, “I don’t need to know how to read the water depth. I have a depthsounder.” But as true as that may be, your depthsounder cannot tell you how deep the water is way over there. And sometimes that is more important than the depth under your keel. After all, if you’re floating you know there’s enough water where you are. The real question is how do you find a channel into an uncharted anchorage without hitting bottom?

Coming into the Albuquerque Cays late one afternoon a few years ago, for example, we could not find the deep-water channel that our chart assured us was there back in the 19th century when the area was last surveyed. A lot can change in that much time and GPS accuracy was low, so running in on waypoints was not wise. Instead, one of us stood on the bow pulpit and looked for deep water. Since depths here ran from “deep enough to float easily” to “shallow enough to hole your boat on a coral head” in a matter of feet, our depthsounder was useless. We followed a blue line of deep water, avoiding black shapes that were likely coral heads, and sniffed our way into the anchorage. We would never have made it otherwise. 

More recently, we were looking for a place to ride out a cold front in south Florida. We were near Peck Lake, an anchorage we really enjoy (especially now that we draw just 13 inches in Walkure, our new sharpie and can sneak into the south cove all by ourselves) but Peck Lake was too exposed to the forecast wind, so we decided to see if we could get into North Jupiter Narrows, a shallow creek just to the north. Although the chart showed a shoal at the mouth of the creek, there also appeared to be some deep water (relatively speaking) to the right of it, so we decided to give it a try.

When we got there, we found the shoal was clearly marked with a stake, but that little else bore any resemblance to what we’d expected. Specifically, the water right of the stake was a much darker brown than the water to the left. Not only that, the brown water to the left appeared to gradually change color as it got deeper, while the water to the right was monochromatic. This told us there was no change in depth from the shoal to the edge of the creek on that side.

An otherwise open stretch of water can speak volumes if you know what to look for

An otherwise open stretch of water can speak volumes if you know what to look for

Because our eyes and the chart didn’t agree, we anchored Walkure short and hopped into the dinghy with the lead line. Sure enough, when we sounded the entrance we soon found that the old channel on the right had silted over and now the deeper water (still only 3 feet deep) was to the left. 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!

As a general rule, when navigating in water that is only shades of brown, assume that shallow water looks lighter, and that what you are seeing is light reflecting off the bottom, as shoals are usually sand. Deeper brown water does not reflect sunlight, it absorbs it, leaving a dark brown trail for you to follow. 

Note that you need not wait until you find yourself having to navigate a tricky channel to practice your eyeball navigation skills. In fact, my husband, Dave, and I often point out shoals to each other to help keep ourselves sharp, even if they are well out of the channel. You can also try to spot shallow spots that are shown on your chart as you sail by them, even if they are nowhere near your rhumb line. If, for example, you see there is a straight line of 3-foot soundings on your chart (as in a spoils area along the ICW) try looking for it and identifying where it begins and where it ends. Similarly, you can practice spotting the channel in areas where you know there is a great change in depth right outside the channel. By honing these skills, you will ensure you have the skills to safely sail in those uncharted areas that often have the best anchorages. 

As you are practicing your eyeball navigation, you’ll soon notice that shallow water is typically much easier to identify when the sun is behind you, preferably over your shoulder. 

You may also notice that it’s sometimes possible to spot shoals by watching the shape of the water. Waves get steeper and break when the water becomes too shallow to support them, so if you see a spot where the water is heaping up and sometimes breaking, you know it is shallower there. We sometimes use 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 then watch for where its wake breaks. That’s where the shoal is. 

See those birds standing over there? Birds can’t stand on water

See those birds standing over there? Birds can’t stand on water

Then there are the birds. Long before we started cruising, Dave was a mate on a sportfish boat. The owner one day asked him, “See those birds standing over there?”

Then there are the birds. Long before we started cruising, Dave was a mate on a sportfish boat. The owner one day asked him, “See those birds standing over there?”

“Yeah.”

“Birds can’t stand on water. It’s a shoal.”

Since then we have always kept an eye out for birds “standing” where they shouldn’t. Even if there is no shoal, the birds are standing on something—a stake or maybe an old piling just below the surface of the water—and you don’t want to be there.

Connie McBride and her husband, Dave, currently cruise aboard Walkure, a wood-epoxy Bolger AS-29 sharpie built in 2005.
Follow Connie’s blog at simplysailingonline.com

ConnieMcBride

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