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A Deep Subject

You may be out of your depth if you only rely on a chart to keep you out of the shallows
Spring tides, wind shifts, storms, shifting sands and outdated charts or plotter chips are just some of the factors that can lower the amount of water beneath your keel and put you aground.

Spring tides, wind shifts, storms, shifting sands and outdated charts or plotter chips are just some of the factors that can lower the amount of water beneath your keel and put you aground.

Askids, we’d ask my father, “How deep is it here?” Looking us straight in the eye, he always gave the same sardonic reply. “All the way to the bottom!” It’s still true today—the topic of water depth seems so simple on the surface, but there’s a lot more to it.

Knowing the depth and making decisions about avoiding submerged hazards is definitely worth a closer look. Charted depth is rarely the same as actual depth, or exactly what your depth sounder reads. The stage of the tide, the wind, the cycles of the moon and the earth all affect the height of the tides and the real depth of the water. Complimenting the information from our charts and plotters will make us better seaman. Using our own senses will make us better yet.

Depth soundings, representing the distance from the surface to the bottom, are depicted clearly on our charts, and uniform depths are connected with contour lines. This gives us a general overall picture of the bottom. These depths are measured by soundings that are acquired by hydrographic surveys and satellite imagery, normally representing the depth at mean lower low water (MLLW). Without getting too wonky, for most of us recreational boaters, this means when the tide predictions show a negative low tide that day, actual depths will be less than the chart indicates. At high tide, the water depths will be greater than soundings shown.

Many other factors can alter the depth of the water depicted. After all, actual soundings were measured years before they were recorded on charts and plotters. Storms move sand bars around, particularly around inlets and ocean-inland interfaces. New wrecks and obstructions occur, which may be marked only with temporary warning buoys until the data travels through our reporting system and onto our charts and plotters. Be alert to the real world and keep in mind that charted information may not depict absolute reality. Keep charts and plotter chips updated. Be aware of the chart datum and plotter settings. It sounds silly to note, but be sure not to confuse feet with fathoms. It happens. Verify charted soundings against the depth sounder. In other words, be prudent.

Variations of the tides directly modify chart information, mainly associated with the phases and ecliptic of the moon. Spring tides, producing higher-high and lower-low tides, occur at new and full moon. This is called syzygy–a brilliant word, right? Neap tides occur during the first and third quarters of the moon, usually producing about 20 percent less than the mean tidal range. During the vernal (March 21) and autumnal (September 21) equinoxes, when the apparent sun crosses the celestial equator, tides are affected further.

As an example, the Hurricane of 1938 swept across Connecticut, Rhode Island, and New York’s eastern Long Island with a 15-foot storm surge. Why? In addition to the wall of seawater normally associated with a storm track, it made landfall at high tide, on the full moon, during the autumnal equinox–a triple whammy. In 2012 Hurricane Sandy also occurred during a supermoon, a full moon at its closest point to the earth.

Even normal storms move sandbars and forge new coastlines, putting the edges of channels and spits of land not necessarily where your chart says they will be. Shoaling is common at ocean inlets and on river bends. Fast growing shore vegetation such as encroaching mangroves can rapidly alter the shoreline, trapping built-up sand and sediment. While cruising in little-explored foreign areas, I’ve been startled to discover whole islands that are not on the nautical charts.

We have to trust our senses too. Sailors have always taken
clues from nature itself to assess water depth. In clear water, like in the Bahamas, “reading the bottom” means just that, determining the depth based on the distinctive colors of the grass, sand and coral. A good navigator can choose the deepest path to transit through a minefield of shallow patches. Elsewhere, even if we can’t see the bottom, there are subtle clues. The expression “still water runs deep,” is a truism. A disturbed or turbulent appearance on the surface often reveals the edge of a rising bottom. Rips occur where the current collides with the edge of a shelf or sandbar.

Sometimes, the tips of seaweed appear as a first warning. For a decade, my husband and I cruised an engineless 28-foot Rozinante ketch. She was beautifully well behaved and serene, lacking alarms, instruments and gauges. We loved the teamwork, tacking in and out of inlets and through narrow passages. The absence of a depth sounder taught us to rely on our senses and read the signs. I remember tacking up Cuttyhunk Inlet, my husband laying on the foredeck watching for the edge of the eelgrass beds. “Tack now!”

On our powerboat, too, we learned to feel the boat’s own shallow water warning, even in murky waters. First, an odd quavering, then the boat slowed and the wake changed slightly. In shallow water, if you proceed slowly, you can easily feel this kind of telltale squat. It occurs because the water compresses between the bottom and the seabed, sucking the whole hull down.

I was once travelling on a small freight boat, returning to the mainland from an island in the mouth of the Amazon. Even though we passengers had to sit on sacks of cargo, I felt the unmistakable feel of shallow water. I said to no one in particular, “We’re smelling the bottom.” Not having a clue what I meant, everyone looked at me like I was crazy, but within a minute or so we came to a dull stop. The captain turned off the engine and the crew rigged their hammocks for a tidal siesta. It wasn’t long before the water ebbed out from under us and we were stranded high and dry on a sandy table. All around us, the former sea transformed into an expanse of undulating water-filled depressions and sandy knolls.

When cruising, we all know that occasionally there’s an unfortunate anchorage, sometimes because there’s no better option. I’d be lying if I said I’ve never woken in the middle of the night sitting on the bottom at low tide. Even after applying a tidal correction to the chart before choosing an anchorage spot or mooring, sometimes-unforeseen local currents act mysteriously. Just swinging into a new position can be enough to meet an isolated shoal. Choose your anchorage wisely.

I met a savvy sailor recently, who just moved to my area and set his own mooring. He said his gear was stout, and it went over the side without trouble, but he forgot one thing–local knowledge about wind-driven tidal effects. It didn’t take long for him to realize his mistake. He was onboard during a hard northwest wind, which blew a great deal of water out of the cove. On the third day, his sailboat started pounding on the bottom an hour before predicted low tide. If you’ve never been on a boat when the keel was having an argument with the bottom in a steep chop, there’s no way to comprehend the violence. Pounding is indeed the right word. He moved his mooring.

On many boats, the depth sounder indicates a slightly different reading than the charted depths. This makes perfect sense, because charted depth is measured from the surface to the bottom. How far beneath the waterline is your transponder? Particularly on a deep draft sailboat, applying this correction to your depth sounder is essential. Properly calibrating the sounder can eliminate the error.

Amusingly, sometimes we must think about what’s above as well as what’s below us. When transiting waters with bridges or overhead power lines, air draft is a constant reality. If wind, tide or heavy rainfall raises the water level, the distance between the water’s surface to the bottom of the structure could be substantially less. Don’t assume. Check individual vertical clearances on the chart. Real-time clearance on fixed bridges is usually indicated on water level gauges, easily seen with binoculars.

Even with the best-updated charts and equipment, the more we cruise, the more likely we are to leave the well-marked waterways in pursuit of new experiences. Remember, conditions change over time, even from one storm to another. Tides and wind uniquely affect different bodies of water. Privately maintained channels are not always maintained, and hazards might not be predicted. So be prudent, use your senses and be mindful of the ever-changing environment under us.

This article originally appeared in the September 2020 issue.



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