Search Google

Volume 16, Issue 23 - June 5 - June 11, 2008

Voyages of Discovery by Lynn Teo Simarskiand Guy G. Guthridge

Sounding the Depths

Navigating safely takes a lot more than GPS

Nautical charts do not depict the near-Biblical truth some boaters have come to expect. Around the Chesapeake aboard our trawler Bright Pleiades, we’ve noticed many depths on charts dating from the 1970s — if not decades earlier.

Coming up the Bay one recent day, we spotted an intriguing marsh area on our chart called Winter Harbor. With six feet of depth shown at the entrance, it looked like the perfect spot to duck into for the night. Checking on-line, however, we discovered an archived Notice to Mariners that reports a two-foot depth and a channel no longer maintained. Since our boat draws three and a half feet, we were glad we checked.

Boaters may install the most modern Global Positioning System software and the latest electronic instruments, but safe navigation still comes down to the accuracy of charts. GPS indeed gives a boater a precise fix or position on the surface of the earth. But that location — shown as a safe depth on a chart that could date back to very old surveys — may now be an unnavigable shoal.

From Mark Twain to Multibeam Sonar

Almost half the soundings on charts for sale today by the National Oceanic and Atmospheric Administration date from lead-line drops before 1940. Until mid-century, navigators used the same technology Samuel Clemens did — which gave him his pen name, Mark Twain — to measure depth. They dropped a line weighted with lead over the side; the shout of mark twain! marked two fathoms: a safe, 12-foot depth for a steamboat. Tallow or animal fat stuck on the plummet brought up a sample of the bottom.

The old method gave a pincushion view of the bottom. Depths between the drops remained a mystery. Even today, NOAA notes that many areas on charts have never been surveyed with modern methods.

On board the ocean agency’s survey vessel Bay Hydrographer, we got a lesson on modern methods of sounding. The Solomons-based Hydrographer sees the bottom rapidly and seamlessly with technology developed in the last 15 years.

Running up the mouth of the Patuxent River off Drum Point, we passed 50 feet over an old shipwreck some 160 feet long. Its image took shape in colorful 3-D imagery on multiple computer screens, the rough but unmistakable sketch of a ship’s hulk shading in as we glided over. Oyster beds looked like mottled leopard-skin on the monitor. It takes training to interpret such blobs and shadows.

Off the side of our boat, multibeam sonar took many hundreds of soundings across a wide swath.

No Shallow Priorities

“Hundreds of years ago, it was the shallows, the nooks and crannies close to shore, that were important,” the Hydrographer’s captain, Michael Davidson, explained. “Big ships drew only 10 to 12 feet even up to the 1930s.”

Charting goals changed as maritime economics evolved. Today the deep channels and approaches to large harbors of the Chesapeake are the top priority of the surveys. Goods must transit safely through U.S. ports to support the nation’s competitive standing; NOAA sounds the bottoms to help that happen.

Recreational boaters’ needs are met “as available resources allow,” the agency says.

Translation: in the Chesapeake, “We typically don’t survey inside of 12 feet,” Captain Davidson said. “We seldom survey the small coves and rivers that flow into the Bay if there is not a significant economic force driving the survey.”

Shallow water also takes longer to chart; that’s another reason we’re more likely to go aground.

Mowing the Ocean

A century ago, several hundred ships busily charted U.S. waters. Now, only a handful are at work. With that data, NOAA has published about a thousand sophisticated nautical charts, covering 95,000 miles of coastline and 3.4 million square nautical miles of the U.S.’s Exclusive Economic Zone.

“It would take 200 years to survey all of the backlog with the staffing we have right now,” Davidson said.

That’s an estimate. There’s no anticipating how long a survey requires, since each stretch of river and bay is unique.

“The more complex the area, like surveying around islands and shoals, the longer it takes,” said Davidson.

Recently, the Hydrographer and a companion boat surveyed about eight square nautical miles of the Potomac River’s deeper channels. The last survey dated to the 1970s, and the ocean agency wanted to be sure its ship, the Thomas Jefferson, could get up river to an open house in Alexandria. Even though the abbreviated, three-week survey identified new “dangers to navigation” on the Potomac, a survey of the entire river from bank to bank

wasn’t made; it might have taken four or five months.

As the Bay Hydrographer plotted a picture of another wreck below, Davidson allowed that a day of surveying — repeating exacting tracks across the waters — can be akin to “mowing the ocean.” But new bottom hazards or a lot of traffic, wind and current break the monotony.

“No two days of hydro are the same,” he said.

Boaters Beware

For boaters eager to follow the first rule of navigation — Don’t hit anything — Davidson has advice: Consult a chart’s “source diagram,” which gives the origins and dates of the data it reports. Read all chart notes; subscribe to the Coast Guard’s Notice to Mariners; get an electronic charting system; and download the latest updates.

On Bright Pleiades, we haven’t gotten out the lead line. But we won’t ever again look at charts with the same confidence.

© COPYRIGHT 2008 by New Bay Enterprises, Inc. All rights reserved.