Tuesday, January 19, 2010

How Deep Is It Here?

Certainly one of the questions most commonly asked by passengers on the vessels I work on is "How deep is it here?" For most of them, it is simply a matter of curiosity, but for the mariner knowing how much water is "under the keel" is essential for safe navigation.

Charted depths. A nautical chart is a good place to start when you want to know how deep it is. The section of chart above provides a lot of information, for example. The depths, in feet, are the little numbers printed throughout the chart. The central channel depth (40 ft) is probably maintained artificially, by dredging. Different charts use different measures, depending on the conventions of the country that produced it and the most convenient unit for a given area. US charts, like the one above, may use feet for charts of harbors, rivers, and other relatively shallow areas, but fathoms (six feet) in deeper areas. Many other countries will use meters on their charts. Most depths are measured from a datum, usually the average depth over the course of hundreds of low tides.

Depth finders. Most vessels will be equipped with a depth finder, an instrument that uses sound waves to detect the distance to the bottom. Depth finders can range from hand held devices the size of a flashlight to sophisticated systems costing millions of dollars. Some models double as fish finders on commercial and recreational fishing boats.

Lead lines. Lead lines are long ropes with the depths marked out in given intervals along their lengths. Many lead lines have a weight on the end with some kind of sticky material on it to bring up part of the the bottom to determine its composition. Lead lines also have a terminology of their own, the most famous term being "mark twain," a way of saying "two fathoms." Writer and river pilot Samuel Clemens took the term as his pen name. Although largely made obsolete by sonar technology, American merchant seamen may still be tested on lead lines before they receive their documentation.

Soundings. Traditionally this was a term for any water shallow enough to be measured by a lead line. In Twain's day, this was 24 feet, but modern electronics have expanded that range to 1500 feet or more. Getting a depth reading gets harder as the water gets deeper, as the water attenuates the sound signal. A vessel traveling at speed may also outrun the return signal, and thus get no reading on the depth finder.

Deepest and shallowest water. The Challenger Deep, in the Pacific Ocean's Mariana Trench, is more than 35,800 feet deep. If Mt. Everest were placed at the bottom of the deep, it's peak would still be a mile under water. Tour operators on Alaska's Chilkat River claim it's "North America's shallowest navigable river," at places shallow enough to both stand in and operate a twin-engine boat carrying 20 or more passengers.

20,000 Leagues Under The Sea. A league is a term measuring distance, not depth. Although its exact length has varied at different times and places, it usually refers to a distance of about three miles. Jules Verne was not referring to the depth of the ocean in the title of his 1869 novel, but to the distance Capt. Nemo's submarine the Nautilus had cruised throughout the world's oceans.

"Full Fathom Five Thy Father Lies." Also known as "Ariel's Song," this poem comes from William Shakespeare's play The Tempest:
Full fathom five thy Father lies,
Of his bones are Corrall made:
Those are pearles that were his eies,
Nothing of him that doth fade,
But doth suffer a Sea-change
Into something rich & strange
Sea-Nymphs hourly ring his knell.
Harke now I heare them, ding-dong, bell.

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