Depth Measurements - Fathoms - Soundings
History of Depth Measurement A sounding line or lead line was a thin rope of a certain length, with a lead plummet on its end. They were swung or cast by the “leadsman,” usually from part of the ship called “the chains” which were small platforms, built on either side of the hull of a ship. Sailors tied marks made of leather, calico, serge or some other materials and placed at certain intervals. They then made them shaped for ease of reading day or night. Marks were placed at every second or third fathom, for example: at 2, 3, 5, 7, 10, 13, 15, 17, and 20 fathoms. After dropping the lead, the leadsman called out the depths. If a particular depth was exactly at a mark, he would say: “by the mark,” and then say the number. If the depth was somewhere between two numbers, he would say: “by the deep” and then say an estimated number of fathoms.
Traditionally, there were specific terms used for some of the depth measurements, such as “deep six” (a sounding of six fathoms), or “by the mark twain!” (the depth of two fathoms). The famous author Mark Twain was a river pilot on the Mississippi. This sounding term is probably where he adopted his pen name from.
History for measuring depth or sounding Seabed mapping has progressed over time. This video below explains how mapping agencies like the Marine Institute survey vast swathes of the ocean floor. This movie animates this progression and how marine charts have evolved.
Sonic Depth Finders
Depth finders are used today to determine depths of water. They measure the time it takes a sonic pulse produced just below the water surface to return, or echo up from the bottom of the body of water. Also known as sonic depth finders, they are in operation on a range of vessels including ships, both naval and merchant, and on small, privately-owned craft.
Sonic pulses detect underwater objects like fish by the same principle. During World War II sonar was used to detect submarines. In addition to protecting ships from shoal water, sonic pulses are used to determine the thickness of ice in Arctic regions and oceanographic charting.
These sonic depth finders can be operated repetitively, recording thousands of soundings per hour. They can record a profile of the ocean floor.
One of the first practical depth sounders was the so-called Hayes sonic depth finder. It was developed by the U.S. Navy in 1919. It also had a timer calibrated at the speed of sound in seawater. About 1927 a similar device was manufactured under the trade name Fathometer and these devices have not been significantly changed since.
This work was funded by Ireland's Marine Institute for SeaFest 2016
Footage: RealSim Movies
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