How Deep is a Fathom in The Ocean? How to Convert Fathoms



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Reading nautical charts can raise many questions that directly affect the correctness of the decision.

One of the main tasks of a navigator, whether a duty officer on a merchant or navy ship or a sailor on a recreational or sailing boat, is to plan a safe voyage and ensure safe navigation. Both tasks require proper map reading and in particular – depth.

How Deep is a Fathom in the Ocean?

A fathom is a unit of length that is commonly used in maritime contexts to measure the depth of water. It is equivalent to 6 feet or approximately 1.8288 meters. This unit of measurement has been used for centuries and continues to be a standard in nautical measurements today.

Depth on nautical charts where the imperial system is used is indicated in fathoms. Earlier in English-speaking countries, fathom was used as a measure of distance and depth.

For example when measuring wells and mines, but over time, this value has become used only in maritime affairs to measure and indicate depth.

Nautical paper charts with depth examples reading which is essential to know deep is a fathom in the ocean

How Deep Is A Fathom In Feet And Meters?

When it comes to the depth measurement on nautical charts, a fathom equals 6 feet or 1.8288 meters.

Here are examples of the most common fathom lines in use:

  • 1 fathom = 6 feet = 1,8288 meters
  • 10 fathom = 60 feet = 18,288 meters
  • 20 fathom = 120 feet = 36,576 meters
  • 30 fathom = 180 feet = 54,864 meters
  • 50 fathom = 300 feet = 91,440 meters
  • 100 fathom = 600 feet = 182,88 meters

On a nautical chart, the line that connects the depths of 100 fathoms is called the 100-fathom line. Depending on the scale of the map, such lines may connect at depths every 5, 10, or 100 fathoms, but it is this depth that is most often mentioned.

What makes it special? Firstly, in the days of navigation, when sailors had to determine the depth themselves, it was this that was the reasonable maximum depth that could be determined.

To get 100 fathoms or 600 feet leadline out of the water, the sailor simply ran out of steam and got exhausted. In navigation, the sailors said that this was the end of the road because it was simply not possible to determine the depth in the open sea.

However, in modern times it has practical meaning and is a popular term for fish, both a commercial and recreational hobby, and also has an important legal meaning. Sea water column up to the depth of 100-110 fathom is where the daylight can penetrate, and photosynthesis is possible and is the richest and most diverse underwater life and is a highly desirable place for fishing.

Certain types of fish species live at certain depths along cliffs, drop-offs, or sharp depth changes, which increases the chances of catching the specific species you are looking for.

Also, these species can be food for deeper predatory fish species and therefore be used as baitfish. For example, the most popular you can fish on 30-fathom lines, but offshore fishing at a depth of 75 to 100 fathoms will increase the overall chances of catching such trophies like tuna, wahoo, marlin, cowcod, giant rockfish, or even swordfish.

Here comes a 100-fathom line as a legal term in the Code of Federal Regulation (Title 50) and in § 660.73. The 100-fathom depth contour is used between the U.S. border with Canada and the U.S. border with Mexico. The paragraph lists coordinates to plot the borderline, but for offshore fishing 100-fathom line also prohibits taking off all species’ seawards.

There is a list of popular areas for offshore fishing for big fish that require both equipment and good legal advice in US waters:

  • Gulf of Mexico
  • Fishing Galveston
  • Panama City Beach
  • Orange Beach or Gulf Shores
  • Tampa Bay
  • Chatham
  • Charleston
  • Hawaii
  • Cabo San Lucas

Most of these places will require traveling 20-50 miles off the coast to reach the 100-fathom line.

What is a Fathom in the Imperial Measurement Unit?

The word fathom originates from the Danish and the Viking’s word “favn” which means embracing arms or a pair of outstretched arms which is about 6 feet in a typically grown adult male. Check the length of your arms!

A fathom is an imperial unit of length used as a nautical unit to measure the depth of water or the length of a nautical rope or cable.

Now a fathom is standardized to six feet, or 1.8288 meters, but earlier it was used by The British Admiralty as 1⁄1000 of a nautical mile, which corresponded to 6,08 feet unless an international nautical mile was adopted.

Why Is A Fathom Used For Measuring The Depth Of Water?

As with many other maritime traditions, the use of a fathom as a maritime depth unit has a practical origin. A leadline was used to measure the water depth around the vessel.

The line was dropped overboard to reach the sea bottom and hauled up and coiled for the next measurement. The two-arm coils were used to count the number of coils and therefore the water depth that became known as a “fathom”.

Mariners carry leadline onboard ships to measure the depth that still is found onboard ready for use. Leadline is counted as one of the first and most valuable inventions in navigation.

The leadline is a very simple tool that consists of a lead weight attached to a rope that is marked at measured lengths and used to measure the depth of the water and determine the characteristics of the seafloor.

To find out the type of seafloor, the sailors had to “arm the lead” or apply something sticky like tallow made from animal fat or grease to the cupped inward bottom of the lead. By healing up the lead and bringing the sample from the sea bottom, sailors could find out how the sea bottom was covered.

Once reaches the bottom, the measurement of the depth could be taken as well. For easier and faster depth reading the leadline is marked at intervals with leather, a piece of white or red rags cloth, and a line with knots.

Here is a traditional marking used on leadline:

  • 2 fathoms – 2 strips of leather.
  • 3 fathoms – 3 strips of leather.
  • 5 fathoms – a piece of white cloth.
  • 7 fathoms – a piece of red cloth.
  • 10 fathoms – a piece of leather with a hole in it.
  • 13 fathoms – a piece of blue cloth.
  • 15 fathoms – same as for 5 fathoms.
  • 17 fathoms – same as for 7 fathoms.
  • 20 fathoms – small line with 2 knots.
  • 25 fathoms – small line with 1 knot.
  • 30 fathoms – small line with 3 knots.
  • 35 fathoms – small line with 1 knot.
  • 40 fathoms – small line with 4 knots.

To make a nautical chart a huge number of measurements had to be done and recorded and called as was sounding.

Nowadays, merchant ships and even leisure boats are equipped with echo sounders that can continuously read the depth and display or even record the profile of the sea bottom.

But even having a modern and accurate echo sounder, the leadline remains a widely used tool on board merchant ships.

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