The Nautical Mile – Defined!

If the Neuse River is 18,714 feet across between Windmill Point in Oriental and Winthrop Point at Adams Creek, how many miles wide is it?   Well, it’s both 3.1 “nautical” miles and 3.6 “statute” miles.   A nautical mile is 6,076.1 feet and a statute mile is 5,280 feet.  A nautical mile is roughly 15% greater in length than a statute mile.  For an approximate conversion, you can multiply statute miles by .87 to convert to nautical miles.  Multiply nautical miles by 1.15 to convert to statute miles.

Generally, statute miles are used on land and fresh water.  Nautical charts of saltwater bodies express distances in nautical miles, with some exceptions in inland coastal areas.  One such exception is the Intracoastal Waterway.  On ICW charts, there is a magenta colored line, which marks the ICW route.  The line is labeled with “St M” every five statute miles along its route.  In Gale Creek, south of Hobucken, you can see the designation “St M 160” and in the Neuse River the designation “St M 180” offshore of Oriental.  Not all charts show the ICW route.  Those that do are labeled with the term “Intracoastal Waterway” in the title block.

When calculating your speed, be sure to know whether it is based on nautical or statute miles.  Speed in nautical miles is expressed in “knots.”  A knot is one “nautical mile per hour.”  By the way, there is no such thing as “knots per hour”, since that would actually be “nautical miles per hour per hour.”  Speed in statute miles is expressed in “miles per hour.”  Based on the approximate conversion factors, 10 knots is equal to 11.5 miles per hour, and 10 miles per hour is equal to 8.7 knots.

The 15% difference can be significant when calculating things like your estimated time of arrival and fuel consumption, just to name a few.   15% may not seem like much until an erroneous calculation causes you to run out of fuel, miss the last bridge opening of the day, or watch the sun go down before you reach your destination.  As in all areas of good seamanship, any lack of accuracy can cause you some sort of trouble.

The length of a nautical mile has been disputed over the years.  Through the course of history, based on differences in the “assumed” size and shape of the earth, the nautical mile was measured in different lengths around the world.

The dispute over the length of the nautical mile is understandable.  There are a total of 360 degrees and 60 nautical miles per degree.  Each time the circumference of the earth was calculated differently, the length of the nautical mile was redefined.  The earth is approximately an oblate spheroid.  It is not a perfect sphere and has a greater circumference at the equator than it does at most other places.  Depending on what region of the world was being studied, the circumference measurement varied.

In 1954, the U.S. Departments of Commerce and Defense adopted 6,076.1 feet as the length of the nautical mile.  Most maritime nations have agreed and adopted the same length.  That leaves the rest of us with one less thing to worry about.

Until next time, we wish you clear skies, fair winds and calm seas!

 

World Wide Marine Training, LLC, is a U.S. Coast Guard Approved facility authorized to give examinations for captain’s licenses up to Master 200 Tons, Able Seaman up to Unlimited, STCW Basic Training, Radar, ARPA and other Endorsements. Please visit www.worldwidemarinetraining.com or call toll-free 866-249-2135.