NAUTICAL MEASUREMENTS:

ONE SHOT AT A TIME

By Mike Descheemaeker of SV WHIRLWIND

The nautical mile, not to be confused with the statute mile, is the basis for ocean navigation … but why?

I thought it would be fun to explore the different nautical words and the variations of actual lengths and depths of different forms of sea people measurements. The ‘knot’, the ‘fathom’, the ‘cable’, the ‘shot’, the ‘shackle’ are all different ways to look at length, distance, and speed. In our modern way of navigating our vessels many of these expressions have really become archaic and less important to the modern navigator.  Nonetheless, a general idea of there usages and their origins is quite interesting.

 

THE KNOT: UNRAVELLED

Obviously, we all are familiar with the knot as a measurement of speed.  One nautical mile per hour is equivalent to one knot. The nautical mile deals directly with measuring a curved surface. From my trusty Celestial Navigation: A Step By Step Procedure For The Complete Idiot I will borrow this excellent description of measurement:

“We measure straight lines by inches, feet and statute miles. 12 inches equals one foot, 5,280 feet equals one statute mile. We measure angles and arcs by seconds, minutes and degrees, 60 seconds equals one minute, 60 minutes equals one degree and 360 degrees equals one complete circle.”

So because the earth is a big round sphere it is measured using minutes of arc.  If we take the 360 degrees the sun appears to travel around the earth each day and divide by 24, the sun appears to travels 15 degree of arc every hour or 900 miles per hour.  This rapid speed rotation of the earth makes the sun appear to move.  900 (miles/hour) times 24 (hours/day) is 21,600 miles/day…the entire circumference of the earth sees the sun each day.  To check the numbers another way, one can multiply 360 degrees times 60 minutes of arc to get 21,600 minutes.  One minute of arc is equivalent to one nautical mile based on the circumference of our planet.

The term knot to designate speed derived from the common log used on board sailing ships beginning in the 17th century. A spool of knotted line was lowered over the stern with a wooden triangle attached to the end upon release it was timed with an hour glass the number of knots that came out in the time on the glass was how many knots or nautical miles per hour the the ship was traveling thru the water.

 

THE FATHOM: UNVEILED

As mariners gathered information underway like their speed in combination with a dead reckoning and a noon site hopefully every day, the ancient mariner could keep a pretty good idea of their place on earth. Knowing one’s location on the earth is of great importance and so is knowing how much water is under the keel. This notion of depth was traditionally expressed in fathoms.  This word came from various ancient languages in what is now Europe and the Mediterranean.  A fathom meant outstretched arms or about six feet. This measurement could vary from five to seven feet depending on nationality and merchant or naval service. The term deep six comes from burials at sea.  Naval Service required that sailors be buried in waters over six fathoms.  ‘By the deep six’ would signify almost seven fathoms.  Eventually, the British formally declared a fathom as ” a thousandth of a nautical mile”.  Back in the 17/1800’s, when ships were made of wood and men were made of iron, finding the bottom involved having someone in ‘the chains’ which was a platform where the shrouds terminated at the hull.  A person could stand on the platform and swing a lead line to find soundings.  With a breast rope over his chest to hold him on board, he would swing and cast.  This was exhausting and wet work.  One had to swing, cast, and retrieve a heavy lead bell shaped 7-14lb weight repeatedly to continuously monitor the bottom.  The weight would need to increase depending on depth and speed of the vessel.  Fathoms were used to simplify the process of assessing depth.   The leaded lines were as long as 20 fathoms marked at 2,3,5,7,10,15,20 fathoms.  This development gave a quick and fairly accurate way to sound the bottom.

Taking the depth a step further mariners developed a way to asses the bottom composition as well.  A shallow cup at the bottom of the lead could hold a scoop of tallow.   This way when the lead hit the bottom the makeup of the bottom could be determined by looking at what stuck in the scoop upon retrieval.  This information was was useful anchoring as well as determining location such as distance from shore or a river mouth.  Sometimes multiple men would be swinging leads to keep up with boat speed.  Remember, the British Navy didn’t take kindly to their Captains sticking ships on reefs. Losing a King’s ship due to negligence was a sure way to find your way high and dry begging on the docks if you weren’t introduced to the hangmen’s noose first.  While the use of a lead line to asses depth is an old technique made obsolete by modern depth sounders, we keep and use a lead line on SV WHIRLWIND to measure depth in anchorages that we recon in our dinghy.

THE CABLE, SHOT AND SHACKLE: UNHOOKED

Anchors in these days of old were attached to the ship with cables which were large hemp ropes laid up left handed with three right hand laid hemp ropes. A cable in England was 85-100 fathoms long, the length of the rope walk where the ropes were made being the determining factor. A cable length was 1/10 of a nautical mile and used to express measurements of distance under one mile.  People may have said, “We are 2 cable lengths from the anchorage or the reef lays 3 cable lengths off the starboard beam.”

As hemp gave way to chain to make up anchor cables the designation of shots and shackles came into use.  A shot of chain was used by the US Navy as 15 fathoms or 90 feet and a shackle of chain was used in the British Navy was 12 fathoms.  Shots or shackles were connected together to create chain cables of up to 100 fathoms or larger.  Because of its weight, chain requires less scope than a rope rhode and certainly a lot less worry around sharp rocks and coral.  As chain came into favor, however, Captains found it prudent to still carry hemp cables for certain circumstances like sending out a kedge anchor or taking anchorage near a drop off.  Near a subsurface dropoff the possibility of the anchor sliding off the ledge before or during retrieval made bringing up the anchor much harder if it was all chain and hanging anchor so they may use hemp cable instead.  In general. fighting ships could retrieve an anchor much faster with a hemp cable than chain because anchors were weighed by man power.  Capstan hemp was much lighter therefore much faster.

While many of these terms and technologies have been left behind and traded for modern convenience and a rapid acquisition of data, it is still a thrill to remember the past mariners, their ways, and their speech that got us where we are today.  As we take a glimpse into the past examining terms and technologies when speed was measured by knotted line and bottoms were found with tallowed lead may we hold fast like an anchor to the spirit of adventure and exploration.