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"Let us strive to make each moment beautiful."
Saint Francis DeSales
painting by Katie Bergenholtz

Explorers and Pirates (1420-1779) Part V: Navigation: Longitude

Navigators used a tool called a log line to determine the ship's speed. Log lines were made up of a wooden roller, a long length of rope and a wooden triangle which was attached to the end of a line. There were knots tied into the rope every fifty feet, eight inches. The triangle, or "log" would be thrown behind the boat. As the ship moved forward, the line came off the roller. Crew members kept track of how many knots went overboard in 30 seconds. The number of knots counted represented the speed of the ship in knots or nautical miles per hour. By knowing the ship's average speed over the course of the day, the captain could determine how far the ship had traveled. Log books were so named because they recorded the speed by the measurements of the log line. 
We made a model with the scale much smaller. I cut two pieces of cardboard in a triangle shape with a slightly rounded bottom edge and glued them together. I punched a whole in the top and tied a string to it. I then made a knot every foot (instead of every fifty feet).  I measured out fifty feet so that they could see how far that was in comparison.

They took turns being the ocean, so that the log line would stay in place, and the navigator, who unwound the string and counted the knots. Sam was the timer. We only used 10 seconds rather than 30 because our rope was in a smaller scale.

The navigator backed up as he unwound the string.

They averaged about 4 knots.
They had fun playing with the log line some time after the lesson was over.


Older students (Ages 10-up) might like to read the Longitude Prize, tale of the scientific contest for the Longitude Prize, which was offered through a 1714 act of the British Parliament in response to the devastating loss to the British navy of four battleships and hundreds of sailors.

1 comment:

  1. Great explanation of this, my dad tried to show me this when I was just a kid. This post reminded me of that time.

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