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.

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.

We read together the Longitude Prize, which is a tale written for ages 10 and up 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.