Why is speed at sea measured in knots?

"When you're at the end of your rope, tie a knot in it and hold on" - Thomas Jefferson

It's a nice metaphor, but, before GPS came along, sailors had to do a lot more than just hold on. With no landmarks to judge how far or fast their ships were travelling, calculations involved knotted rope and mental maths. 

In the 15th century the nautical mile was introduced (equating to 1.852km). Shortly after this, sailors worked out that they could operate the first maritime speedometer using just a wedge of wood, a glass timer and a really long rope. Knots were tied at intervals of 14.4m and, with one end tied to a take up reel and the other to a wedge of wood, the rope was thrown overboard. As one sailor kept an eye on the sand timer, the other let the rope pass through his hands, counting the knots as the rope disappeared. 

With the time and number of knots logged, sailors were able to measure speed using the equation 14.4m/30s = 1 nautical mile/hour (or one knot). 

This practical thinking, as well as the processes and the knots themselves, inspired our Marine collection of cufflinks and lapel pins.