Marine Science Story

Why is the speed of ships are measured in knots

A knot is one nautical mile per hour and is the same as 1.852 km/hr (1.15 statute miles/hr), and a nautical mile is about a minute of arc (one-sixtieth of a degree) of the latitude or longitude of the Earth.

Consider the Earth as a perfect sphere and cut the circular cross-section into 360 equal sectors, gives a degree. Again dividing the degree into 60 equal sectors gives a minute of arc. So approximately, sixty nautical miles is a degree of circumference around the earth.

Then why named knot? Out there in the big vastness of the ocean, to measure how fast the ship is going was a difficult task in an earlier time. Thus around the 16th century, the navigators were using a technique called chip log or log-line to measure the speed of ships. A chip or a piece of wood (or any floating object) where one side weighs more than the other to keep the chip in a vertical position to increase the friction, later standardized into a quarter circle shape attached to one end of the rope where knots were tied at uniform intervals.

Model of chip log (Wikipedia/RĂ©mi Kaupp)

Sailors used to drop the log over the ship’s stern and let the log-line run out for a fixed time while counting the knots that passed over. The number of knots was treated as the speed of the vessel.