Why does boats measure speed in knots and not mph/kmh?

Question

Why does boats measure speed in knots and not mph/kmh?

In: Physics

Answer 1

This story has two parts: how we measure distance, and how we measure speed.

Measuring long distances on the ocean used to be very hard. There were no fixed objects to get your bearings from and with the currents, who knew if where you were now was the same place an hour ago? But they did have two things they could measure reliably: latitude and direction. Latitude (how far up or down you are from the equator) can be measured by looking at how high the sun gets in the sky at noon. You can find your direction (north/south) using a compass.

So sailors started defining distances in terms of these two things that everyone could use. 1 nautical mile was the distance covered if you travelled perfectly north or south until you had moved by 1 arcminute of latitude. In this case 1 arcminute doesn’t have anything to do with time, it’s an old-fashioned way of dividing things. If there are 90 degrees from the equator to the north pole, 1 degree is a really long way. So you have 1 arcminute, which is a 60th of a degree, and 1 arcsecond, which is a 60th of a minute. There are 324,000 arcseconds between the equator and the north pole.

So that’s the distance part of the story: 1 nautical mile is something that everyone can calculate based on some fundamental things like how high the sun gets in the sky and which way the compass points.

Then there’s speed. Well, if you’re on an ocean, you can’t just stick a peg in the ground and run a string from it, then see how long the string is. Instead, you get a rope with a big flat board on the end. This flat board has a lot of drag (like when you swish your hand flat in your bath water, versus when you swish it side on), so it’s more likely to stay in one place and not float away. You drop the board over the side of your ship and as the boat moves, it hopefully stays in one place, and you can measure the distance travelled by how much rope you have to let out between the ship and the board. Sailors would tie a knot in the rope every nautical mile, so they would count how many knots they reeled out in an hour and use that to calculate their speed. That’s why a speed of 1 nautical mile per hour is called 1 knot.