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



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

In: Physics


In the open ocean, its hard to judge speed when you dont have a speedometer.

So sailors would drop a log into the ocean tied to a rope, rope had knots on it all set a certain length apart and the log would pull the rope off the deck as it went. after a period of time they would pull it in and count the knots that were pulled off. The more, the faster.

Now, its purely tradition.

Earth is measured that way (nautical miles). Each degree of latitude is 60 nautical miles. Each second is 1 nautical mile. So the earth is 360 x 60 degrees (or nautical miles).

Therefore speed matches that unit for navigation purposes. Also, it becomes an important factor when choosing map projection styles (converting a globe to a 2D map)…but that is a tangent.

The term “knots”, as mentioned refers to a old low-tech way of measuring speed by counting knots tied in a rope. The term translates to “nautical miles per hour” (so a knot isn’t some other unit of measure).

Nautical Miles are different from Land Miles because land miles can be easily measured directly (with a string or a wheel or anything really). Nautical miles aren’t measured but *calculated*, a sailor would take measurements of stars at night and use those measurements and some math equations to determine where on the surface of the Earth they are. A nautical mile is defined as 1 Minute Division (1/60) of a Degree of Latitude (the horizontal lines on the globe, like a ladder).

ELI15 – Land miles are easily measured in an X-Y-Z (cubic) 3D axis, this means that any point in space is defined by 3 points, one on each axis. Traveling the sea is more like walking across the surface of a ball, so rather than an X-Y-Z system, we use a spherical axis system where each point is defined by degrees around the origin, like arms on a clock, and a single distance (radius) from the origin to the point.

Because that’s how it started out, and inertia is a powerful thing.

You’ll find descriptions of “knots” elsewhere in this thread, But think about it. You’re in a 17th century sailing vessel on the high seas in the middle of nowhere. All around you is nothing but water. You have no reference points. You can measure your latitude using objects in the sky – when you can see them – but that only tells you part of the answer, and even that part is about the past. You want to know what’s happening NOW. How, exactly, do you intend to measure your speed?

Answer – leave something behind, and see how far away from it you get in a given time. And you need to be able to measure the distance reasonably accurately, and in bad visibility. So (as others have described) that “something” is a board on the end of a rope that pays out freely. The rope has knots at regular intervals so that you can measure how much rope has been paid out. You could do that at night or in a fog, with your eyes closed. Paying out a single knot corresponds to moving at one nautical mile per hour. And when you’re done, you just reel the rope, and board, back in, ready for next time. It’s clever, ultra-low tech, really simple, and really effective.

(And if you’re paying attention, you’ll have realised that water doesn’t always stay still. There are things such as tides and currents – sometimes quite fast ones. What you’re measuring isn’t, and can’t be, an absolute speed – it’s your speed relative to the water you’re afloat in.)

Knots = short for Nautical Miles or aka As the Bird Flies. We do not always drive in a straight line but boats usually do so land miles is different than nautical (sea) miles as far as how we measure them, not that the actual distance is different.

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.

On land, you can measure distances easily. In the sea or in the air, it is near impossible to measure on the basis of straight lines. Also, when you are flying, there is a third dimension, which can render distance meaningless. I can drop 1 Km straight down, I have not moved in reference to the ground but in absolute terms, I moved 1 Km.

This is the reason a measure in reference to the curvature of the earth is used to measure the distance travelled. Since the distances are measured in knots obviously it makes sense to measure the speed in the same terms.

Think about the old days. You don’t have a speedometer. There are no landmarks on the sea to figure out your speed. Yes, the waves, but the wind can be blowing them. How can you tell how fast you are?

The solution? Create your own landmarks. You do this with a long rope, with knots tied into it at fixed intervals. You lay the rope down floating on the water, then see how long it takes your ship to pass the next knot.

Then you measure your speed in knots. “My ship was going 8 knots”. Which means you passed a knot in just 1/8 of an hour.

Now we do have speedometers and the like the but the old knots lived on because of A) tradition and B) it’s just as useful as “kilometers per hour” and is a lot faster to say.