Why does boats measure speed in knots and not mph/kmh?
In: Physics
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.)
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