Why Is Speed on Water Measured Differently Than Speed on Land?

Before Global Positioning Systems took all the fun and mystery out of navigating on the big blue ocean, sailors had to come up with a way to measure the speed of their boats.

These salty seafarers realized tying evenly spaced knots on a piece of rope and attaching it to a wedge-shaped plank of wood (weighted on one end so it would float perpendicular to the water and increase drag) would give them a measuring stick they could pull alongside the ship.

This photo is dubbed "Old Salts" and I guarantee they knew how to tie some lines. (U.S. Navy)

Unfortunately the knots-on- a-line technique is only as accurate as the sailor tying them. (Shutterstock)

Daven Hiskey explains on his site, Today I Found Out:

The wood would (then) be tossed into the water and the line let out while a sailor used a sand-glass to time the number of knots let out in the given timespan.

As for the interval and the time-span, this varied somewhat in the beginning, but for reference, one mid-eighteenth century version (attested in A Voyage to South America by Jorge Juan and Antonio de Ulloa) had the knots at 1/120th of a mile with a 30-second timer.

Of course, hand-tied knots on a rope aren't as accurate as our current navigational standards, which are all based on the nautical mile (today equaling 1.852 kilometers). One knot then equals one nautical mile per hour. In landlubber terms, this is about 1.15 miles per hour or 1.852 kilometers per hour, according to Hiskey.

To picture it even more clearly, grab the closest globe or map. Oh wait — no one has those anymore? Right, so pull up a map on your computer. Look for the latitude and longitude lines; assuming the Earth is a perfect sphere (we know it isn't, but it's close enough) today's nautical miles are exactly 1/60th a degree of latitude or longitude - or one minute of arc around the globe.

That means if you were sailing along at one knot, it would take you roughly 60 hours to go 1 degree of longitude or latitude. And then your next step should be to buy a faster boat.

Oh, and since my Navy friends will likely make a fuss if I don't point this out; especially if you are around military sailors — it's a ship, not a boat. And a line, not a rope.

Today's midshipman may learn how to tie some ropes, but they definitely learn how to use GPS (Image via USNA).

(H/T: Gizmodo)

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