Guest Author - Lisa Shea

Cruise ship speed is usually measured in knots - as in 20 knots top speed. Just what is a knot and how does it relate to miles per hour?

Back in the days before GPS units, ships had a hard time knowing how quickly they were going. Especially when they were out of sight of land, there was no way to know how quickly they were crossing the ocean. They came up with a system involving a rope with knots in it, a weighted anchor and an hourglass. They would turn the 30-second hourglass over, and toss the anchor in the ocean. The rope would start to play out, and it had a knot in it every 50 feet. They would simply count how many knots were let out by the time the hourglass was empty, and that would tell them their speed.

Every 50 feet per 30 seconds would equal one nautical mile per hour. If a ship registered 20 knots in those 30 seconds, the speed was 20 nautical miles per hour.

To equate that to "real" land miles, you first have to realize that the definition of a land mile has changed greatly over the years. It was only 1958 where we had a real international agreement as to what a mile was equal to. Back in the days of the Roman Empire, a mile was how long an army marched with 1,000 double paces. Over the years, different countries have come up with different definitions of this distance, ranging from around 1,000 yards to 12,000 yards!!

In modern times, all countries have agreed that a mile is exactly 5,280 modern feet, or 1,760 yards.

So this means that 1 knot/hour is about 1.151 mph. So it is ALMOST the exact same thing - to go 20 knots/hour is about like going 22 mph.