Knots, Miles, and Minutes
Answering an FAQ about distance on the water
One of my favorite qualities in a writer is the ability to make a seemingly boring or unpromising subject absolutely captivating (see John McPhee’s Oranges). It’s good practice to write without the crutch of a gripping subject.
So this week, as an exercise in minutia, I’m going to answer a question I get asked surprisingly often:
What is a nautical mile?
If you just want conversions:
1 Nautical mile = 1.852 Kilometers = 1.15078 miles (or statute miles)
The cool thing about nautical miles is that they’re based on the dimensions of planet earth. A nautical mile is one minute (1/60 of a degree) of latitude. 60 minutes x 360 degrees is 21,600 nautical miles, pretty close to the earth’s polar circumference (it’s not exact because the earth isn’t a perfect circle).
It makes sense to measure this way when you’re traveling long distances across vast, empty stretches of ocean. Nautical miles are also used by pilots and astronauts. The moon is 207,600 nautical miles away.
We talk about speed on the water or wind speed in “knots”—nautical miles per hour. The term originates from the device 17th-century mariners used to measure the speed of their vessels, towing a line attached to a piece of wood in their wake and recording the time it took for knots in the line to run out.
Nautical miles are easy to work with on a chart or map because all you need is the latitude scale on the side to measure distances at any scale. They can also be worked out by someone with a sextant and knowledge of celestial navigation.
Imray Charts, which I’m using to plan this trip, take it a step further with handy little 1 mile x 1 mile grids printed on the chart, the way hiking maps have 1km x 1km squares.
You can mark a known distance on a piece of string, or use an “anatomical scale” like the width of three fingers or the length of your thumb. In Jeff Allen and Harry Whelan’s round Ireland documentary, they had a slice of cheese that was exactly 8 miles on the chart.
How fast, how far?
Kayaking all the way around Ireland is a journey of roughly 810 nautical miles or 1,500 kilometers. Whenever I mention miles, in training or on the trip, I’m talking about nautical miles.
My kayak, the Rockpool Taran is built to go fast and far, easily maintaining 4-4.5 knots. I estimate 4 when dead reckoning. A more traditional kayak averages 3 knots. That might not sound like a big difference, but it adds up over a long day or a long trip.
An ocean-going sailboat averages around 5 knots.
“10 miles before 10:00”
A good day on this trip would be 30 miles. At that rate, you’d get around Ireland in less than 30 days (assuming you can get on the water every day and maintain that pace).
40 miles would be an outstanding day and doable with the tide and/or conditions in your favor.
20 miles is good too.
Some expedition paddlers swear by the practice of putting in 10 miles before 10:00.
I’ve found that it helps to practice “distance deflation” and trick yourself into squeezing out a few extra miles, the way your parents would lie when you asked “are we there yet” on a family roadtrip.
Do you know why pirates wore eye patches?
For a fascinating deep-dive on “the longitude problem,” the development of accurate timekeeping, and what navigation has to do with pirates wearing eye patches, read Longitude by Dava Sobel.
P.S. When I’m not having adventures, I’m helping other people write about theirs. If you or someone in your circle has an amazing story to tell, I'd love to hear it. I have some availability for new ghostwriting and editing projects.
— Charlie
The Lap is a fundraiser for The Michael J. Fox Foundation for Parkinson’s Research.
Kokatat is the official gear sponsor of The Lap.
Ration packs and fueling strategy by Resilient Ops.
CH Marine will be providing a VHF radio and other safety equipment.
Camp kit and cooking gas provided by Paddle & Pitch. Wool baselayers by Aclima. Trolley by KCS. REAL Field Meals at a discount from Adventure.ie. Discounts from GreatOutdoors.ie. Expedition coffee by 3fe.
Interesting post Charlie.
It's worth noting that the kilometre (and by extension the whole metric system) is also based on the circumference of the Earth.
Instead of dividing the pole to the equator into 90 degrees, divide it into 100 gradians (or 100%, as cyclist might describe it). Then if you divide each gradian by 100, into centigrades, each centigrade is equal to 1km distance on the surface of the Earth. (In the same way that 1 second of arc is equal to 1 nautical mile).
All other measures in the metric system were extrapolated from this fundamental measurement, even though they were late standardised by more accurately definable measurements.
Even though accuracy of the measurement of the circumference of the Earth has improved since the metric system was developed, it's still quite reasonable to estimate the circumference of the world at 40,000 km.