Sunday, March 23, 2014

It's time to switch your km's to n.m.

A couple of days ago I found myself on r/kayaking (the kayaking specific sub-reddit. It is unfortunately very recreational kayaking, and very little touring kayaking. There is r/seakayaking but it gets very little traffic and most of it is me.) and someone posted an awesome photo of a sea arch.  It was titled "kayaked over 10kms for this, So Perfect"

I did the math - actually google did the math - and "10kms" is 6.2 miles. Now I am going to get a little nit picky, because I am a bit of a navigation geek, and I should point out I teach a lot of navigation classes. I have read the history of the compass, as well as the history of longitude - a great little book, aptly titled "longitude". I taught myself to use a sextant, even though it is impossible to use a sextant in a kayak. So here comes the nit picking.

First, paddling 6.2 miles isn't that far. I applaud anyone who gets in kayak, sets their sights on a goal and accomplishes it. But if you are traveling 2 miles an hour, that is only 3 hours in the cockpit. That isn't really that much time.

Second, If you are starting look at distances, and set goals you are going to do much better if you choose not to use Kilometers. Now I like the metric system as much as anyone but kilometers just aren't meant for use in a boat. And you shouldn't be using statute miles either.

You should be using Nautical Miles, here is why:

One nautical mile travelled in an hour, is referred to as one Knot. And the origin of the name knot, is this:

Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was "cast" over the stern of the moving vessel and the line allowed to pay out.[5] Knots placed at a distance of 47 feet 3 inches (14.4018 m) passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation.[6] The knot count would be reported and used in the sailing master's dead reckoning and navigation. This method gives a value for the knot of 20.25 in/s, or 1.85166 km/h. The difference from the modern definition is less than 0.02%.

So why would you possibly want or need to use a measuring system from the 1800's? Because if you are using a chart, that is the system they are using, so why would you want to use something like kilometers, on a chart designed for nautical miles. You are making work for yourself. Of course, we are used to thinking in terms of kilometers or miles, and it will take a little while to get used to it.

For reference, a kilometer is .62 statute miles, and .54 nautical miles. That is right, a kilometer is pretty close to half a nautical mile.

But that still doesn't tell me why I should be using nautical miles? Well, here is the reason. Because one nautical mile is equal to one minute of latitude. Which means you can measure a distance on your chart, and then determine the length of that distance - either with string or dividers - right on the side of the chart. It makes life super easy.

I think it is important to set goals and paddle towards them. But you should be able to measure the distance when you are looking back.


  1. You said a n.m. is just about half a kilometer :)

    1. I did, because the first part of switching the way you think, is making a connection to what you already know.


    2. a kilometer is about half of a n.m. not the other way :)

      but very good article, I like your blog

    3. Oh, I see what your saying. I corrected it. Thanks.


    4. Sorry but, one kilometer is about 0.54 (half) n.m., so one n.m. is 1.8 kilometers, (not half but a bit less than two kilometers)

  2. apparently I can't do anything right today. Ill fix it tonight. Sorry for the confusion.