March 27, 2012

Metric nightmares

I HAVE A RECURRING NIGHTMARE in which hundreds of chart makers are erasing all depths marked in fathoms and replacing them with depths marked in meters. It's the worst kind of nightmare a person can have, because, basically, it's true. It's happening. We are going metric, slowly but surely, and I can only express dismay and astonishment that such a useful natural measure as the fathom should be ousted by that most unnatural and artificial measure, the meter.

As you probably know, the meter was a blunder. The French thought it was one ten-millionth of the distance from the equator to the pole, measured along a meridian.  Well, their math wasn't too good. They got it wrong. So in 1960 the length of a meter was defined as 1,650,763.73 wavelengths of the orange-red radiation of krypton 86 under specified conditions. As you can imagine, not everybody has a piece of krypton 86 handy in case they want to check a meter or two, so, in 1983, the meter was officially re-defined as 1/299,792,458 of the distance light travels in a vacuum in one second.

Now, if you have a boat drawing 5 feet, and your chart says you are in water 2 meters deep, how do you know whether you're going to go aground or not? Where are you going to get a vacuum from for a start?  Well, okay, say you have a Thermos flask and a flashlight to shine inside it and a chronometer to measure one second exactly — how do you measure 1/299,792,458 of the distance the light travels?  I mean, it travels so fast. So what use is a chart marked in meters?  What use, in fact, is the meter? You'd be aground before you could find the Thermos.

Now the fathom, on the other hand, is something you can get to grips with. The word was derived from another word meaning two-arms'-width, or an embrace. That, in turn, came from the Latin root for arms.  Not guns and things. Arms with hands on the ends. In fact the Latin languages still make its derivation clear.  In Portuguese, for example, it's braca.  In Spanish it's braza, in Italian, braccio.  English had to be different, of course.  Our word fathom comes from the Old English faethm, which meant the outstretched arms.

You'll recall that the old lead-line used to find the depth of water was measured between the outstretched arms. Now, it might occur to you that different sailors had different-sized embraces, of course, so in the end, to keep things nice and tidy the fathom was standardized at 6 feet. And nobody tried to pretend it was any kind of portion of the distance between the poles, and nobody sat it down next to a piece of krypton 86 or tried to measure it in a vacuum.

At 6 feet, the fathom is a subdivision of the cable, which is 100 fathoms or 600 feet.  The cable, in turn is a subdivision of the nautical mile, which for all practical purposes is 6,000 feet (or 10 cables).  These are lovely, natural, easily remembered measurements.  There is no Frenchified pretense about them. They're good old Anglo-Saxon stock and they have served us well for many centuries.  I can hardly believe we are doing away with them in favor of such vague and inferior replacements.

We must have lost our senses. If were President, I'd insist that people keep on embracing the good old fathom. And it would be flogging and keel-hauling for stubborn recidivists who tried to re-introduce the meretricious meter.

Today's Thought
O coz, coz, coz, my pretty little coz, that thou didst know how many fathom deep I am in love! But it cannot be sounded; my affection hath an unknown bottom, like the bay of Portugal.
— Shakespeare, As You Like It

"How did you get that flat tire?"
"Ran over a Coke bottle."
"Jeez, didn't you see it?"
"Nah, the idiot had it in his coat pocket."

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


Stefan Decuypere said...

Being from a metric country, I guess it all depends on what you grew up with. For me fathoms is a foreign concept. I also have a nightmare: that they would convert all the metric signs on our roads to imperial measures...
Stefan from Belgium

Anonymous said...

Convert your boat's draft to meters rather than trying to convert the chart markings. Memorize it. Then you're good to go no matter how the chart is marked.

Matt Marsh said...

SI units make perfectly intuitive sense if you use them on a regular basis. This is doubly so when you try to do engineering calculations.

If you rarely use them, or if you make a point of converting to some other system before thinking "Ah, that's X far from here", it will continue to seem confusing.

Now, if you want a bit of confusion, tell me what a gallon is. (It has three different definitions.) Or a pound. (It has at least three definitions in current use, and the same term is used for two different quantities.)

And if you want real confusion, try converting your depth soundings to any of the systems of true natural units. We could print depth soundings as:
"1.24e11 YlP" (yottaplancklengths in the Planck system), "10.1 MeV^-1" (inverse megaelectronvolts in one natural system preferred by particle physicists),
"37 GlA" (in Hartree atomic units).
These are units that truly do come from the fundamentals of nature. And yes, your 5-foot keel is safe in all three cases.

Anonymous said...

Given the US is about the only country in the world still using the old measurements on any regular basis, the advice to find out your boat's draft in metres seems to make sense.

By the way, rather than worrying about the speed of light in your vacuum flask, just remember there's 30.5 cm in a foot. So a yard is 91.5 cm. If your boat's draft is 5 feet, then it's 152.5 cm, or 1.525 metres. Easy.

And it's all based on the most natural measurement of all: you have ten fingers, so everything's in multiples of ten. Much easier than all the different multiples in imperial measures.

Wasn't the Hubble telescope a mess because someone couldn't convert feet to millimetres?

Matt Marsh said...

Anonymous- You're probably thinking of the Mars Climate Orbiter. NASA uses SI (metric) for everything, but someone gave the thruster specs to the navigation computer in pound-seconds instead of newton-seconds. The computer dutifully calculated a correct course using the incorrect engine specs, which the orbiter dutifully executed- thereby flying straight into the atmosphere.

Hubble's mirror problem was a little more involved (the root cause was a management culture issue), but the technical glitch that made the mirror warped was a scratch on the tip of a measuring rod used to check the mirror curvature. When later QA checks hinted that there might be a discrepancy, everyone thought the QA test (rather than the mirror itself) was wrong.