April 22, 2010

When down is not down

YACHTIES AREN’T ALL purposeful liars. They don’t deliberately exaggerate the size of the waves they survived at sea. It’s just that it’s very difficult to judge wave height from the cockpit of a small boat.

And the main cause of this difficulty is the strange fact that down isn’t always down at sea. The scientist William Froude started investigating this phenomenon in 1861 and he found that waves produce accelerations that combine with gravity to produce a local down that is always square to the face of the water.

Now, in a large swell the face of the water is horizontal only in the trough. That’s the only time that the down you’re experiencing is truly down. On each side, the face of the water bends up — but if you are on one of those sides, down, to you, will still be square to the face of the water. In other words, no matter where you are on the slope of a wave, down always appears to be at right angles to the surface.

No amount of mental compensation can get things into perspective because the illusion of down arises from physical forces to which humans have been conditioned to respond over millions of years.

This explains why an apparently near-vertical avalanche of water rearing up astern seems somehow to flatten out and pass harmlessly under the hull. It’s steepness is an illusion.

It has therefore proven reasonable to assume that the real sea is probably not much more than half as high or as steep as it looks at its worst moment from the vantage point of a small yacht. But it’s not the yachties’ fault.

Today’s Thought
Laws of Nature are God’s thoughts thinking themselves out in the orbits and the tides.
— C. H. Parkhurst, Sermons: Pattern in Mount

Boaters’ Rules of Thumb, #42
Coral reefs. Those of you who dream of sailing where there are coral reefs will want to know that the rule of thumb when navigating in these areas is to wait until the sun is high and behind you, from about 10 a.m. to 4 p.m. Height above deck is an advantage to the reef spotter, who must tell the helm which way to steer. Dark-blue water is deep, 20 fathoms or more, and the lighter the shallower. Turquoise, a vivid green-blue, usually means coral sand covering a flat expense of reef with 4 to 6 feet of water over it. Dark-brown indicates coral heads. Yellowish-brown means a reef covered by 3 0r 4 feet of water. Greenish-brown indicates a grassy bottom, and white means water too shallow to sail in.

Tailpiece
“Can I hire this horse, my good man?”
“Sure thing, ma’am. You’ll find a jack under his saddle.”

3 comments:

bob said...

Bravo! I've never seen this as well described.

Bob
s/v Eolian
Seattle

Aaron Headly said...

Welcome back.

This got me thinking: a GPS can calculate altitude as well as location. All a GPS would have to do is track a boat's altitude for a while and then subtract the average lows from the average highs, et voilà — average wave height.

I don't think a GPS updates quite fast enough to do this 'instantly' (often the calculated error is higher than most waves), but over a range of points it would probably work.

My GPS runs on an old 'ruggedized' PC tablet, so I could try this out (altitude tracking PC applications are pretty easy to find). Most marine GPSs don't pay much attention to altitude, though.

Apparently some guy has already patented this idea. I hope he can't come after me if I pull off the trick myself.

Oded Kishony said...

I tried to do this on my GPS a few years ago when I noticed that altitude numbers changing, but there was no way to verify if the reading I got was the actual wave height or not.

Oded