Thursday, May 26, 2011

"By the Mark, Twain!"

"By the mark, twain," indicating a sounding of two (twain) fathoms, or 12 feet - safe water for a riverboat's passage. And so Samuel Clemens - Mark Twain - filled our childhood memories with exciting Mississippi River boat pilots and the challenges they faced. Of the river adventures of Tom Sawyer and Huckleberry Finn.

With the Mississippi River running at record levels, I grew curious as to how this impacts the passage of river "tows."

As one who grew up in a "tugboat" family, I cannot help but smile a little, every time I think about a quirk in maritime terminology between coast-wise towing and river "towing." Here in the Pacific Northwest - as elsewhere - a tugboat tows, that is to say, pulls a barge.

On our river systems, including the mighty Columbia, a towboat pushes, rather than tows barges. It would be more sensible to call them push boats, but we live in a world full of peculiarities.

It takes years for a riverboat pilot to earn his "ticket," the U. S. Coast Guard document that certifies minimum knowledge has been acquired, to allow the person to operate a towboat on the river.

Pilots License, Samuel Clemens

In "Life on the Mississippi," Mark Twain provides a provocative insight as to the extraordinary memory a skilled riverboat pilot needed, in his day, to earn a license to operate a steamboat the 1,200 miles to New Orleans:

One cannot easily realize what a tremendous thing it is to know every trivial detail of twelve hundred miles of river and know it with absolute exactness.

If you will take the longest street in New York, and travel up and down it, conning its features patiently until you know every house and window and door and lamp-post and big and little sign by heart, and know them so accurately that you can instantly name the one you are abreast of when you are set down at random in that street in the middle of an inky black night, you will then have a tolerable notion of the amount and the exactness of a pilot's knowledge who carries the Mississippi River in his head.

And then if you will go on until you know every street crossing, the character, size, and position of the crossing-stones, and the varying depth of mud in each of those numberless places, you will have some idea of what the pilot must know in order to keep a Mississippi steamer out of trouble.

Next, if you will take half of the signs in that long street, and CHANGE THEIR PLACES once a month, and still manage to know their new positions accurately on dark nights, and keep up with these repeated changes without making any mistakes, you will understand what is required of a pilot's peerless memory by the fickle Mississippi.

During the high water on the Mississippi, pilots face an extraordinary challenge due to the increase in water flow.

As an example, in order for the rudder to steer the vessel, water has to flow past it.

In still water, once a vessel begins forward movement, two to three miles per hour, she will begin to respond to directional change.

In addition to water flowing past a rudder, two additional laws of physics act on the vessel; "Advance and Transfer."

Forward travel, from the point the rudder is turned, is referred to as "Advance." This is simply a function of speed and mass of the vessel, which wants to continue on a straight line.

The distance it takes for a vessel to finally adjust to a new course is called "Transfer." For many vessels, extensive "sea trials" are conducted to create "Advance Tables" for given speeds and rudder settings.

Now consider the radical changes
in the physics of rudder and "Advance and Transfer" when the body of water is moving! If that body of water is moving at 10 or 12 miles per hour, a vessel at rest will not respond to rudder movement, but drift at the speed of the current.

There can be no meaningful response to the rudder until the vessel is moving, up to a third faster that the current!

Here is a video taken last year, a pilots eye view clearly demonstrating advance and transfer as he lines up to pass under the Vicksburg bridges.

The first bridge is the 1930 Rail/highway Bridge, the second is the I20/US 80 1973 structure. Keep you eye on the changing angles by watching the bridge piers.

Remember as you watch these videos: The towboat and tow HAVE to be moving at least a third faster that the rushing river, for there to be ANY steering control with the rudders! This occupation is not for the faint of heart!

I located another remarkable video of a towboat and her barges, as the river dashes her past Vicksburg last week. Care to take a guess as to how fast the river is flowing? Can you imagine being the pilot moving this tow - in this maelstrom? As you watch the video, two things become apparent, rapidly.
  • First, the woman who shot the video suddenly realizes that she has to change camera locations quickly to watch the drama unfold, as the pilot navigates the curve in the river and the bridges!
  • Second, pay attention to the effects of the river as the barges are caught broadside by the current. And remember the physics of "Advance and Transfer." This is a remarkable record of a remarkable feat of piloting on the Mighty Mississippi River.
Battling the current upstream presents another set of problems. The navigation channel through many of the bends on the Mississippi River becomes so narrow and the current so swift that a barge tow has to use a method of maneuvering called flanking.

Flanking involves a series of engine thrusts against the current. The barge tow actually pivots through the bend instead of directly steering. Thus, long periods of time are required to navigate through relatively short stretches of the river. More fuel is required by these towboats because of this flanking.

And this does not begin to address the issue of "How do you corral a runaway barge, careening downriver?

"When I was a boy, there was but one permanent ambition among my comrades in our village on the west bank of the Mississippi River. That was, to be a steamboat man."
[from "Life on the Mississippi" by Mark Twain]

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