Friday, June 27, 2014

To scrape or not to scrape

Last Fall I did two visits to Mississippi Lock and Dam #18. During the first visit, I did not see any tows at the dam, from the bridge in Burlington, nor from the bridge in Fort Madison. And then my wife remembered that the federal government was shutdown. We concluded that the Corps is one of the services that was suspended.

When I arrived for the second visit, an unusual locking operation was in progress. Unfortunately, this was BC (before camera), so I'm going to use satellite pictures to illustrate the procedure.

The upstream tow was five barges in a single file. And the barges were wide enough so that only two fit side-by-side in the lock. Normally 3 barges can be fit side-by-side. When I arrived, the towboat was shoving the first 3 barges into the starboard side of the lock. After they tied those barges to the slip anchors, they untied the fourth barge from the third barge. The tow of two barges backed up and moved over so that it could enter the lock on the port side. Actually, it didn't back up that far. It first swung the front end over. It could do that by running its starboard propeller forward while running its port propeller in reverse. And then it pushed its rear end over. I still haven't figured out how it did that. I wondered if he had sideways thrusters. But I did not see any prop wash coming from the side of the towboat. Thrusters are powerful enough to create a visible wash?

The second half of the tow entered the lock very slowly. There were just inches on each side, but it never touched the other barges or the lock's wall as it entered the lock. Below, the red indicates the position of the 5 barges and the blue is the position of the towboat.


They then lashed the two halves together so that when the lock filled, the towboat could shove all the barges out of the lock. They exited very slowly so that nothing scrapes the walls. They then unlashed the front half and backed up the rear half to the following configuration assuming that the upstream gates are opened.


The backing up was very slow so that nothing scraped the wall or the other barges. Or if something did hit, I did not notice (hear or see) even though I was watching closely for contacts.

He then twisted the front of the second half to the right behind the first half by going forward with the port prop and backwards with the starboard prop. I was able to watch the prop wash and confirmed that he was pushing hard with the port prop and pulling hard with the starboard prop. The prop wash in the following webcam picture of a tow leaving Mississippi Lock #15 illustrates that the props are near the edges of the boat to maximize the distance between them to maximize the torque that they can apply to the tow.



And then he moved the rear over. Again, I can't figure out how he did that maneuver because he did not move forward or backwards, just sideways. It took a while to lash the back half to the front half. After the deckhand cranked the winch as hard as he could, he would stand on the cable, and it would go down to the deck. Then he would crank that slack out and stand on the cable again. He repeated the stand/crank cycle until the cable would not go do down; he could jump up and down on the cable and it just bounced.

When I arrived, there was already a 15-barge tow moored upstream waiting to use the lock. And while this 5-barge tow was locking, a little tow arrived on the downstream side. After the 5-barge tow was clear of the lock, the water was lowered to let the little tow lock through before the lock was tied up with the 15-barge tow. I'm glad to see that they make exceptions to the first-come-first-served rule because the small tow had plenty of room on either side so it locked through quickly because it was able to remain intact and it entered and left the lock quickly since it did not have to worry about scraping the walls.

Note that the 5-barge towboat maneuvering slowly to avoid scraping the walls or barges was costing 3 crews precious time. I had assumed it was important to avoid scraping until I saw the 15-barge tow approach the lock. As explained in The Force of Outdraft, a second towboat was used to keep the tow tight against the guide wall. So the barges were scraping the wall during their approach.

So we have two extremes, one tow using a lot of time to avoid scraping something and another tow deliberately scraping the guide wall with quite a bit of force.

The guide and lock walls do wear out, but it is not clear if scraping is the issue compared to weathering. Below is the input bay to the Lockport Powerhouse on the left and the approach to the Lockport Lock on the right. The "circles" are fenders that should catch any barges that get in trouble because of the outdraft caused by the flow through the powerhouse.

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You can see that the wall on the right has been refurbished. The wall on the left has not. But note that there is no shipping activity along the old wall. All of that deterioration is due to weathering. A closeup of the refurbished wall indicates that it has already been scrapped quite a bit even though this part is still a ways from the Lockport Lock.


And the outside wall of the old 1907 lock, which is no longer used, also shows that walls can deteriorate even if nothing has scraped them. (When it was built, it had the highest drop in the world---38 feet.)


So it seems that a captain going very slowly to avoid any scrapes is more of an ego/pride thing than a real need. And the slow speeds reduce the capacity of the locks.

Update:
A video of the 641' Tecumseh rubbing the fenders while entering the Iroquois Lock (Satellite). Judging from the paint, this is not the first time the Tecumseh has scraped its side.

Screenshot from video




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