|A Photo by Ken Derry|
Saturday, April 29, 2017
Big Prop Wash (and Threading a Needle)
The former CN/EJ&E Bridge over the Illinois River was very narrow. That is why these pictures show it being replaced by a bridge with a much longer lift span. While Ken was documenting the construction, he caught a couple of photos of a tow going upstream. I was surprised that the hydraulic jump caused by the propwash was so high because I had assumed that the tow would move slowly through such a tight space. They certainly enter locks moving very slowly. Then it occurred to me that it might have been moving slowly as far as the bridge was concerned because he was going upstream and the river may have had a heavy current flow. When they enter locks, they are more sheltered from the main flow of the river.
Then I saw this photo. I think this propwash is the highest that I have ever seen. The pilot probably gunned his engines when the barges cleared the bridge because he could safely go faster. Those big towboats have thousands of horsepower and making a lot of water move fast is how they translate that horsepower into kinetic energy that causes an equal and opposite force to move the tow forward.