Monday, July 9, 2018

Z-drives on Towboats and Flanking a River Bend

I've noticed that the inland waterway industry is replacing "towboat" with "pushboat." Since pushboat is a much more accurate description of what the boat does, I'm going to start using the "pushboat" terminology. But, since I already have 30 postings labeled "towboat," I'm not going to change the label. (Update: it finally occurred to me that "towboat" describes what they push, not how they move it. So I'm switching back to towboat in new posts.)

John W. Coke posted
Dennis DeBruler A nice view of two Z-drives. They can be turned to produce thrust in any direction.
When I saw a pushboat at the front of an 11-barge tow on the Des Plaines River in Joliet, IL, I wondered if it had Z-drive because it was facing the wrong way for bow steering. But the Mike Planche was built in 1975, and Z-drives were not tried on pushboats until 2008.  [MarineLog]

Z-drives have been used a while now on tugboats that handle ships, so they have proven that they can handle the output of high-horsepower engines. I found this video to illustrate what a Z-drive looks like.

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One of the advantages of a Z-drive is that they can apply full thrust in any direction, including sideways. So when used as a bow thruster, a lot smaller (cheaper) pushboat can be used. The video below illustrates that a Z-drive is as efficient in reverse as it is in a normal direction. I assume they are pulling backwards because it allows the captains in the bridge to more easily watch what is happening with the ship. (They are trying to pull a ship off of a submerged bank in the St. Lawrence Seaway. (The ship lost steering control.) They ended up bringing in a third tugboat that has ocean towing power. They still could not free it. So they used the downstream dam to raise the water level a little, and that allowed the horsepower of the ship plus three big tugs to free the ship. It was a tanker ship, but nothing leaked. In fact, I think it passed its hull inspection without needing any repairs.)

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A retired professional brown water mariner from tugster
Maneuvering the large tows that are used on the Lower Mississippi around tight bends requires skill and horsepower. If you search for "RE:" in tugster, you will find an explanation of flanking Wilkerson Point. To summarize, as we can see from the plume in this photo, the pushboat is in full reverse with the plume directed to port to stop the pushboat at the inside shore of the bend. Then the river current that flows along the outside shore will push the front of the tow around the bend. It may need a little nudge to help the current, that is why the little pushboat is headed towards the front of the tow. Note that the tow occupies the entire width of the river when making a sharp turn, so it must use the radio to notify downstream traffic that is headed upbound that they will have to stop until this tow makes it pass these river bends.

If the pushboat has enough power and maneuverability, it can drive the bend rather than flank it. That is, it doesn't need to come to a stop because it has enough thrust to shove the front of the tow around the bend before it runs into the outside shore.
Making a bend while heading downstream is an everyday occurrence on the rivers. Flanking and driving a bend are two common techniques used to navigate a bend. Both maneuvers require the pilot to position the stern of the towboat so that the pilot can overcome the tendency of the current to sweep the tow down on the outside of the turn. In the case of driving the bend, the pilot has to have enough steering power to swing the tow and power out of the bend before the towboat ends up on the outside bank. In the case of flanking, the pilot holds the stern more or less stationary over the ground while the current pushes the head end of the tow around. Flanking requires long periods of time and large amounts of power to navigate through relatively short stretches of the river. This is less efficient and uses more fuel but is required if the towboat doesn’t have enough power and maneuverability to drive the bend. A Z-drive towboat with its superior omni-directional thrust may be able to drive the bend in cases where a conventionally propelled towboat would have to flank. [MarineLog]

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