Industrial History: 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]

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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]

Max Wolf posted
[I noticed that it was riding awfully low. Other commented about almost no freeboard.
The comments indicate that this is a triple screw Z drive. I've learned that it is rather typical for a towboat working on the lower Mississippi River to have three engines. If I understand a comment correctly, it has 6600 hp. Max Wolf comments that he was told it can handle 42 S/B and 35 N/B. Barry Bequeret commented that "Jerry Jarrett has taken 46 s/b from lake providence to Nola."]
Christian Townsend: Ken Stiltz Jr. I designed this boat... Marquette runs these z’s about 4” deeper than we designed her for. They like that ass deep in the water.
Ken Stiltz Jr.: Christian Townsend I understand that for different reasons. But she can get swapped real easy. U would have added a little more length to her.

Update: Brandon Hayes posted two images with the comment:

Down time in Natchez this morning, trying to help the new steersmen grasp the concept of a flank I drew these up. Figured I'd share them for those of you that are curious about why we flank vs steer some bends, and what a flank actually is.
To steer a turn in moving current is the pilot betting himself that he will have the tow pointed in the correct orientation at the bottom of a turn with enough room left over to out run his slide with out colliding with the bank.
That are many sure bets. When the odds are in the pilots favor a steer is pain less and quick.
When the odds are not in the pilots favor, flanks are performed. For me personally, there are several turns that depending on the river stage and quanity of barges in my tow I will flank no matter what. Others I use the #2 pencil trick in the radar, ( thats more in depth, if you are a pilot and don't understand what I mean by that send me a message I'll explain it to you) Finally there's the I thought about it 3 times. Any turn on the river that I have thought maybe should be flanked more than 3 times before I get to it I will flank. It cheap insurance.
So what is a flank?
In short a flank is allowing the river to do the work.
By flanking you bring the tow down to the speed of the current ( how fast the water is moving) and allowing the river to safely bring your barges around the turn.
On the flank photo
#1- the tow is moving at normal operating speed, maybe slightly slower.
#- 2 the pilot begins to back up. His goal is to get the tow down to the speed of the current. Some times it's nessary to get the tow below current speed for the tow to get into the flank, but not always.
#3 with the tow down to current speed, the boat and stern of the tow are positioned into the slacker water while the bow goes down into the bend. The faster water is down into the bend will start to push the bow of the tow around the turn.
#4 very similar to #3 but this is where all the magic happens. The faster water in the bend pushes the forward half of the tow around, the stern beling in slacker water acts as a pivot.
When the tow is between photo 4 &5 the pilot straightens all the rudders and begins to push ahead.
#5 the pilot is shoving out of the flank. The shove out is critical as waiting to long the tow will go into the sand bar, not waiting long enough and you might as well have steered it because you just done alot of work to race the bank anyways. When shoving out the tow needs to get above the current speed as quickly as possible to obtain steerage. When done correctly, althow it takes a considerable amount of time longer than to just steer a turn, it's 100% less stressful and the chances of collision with the bank are dramatically decreased.
Hope this helps some of you out. If nothing more, it gave me something to do while waiting tug service.