Z-drives usher in new era for fleet boats


With Capt. Michael Duncan at the controls, USS Cairo, a new 68-foot pushboat, slowly approached a barge moored along a bank of the Mississippi River.

With a current of about 2 mph pushing directly against the pushboat’s port side, Duncan brought his bow gently up against the side of the barge.

“I just walked it sideways into the current and landed it like a bee,” Duncan explained to a visitor in the octagonal pilothouse. “Did you feel it?” he asked, referring to the pushboat making contact with the barge. “With a normal boat, it’s a controlled collision. With this boat, there’s no collision.”

USS Cairo is not a normal pushboat. As a river fleeting boat, it is the first of its kind to be powered by z-drives. This propulsion system, according to its operator and designer, allows it to do things no other fleeting boat can, while increasing margins of safety and efficiency in all its operations.

“You ain’t gonna find a fleet boat on the river that beats this,” Duncan said. “In layman’s terms, this is a bad-ass boat.”

USS Cairo was designed by CT Marine, a naval architecture firm with offices in Edgecomb and Portland, Maine, for the Carline Companies, which operates fleeting areas on the Mississippi.

Fleeting operations are inherently risky operations, especially in high water. Fleeting boats put together and take apart the large assemblages of barges that larger line-haul boats move up and down the river. That means constantly shifting barges short distances, often in strong currents and eddies that can make it difficult for crews to keep their vessels under control.

“Much more than a line-haul, this is a much more dangerous situation,” said Christian Townsend, naval architect and owner of CT Marine.

The job that faced him and his father, Corning Townsend, the founder of CT Marine, was to create a boat ideally suited to the unique challenges fleeting boats and their crews face. That meant designing a small, tough and highly maneuverable boat that would be both safe and efficient.

What they came up with was a boat 68 feet long with a beam of 34 feet, powered by two Thrustmaster z-drives with 57-inch props in nozzles. For a boat this size, it is powerful — with a total horsepower of 1,500 at 1,800 rpm.

Most crucially, all of this power can be exerted in any direction, giving the boat the ability to extricate itself from tight situations that would leave a conventional boat at the mercy of the currents. “This is basically a tractor tug,” Christian Townsend said.

Capt. Michael Duncan at the helm. The pilothouse is set farther aft by 6 feet more than it would be in a conventional pushboat, placing it at the longitudinal center of gravity and providing much greater visibility.

While Christian and Corning Townsend designed the boat, Clay Harmon, Carline’s vice president of operations, told the designers what his company wanted. And that was a boat that could operate safely in the most challenging situations.

He began considering a z-drive boat when his company had a job coming up involving downstreaming in high water among docks. That would mean positioning the fleeting boat upstream from the barges, coming downstream to land on a barge while surrounded by other barges and docks and then extricating the barge and moving it against the current.

“We didn’t feel safe doing this in any kind of mean river,” Harmon said. “Downstreaming is tough enough as it is.”

The primary danger in downstreaming is that the current will force the pushboat’s stern out of line with the boat’s intended path. If the boat’s conventional props and rudders are unable to bring the stern back into place, the boat can capsize or get pinned under a barge with catastrophic results.

“You don’t have the steering power required in a conventional boat,” Harmon said, while with USS Cairo, “basically the boat can pick up the stern in just about any situation.”

Duncan, who had been piloting traditional pushboats for about 20 years before going aboard USS Cairo when it began operating in December, put it this way: “A conventional boat can’t do what this boat does. Once you get to use it, nothing compares to it. … It’s much safer. It gives you so much confidence. You just know she’s going to pull through without any hesitation.”

While the z-drives set USS Cairo apart from other fleeting boats, a host of other design elements add to its ability to operate safely and efficiently. For example, the pilothouse is set further aft by 6 feet than it would be in a conventional pushboat, placing it at the longitudinal center of gravity. That placement, along with the eight panes of glass giving the pilothouse its distinctive octagonal shape, creates “tremendous visibility” for the pilot, according to Christian Townsend.

Moving the pilothouse aft gives the pilot a much better view of the working deck. “In the old days, looking down you couldn’t even see the H-bitt,” Townsend said. “Now the pilot can sit in the chair and see the entire foredeck.”

With glass all the way around the house, the pilot also has much better visibility of the aft corners of the boat. “It’s literally a greenhouse,” Townsend said.

CT Marine has been designing semi-octagonal pilothouses for about 30 years, and the firm was the first to put them on a towboat, Townsend said.

The Palfinger 1,500-pound capacity crane, used mainly for mooring lines, has a reach of 40 feet. The boom can extend over and beyond a 35-foot barge to reach heavy lines on a dock. Safety is improved because the crew does not have to do the lifting.

Being able to see the aft corners is important for a line-haul boat pushing barges in an H-pattern, with barges attached along both sides, Townsend said. For a crew towing in an H-pattern, “they had to see those corners,” Townsend said.

The ability to see the aft corners is also crucial in a fleeting boat operating in crowded places where it is often surrounded by barges.

In addition to providing good visibility in all directions, the octagon shape (in a sense, an assembly of triangles) is also very strong structurally.

While CT Marine has been building octagonal pilothouses for decades, the details of the designs have been steadily evolving. With 3-D modeling and numerical parts cutting, naval architects can more easily modify and improve their designs to make them stronger and more serviceable. “We are able to do more complex things with the software we have today,” he said.

Townsend is particularly proud of how much CT Marine was able to fit into a boat as small as USS Cairo.

A towboat in the 6,000- to 8,000-hp range would normally be 160 to 180 feet long.  By contrast, the 1,500-hp USS Cairo is only 68 feet long; yet it has the systems and gear of a much larger boat.

 “We have all the same stuff in that boat that an 8,000-hp boat has in a space that’s 100 feet shorter,” he said.

The result is a boat that is incredibly rugged. For its size, “I would say this is the toughest boat on the river,” Townsend said. “It’s a 50-year boat.”

CT Marine was able to build a tough boat without compromising its ability to operate in shallow water. The boat draws 8 feet.

“Draft is the biggest concern of all our clients,” Townsend said. “If it had come in at 8 feet, 6 inches, my father would have considered it a total failure.”

To maintain the design draft, a boat has to be able to keep proper trim. As fuel is burned, the boat would have a tendency to rise at the bow and sit at the stern. To counter this, Cairo has two dedicated non-potable water tanks, one fore and one aft. Water can be pumped from the aft tank to the forward one to keep the boat level.

A Wintech deck winch and Wintech capstan help USS Cairo shift barges more easily. The face lines are 1.5-inch Samson Blue synthetic rope.

This system, more common on larger vessels, is an example of the type of “big-boat feature” CT Marine was able to squeeze into the Cairo design.

Part of the reason the boat’s draft came in on target is its unusually wide beam of 34 feet, as compared with a more typical beam of 28 feet for a fleet boat.

That extra beam is important not just to reduce draft, but to provide the additional lateral stability required because of the z-drive propulsion. The ability of a z-drive to exert full power in any direction would create dangerous stability problems in a boat with a narrower hull.

“You wouldn’t put a z-drive in a 28-foot boat,” Townsend explained.

USS Cairo was built as wide as possible, given the size of the barges it is designed to work around. Cairo needs to be able to squeeze in and out of the spaces created when barges are fleeted up. Since the standard barge is 35 feet wide, Cairo had to be slightly smaller than that. “You need to be able to get in between barges,” Townsend said.

Safety is of course a priority in vessel design. The wide beam is an obvious element, but some are much more subtle. Consider the interior spiral staircase leading to the pilothouse. Serving as a kind of vertical spine on the centerline, the staircase comprises a series of alternating flights of six steps, each flight rising just a half a floor. Anybody slipping will fall only a short distance.

Additionally, the flights alternate in such a way that “you put a hand on the left rail and just keep going down.” In other words, in an emergency such as a fire in the night, a crewmember can quickly and surely make his or her way down and out of the pilothouse to safety.

Another feature to improve safety margins was the installation of push knees in the stern. “If you get under a barge, it gives you the freedom to get out,” Townsend said. “It keeps you from going up under.”

The Palfinger 1,500-pound capacity crane, an unusual piece of equipment for a fleet boat, also helps to ensure the safety of the crew. “It’s mainly for mooring lines. Clay (Harmon) was adamant on keeping the boat as safe as possible,” Townsend said.

With a reach of 40 feet, the crane’s boom can extend beyond a 35-foot barge to reach heavy mooring lines on a dock. Having the crane, rather than the crew, move the heavy lines helps to prevent injuries.

The face lines are 1.5-inch Samson Blue synthetic rope. Much lighter than wire, it too helps to prevent injury.

Steersman Mark Larmeu in the engine room. Main propulsion is provided by two Cummins QSK19-M Tier 2 engines, giving 1,500 hp. USS Cairo packs in the same systems and gear as an 8,000-hp boat.

Other safety features protect the vessel from loss of steering as a result of generator failure. “If you lose power, you lose steering,” Harmon said. “You don’t want to lose power in a tight situation.”

If the monitoring system detects a problem with a generator, it alerts the operator, who can then switch to the backup generator. If a generator actually fails, the system automatically switches to the backup.

Another small but significant contributor to safety is a foot control for the VHF radio. Because the operator of a z-drive needs to keep one hand on each of the thruster controls, his hands are not free to push the call button on the radio. Hence the foot control for hands-free communication.

USS Cairo’s crews will work shifts and will not normally sleep aboard, but the design does not ignore livability issues. The boat has a well-equipped and comfortable galley and mess area.

“This is the Rolls-Royce of fleeting boats,” Townsend said.

While the propulsion system of a z-drive is more expensive than a conventional one, the overall costs of the two types are not that different, according to Townsend. “The costs of building a conventional boat versus a z-drive are similar,” he said. “If you calculate the costs of three rudders, shafting, stuffing box, bearings, tiller arms, etc. … the cost isn’t different enough to have this be a decision-maker,” he said.

From a business perspective, the cost of an asset has to be weighed against the efficiencies it may bring and the revenue it will generate.

USS Cairo, because of its power and maneuverability, is expected to be able to shift more barges in a given period of time. That should mean that USS Cairo will generate more revenue per hour than a conventional boat.

In late February, after the boat had been in operation for less than three months, Lew Parks, Carline’s chief executive officer, was asked if USS Cairo had proved itself. “Absolutely,” he replied without hesitation.

And the boat had yet to operate during the high waters of spring when the river poses the greatest challenges to a fleeting company. “That will be the real test. That will be when the dividends pay off,” Parks said.

By Professional Mariner Staff