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The Evolution of the Z-Drive Inland Towboat


Birth of the Z-drive
Z-drive propulsion is over 60 years old. The Rudder Propeller, or Z-drive as we know it today, was developed and built in 1950 by Josef Becker, founder of the present-day Schottel Group. But it wasn’t until the 1980’s that its use began in North America, when it was found predominately in ship assist tugs, not inland towboats. Since the early ‘80’s the Z-drive ship assist tug fleet in North America has grown from a handful to almost 300, tripling in the last 15 years. It continues to grow and is the dominant propulsion in newbuild ship assist tugs.

The inland towboat industry has lagged in adopting Z-drive technology, but is now on the precipice of the same exponential growth experienced in the ship assist market. In 2008, two inland companies utilized Z-drive towboats. One was Southern Towing Company out of Memphis, TN, and the other a Pacific Northwest company, Shaver Transportation. Less than a half-dozen Z-drive towboats worked the inland river system. Today, we find 10 companies and well over 50 Z-drive towboats built or under construction—and more to come. What caused the change in 8 years?

The inland towing industry is well-versed in navigating the challenging system of inland rivers and canals. It has expertise in adapting to a river’s changing depth, sand bars, currents, and back eddies, and has developed reliable routes and channels that they have followed for years. However, they also expect periodic dramatic changes—such as when a river cuts a new channel to bypass an established one that has silted in. The same is happening with propulsion technology in the industry. Z-drive technology is the cut bank that has broken through to a new and more efficient pathway towards getting the job done. Two companies were instrumental in carving this new path.

The growing popularity of Z-drives in the inland towing industry today is due in large part to two Z-drive pioneers of the distant and recent past—the towboat Miss Nari, and Southern Towing Company.

The Miss Nari, originally named the Delta Cities, was built as a twin-screw conventional towboat in 1951. The boat burned in 1970 and changed hands several times. Eddie Conrad renovated the newly named Miss Nari with Niigata z-drives and diesel engines, and returned it to service in 1982. It is widely accepted that the Miss Nari was the first U.S. towboat to use Z-drive propulsion on the Mississippi River system.

Bill Stegbauer of Southern Towing Company of Memphis, TN, is the more recent pioneer. In the spring of 2006, he began discussions with Ed Shearer of Shearer & Assoc., Inc., to design a new series of 3,200 hp towboats. Following research and discussion, Stegbauer decided to pursue the possibility of a towboat equipped with Z-drives. The first of eight Southern Towing Z-drive towboats went into service in August 2008.

Many factors that led Southern Towing to Z-drive towboats are the same ones driving the change today. However, Southern Towing was the first to step into the deep end of the pool while others stood on the edge, waiting to see whether the Z-drive concept would sink or swim. A key factor in getting others to follow was the empirical data generated by Southern Towing’s fleet, which had multiple opportunities to make side-by-side performance comparisons between conventional and Z-drive towboats pushing identical tows on identical routes.

Inland towboats are tasked with maneuvering large tows, running at 80% load 24/7 year round, navigating bends, currents and restricted channels, meeting other vessels in close quarters and transiting narrow bridges and locks. Key performance criteria for an inland towboat are its amount of thrust, steering power and maneuverability. Z-drive propulsion has inherent advantages over conventional propulsion in all of these categories. It has a high ratio of converting horsepower to thrust, 50% more backing (astern thrust) power, 50-70% more steering force and, of course, 360° thrust capability.

The advantageous Z-drive thrust and steering performance translate into more efficient and safer operations in most inland towing applications. A critical factor to a towboat’s maneuvering success is its ability to “hold the stern” or “not lose the stern.” That concept is extremely important to a towboat pilot. Whether he or she is going into a lock, making a bridge, or going around a bend, his or her fate rests largely on the ability to control the stern of the towboat, its location in the river, its aspect to the current, and its rate of swing and advance.

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.

This is one example of Z-drive performance manifesting as operational efficiency in comparison to conventionally propelled towboats. A Z-drive more efficiently transfers main engine horsepower and applies that horsepower to inland towing applications. Put simply, it can do more with less. A Z-drive towboat consumes 20-30% less fuel, has shorter transit times, covers more distance per gallon and pushes a greater number of barges than a conventional towboat of similar horsepower.

The question remains: If Z-drives are so great what took so long? Skepticism to new technology is part of the DNA of the towing industry. And for good reason. The life cycle of a towboat can be 40 to 50 years, requires a large capital outlay and may not produce a return on investment for 5-10 years. You don’t want to make a bad bet that you must live with for 50 years. Questions of initial cost, reliability and maintenance have always been associated with the introduction of new technology, and Z-drives were no exception. What change agents overcame this skepticism? Although performance advantages of Z-drive propulsion are well documented, economic factors were the final push to get the inland towboat industry to accept Z-drive propulsion.

Economic Change Agents
There are five economic change agents:

  • Empirical Performance Data
  • Fleet Modernization
  • Z-drive Construction Cost Parity
  • Critical Mass
  • Competitive Standard

 Empirical Performance Data
The favorable empirical data generated by Southern Towing provided the “real” towboat data proving that the advantages of Z-drive propulsion are not just theoretical; they are a real.

Fleet Modernization
The inland fleet is in the midst of modernization. Much of the existing fleet is at or near the end of its life-cycle and needs to be replaced. Companies are faced with making large capital investments in equipment. Z-drive propulsion must be evaluated in constructing a towboat today that will remain competitive over the next 30 to 50 years.

Z-drive Construction Cost Parity
The difference in construction costs between conventional and Z-drive propelled towboats turns out to be negligible. It was once thought that Z-drives were more expensive to construct. This assumption was based on the price of Z-drive units. However, it has become apparent that the elimination of reduction gears, shafting, rudders and steering systems negate a large portion of the cost difference. There are also maintenance costs that are reduced or eliminated by removing some of the major mechanical components of a conventionally propelled towboat.

Critical Mass
The fact that there are now 50 or more Z-drive towboats in operation and under construction and more on the way has created a critical mass—whereas at one point a Z-drive towboat was a novelty, it is quickly becoming the norm.

The inland towing industry has gone through many major changes. Just two of them are the transition from steam to diesel and paddlewheel to propeller. These changes have increased efficiency, reduced fuel costs and made the industry safer. Clearly, the use of Z-drive propulsion is another such evolution. While it may not be appropriate for all inland towing applications, what was once a unique and rarely used technology is rapidly becoming an industry standard. Towboat companies positioning themselves for the future will be drawn to Z-drive propulsion. Like the rivers they transit every day, the path of the inland towing industry has cut a new channel.