At the invitation of Southern Towing's president Bill Stegbauer, I boarded M/V David Stegbauer one day in late 2008 on its delivery voyage from New Orleans to Memphis, Tenn. The boat had picked up two empty 295-foot tank barges in Mobile, Ala., and had brought them across to the Mississippi before starting upriver. Capt. Stephen Wage sent engineer Tom McCoin with a Zodiac rigid-hulled inflatable ashore to pick me up in the Industrial Canal at the Port of New Orleans.
As he eased the tow into the lock that joins the Industrial Canal to the Mississippi, Capt. Wage illustrated the ease with which the tow could be positioned. What was immediately remarkable to an observer was the lack of vibration when "backing down." (That may not be the correct term as the operator simply rotates the control units 180 degrees and the thrust is reversed.) "The props are tuned to go just one way so they are more efficient and smoother," said Wage. "They don't need to compromise the pitch between forward and reverse thrust as you would on a conventional towboat. This is a lot easier, you can just turn the drives and ease it in without a lot of shifting and banging around."
Capt. Wage was still perfecting his technique and learning to get the most out of the drives. As with all of the Southern Towing staff that would be piloting the company's four z-drive towboats, he had attended a training program at Seattle's Pacific Maritime Institute.
Wage also noted economies in operation: "With this boat, I steer with only one drive so you still have the other pushing ahead," he said, "I've just come off a conventional 3,200-hp boat and I've noticed a difference in the fuel burn because you can do so much more with less rudder. For handling, this has the equivalent forward thrust of a 3,800-hp conventional boat, but with better backing power."
Once out in the mainstream of the Mississippi, our captain was busy putting together his tow for the trip upriver. Once again the z-drives proved their value in making up to the second barge without the shifting and rudder work of a conventional towboat, as the drives worked both together and independently to align the two barges for the deck hand to lash alongside each other.
Soon after, pilot Kenny Williams came into the wheelhouse to take over the noon-to-six watch. One of the few pilots on the river with extensive z-drive experience, Williams is a huge fan of the system. "I worked on the River Explorer's Miss Nori off and on for a number of years," he explained in reference to the z-drive towboat that formerly took tourist barges along the inland waterways. "If I was coming down that big bend above Memphis and a tow ahead ran aground, I could reverse the drives and hold back my tow."
Williams has also piloted the steam paddle wheeler Delta Queen. He has his First Class License for Steam Passenger vessels. His licensing has required memorizing thousands of miles of inland waterways well enough to draw them from memory in an examination. It is on the tight, nearly 360-degree bends where he demonstrated his mastery of the z-drive system.
Steering around several of these bends between New Orleans and Baton Rouge, Williams demonstrated the ease of working the drives. With hands resting on the console, he used his thumbs only to move the drive controls. "With a rudder boat I would be putting on 40 to 45 degrees of rudder on both sides," he said. "With the z-drive I only put about 20 degrees on the drive that is on the inside of the turn while the other drive pushes ahead."
Williams explained that down-bound with a loaded barge the advantages were equally significant. "With a rudder boat the biggest danger in flanking a bend is losing your stern to the current; with a z-drive you could just turn it back in. But you have to take care not to break your tow apart."
With conventional towboats using six rudders to deflect the water flow there is a significant loss of efficiency. The z-drive needn't deflect flow, but can simply direct it in with one or both drives in any direction. This translates to fuel savings and an increase in equivalent thrusting power. Pushing two empty barges upriver at low water with very little current, David Stegbauer was making 8.7 knots over the ground with the engines throttled back to a comfortable 1,560 rpm. The digital readout was showing only a 60 percent load on the engines. In terms of fuel burn, the people on the boat figured that they were burning 40 to 45 gallons per hour at that rpm. This translated to savings of 500 to 1,000 gallons per day over the fuel burn on a conventional 3,200 to 4,000-hp boat.
As much as Williams and Capt. Wage appreciate the new boat's creature comforts and navigational aids, the talk in the wheelhouse keeps returning to the advantages of the z-drives. The stopping power achieved by propellers optimized for thrust that can simply be rotated 180° is frequently discussed. The ability to maneuver with one prop thrusting to the side while the other pushes ahead or any other combination of thrust is much appreciated. At the same time, the operators stress the importance of the strength of the four Spectra face wires and two wing wires on each side managed by the four deck winches are important to cope with the torque that can be generated when turning with the drives.
Putting the starboard bow of one of the barges into the bank below Baton Rouge in order to take on some supplies for the mid-stream supplier there, Williams said, "There's no sense waking everybody up. With this boat I can just come in nice and gentle and still know I can get out easily. I tell you, once you have worked a z-drive, you never want to go back to a rudder boat."
