457 December 2007
Z-Drive for Inland Towboat
Steiner Shipyard in Bayou LaBatre Alabama have begun construction of the first of four 3200 HP pushboats for Southern Towing Company of Memphis Tennessee. At 120 by 34 feet this is a mid-size inland waterway towboat. What makes it truly remarkable is the use of a pair of HRP model 6111WM Z-drives coupled to a pair of Cummins’ new 1600 HP Tier 2 QSK50 engines that are mounted on the main deck level. Southern’s president Bill Stegbauer feels that the z-drives will give the double-hulled boat the power of a 4000 HP conventional pushboat.
This revolutionary design will give the vessel unparalleled maneuverability while negating the need for steering or flanking rudders. All of the vessel’s controls, including the steering, are electrical eliminating the potential for hydraulic spills. The vessel’s designer, naval architect Edward L. Shearer, explains that the savings on installation of rudders and their hydraulic system as well as gears and the ease of engine alignment all contribute to offsetting the additional cost of the z-drives. “Looking at new construction costs for conventional boats we thought that it was time to look at other designs and ideas,” said Stegbauer, “And this is the next generation.”
In an excellent and comprehensive paper that he has prepared on the subject, Stegbauer asserts, “…In fact lower long-term maintenance costs are possible since a towboat with z-drive units does not have all the normal maintenance problems and requirements found in the usual hydraulic steering gear, drive shafts and bearings, reverse reduction gearboxes, and rudders. The Jeffboat/Aquamaster study demonstrates that a z-drive towboat will be safer and more efficient to operate since it can stop quicker, maneuver better, and make better time. The study also indicates that decreased fuel consumption can be expected from these efficiencies. The use of a skewed wheel will contribute to increased pushing power and decreased fuel consumption.
“The efficiencies to be gained include: better stopping power, decreased time making bends and turns, decreased time making locks, docks, and fleets, greater push, and decreased fuel consumption.”
The nozzled propellers will give added bollard pull and steering power. Southern Towing’s port captain has already attended Seattle’s PMTI for training on their simulator. Although there will be significant differences between a z-drive docking tug and a pushboat arrangements have been made to send groups of Southern captains for training over the next five quarters. To help with the additional torque that the towboat can apply in turning the barge, four deck winches will be installed rather than the usual two. With nearly the whole hull available for tankage the boat will carry in the 10.6-foot molded depth of the double hull 69,500 gallons of fuel, 13,000 gallons of water, 1100 gallons each of slop oil, lube oil and hydraulic oil. The 15-ton HRP z-drive units will be installed through soft patches in the hull’s deck facilitating removal without the use of a dry-dock.
Edward L. Shearer, P.E.
Peter J. Slootmaker