Southern Towing Spread the Word About Z-Drive Towboats

Engineer Tom McCoin provides a better view of his boat underway.

Steiner Shipyard of Bayou La Batre, Ala. has completed three vessels of an unusual four-towboat contract for Southern Towing of Memphis, Tenn.

The 120-foot Scott Stegbauer was delivered April 30, preceded by Frank T. Stegbauer and David Stegbauer.

As has been well documented over the past year, these new 3,200-hp vessels are unusual in that they are among the first towboats on the inland rivers to be propelled by azimuthing stern drive (ASD) propulsion.

Design of the new boats was by Shearer & Associates of Metairie, La.

Based on the performance of the first two towboats delivered, the people at Southern Towing appear to be happy with the performance of the vessels, with most of the accolades directed at the z-drives.

ASD (z-drive) propulsion has become very popular as a means of propulsion for harbor tugboats, but less so for other types of tugs and almost never, or rarely, for pushboats. But z-drive propulsion works just fine for Southern Towing, says the man who built these tugs, because the company values maneuverability as much as raw power.

Stephen Wage, captain of David Stegbauer, pauses with his hands near the port z-drive control in the wheelhouse.

“If you have ever looked at a navigation map of the Mississippi River,” said Russell Steiner, president of Steiner Shipyard, “it should be apparent that the river has more twists and turns than it has straight segments, and that is where the z-drives have shown their worth,” he added.

Bill Stegbauer, president of Southern Towing said the thing about many of these towboats is that they are constantly turning. “For so many of these turns, we set one z-drive for thrusting straight ahead, and turn the boat with the other drive. “We get no resistance from all those rudders dragging through the water. Net result is more towing power and greater fuel efficiency,” Stegbauer said.

The owner also said he anticipates additional efficiencies to be gained from better stopping ability, decreased time making bends and turns, decreased time making locks, and in areas of docking and fleeting activities — not to mention decreased fuel consumption.

Most towboats have traditionally been designed with long horizontal levers in the wheelhouse for rudder control — two for steering rudders and two for flanking rudders. River pilots typically navigate while standing between these levers, two on each side.

Pilots on most ASD tugs, including these new towboats, maneuver their vessels by standing or sitting between vertical controls, sometimes called joysticks, typically with one hand on each control grip.

Because of the nature of z-drive propulsion units — extending down into the water like an outboard engine — the head of the drive unit is usually located near the same level as the tug’s engines. In this case, the designer placed both engines and drive units on the main deck, well aft of amidships, with the surrounding, spacious engine room taking up almost full width of the 34-foot-wide boat.

This model of the new ASD towboat for Southern Towing shows proximity of ASD drive at stern and the Cummins main engine just a few feet forward.

The engines are Cummins QSK50 diesels, producing about 1,600 hp each at 1,800 rpm. A short shaft, leading aft about four feet from the rear of each engine, transmits power to the z-drive units, which were provided by the European company Holland Rudder Propeller. The HRP 6111WM model z-drives put out power through fixed propellers in nozzles. A soft patch in the deck overhead allows the entire z-drive unit to be lifted out vertically, should the need arise. Southern Towing reported it is keeping at least one complete z-drive unit as a spare, should it be needed to replace one of the eight that will soon be in service.

Stegbauer said he is impressed by the simplicity of the z-drive system. “They may have a higher initial cost than shaft/propeller systems, but there are no gearboxes, no big rudders or steering gear, no struts, no tail shafts and you don’t have to maintain parts that aren’t there,” he explained.

The new towboats are almost entirely electric in power source, with no hydraulic power. Thus the pair of 170-kW Cummins/Newage auxiliary power generators are larger than would normally be found in a boat of this size. Most electrical gear is also installed in a full-width compartment on the main deck.

Also on the main deck are the lounge, mess, galley, a large machine shop complete with a washer and a dryer and stores. The main deck living spaces are separated from the engine compartments by the relatively quiet workshop space. Above the main deck are two levels of accommodations, topped by the pilothouse. There are eight staterooms with 14 bunks. Typical crew is five to six people.

Tank capacities are 69,500 gallons of diesel fuel, 13,000 gallons of fresh water and 1,100 gallons each of slop oil, lube oil and hydraulic oil.

The new tugs have a rather plain, unadorned look to them, with exhaust stacks located right aft, and a pilothouse projecting forward of other deck structures below. The extended pilothouse nose is intended to enhance the pilot’s view in all directions.

Southern Towing specializes in transporting bulk liquid fertilizer and also barged cargoes of refined petroleum products, chemicals, grain and aggregates. The company says it operates the largest fleet of anhydrous ammonia barges in the U.S. The company includes up to 20 towboats in its fleet. •

By Professional Mariner Staff