Firm Conducts CFD Studies On U.S. Pushboats

Robert Allan Ltd., a naval architeture firm based in Vancouver, Canada, is conducting studies alongside ZF Marine on the U.S. inland marine market with the use of computational fluid dynamics (CFD) to compare propulsion configurations in a controlled virtual environment. The company said that it is typically difficult to replicate the continually changing river waters due to currents, vessel driving styles and the various builds of towboats.

Robert Allan said it is conducting the studies because it sees great potential to combine its modern Z-drive pushboats now operating in South America with its experience in the U.S. blue-water sector to potentially invest in the U.S. inland marine market.

In one joint study with ZF Marine, a conventional triple-screw 8,000 hp. towboat was compared to a modern 8,000 hp. triple Z-drive towboat to determine the actual performance improvements possible with the implementation of Z-drives on linehaul pushboats.

This study evaluated pushing a mixed tow of 40 jumbo barges while accurately measuring ahead efficiency, turning ability and stopping ability of the two propulsion types, with results as follows:

• Ahead Efficiency—Approximately equal performance as the conventional screw configuration produces greater thrust, but then loses this thrust advantage due to increased appendage drag from the flanking and steer rudders;

• Turning Ability—The Z-drive pushboat has the ability to exert 55 percent more steering momentum on the tow, which allows the towboat to turn the tow quicker and more accurately and therefore power through a turn and stay in the center of the channel instead of having to perform flanking maneuvers through the corners; and

• Stopping Ability—A Z-drive pushboat has the ability to stop the tow 38 percent faster and in a more controlled manner due to the ability of the Z-drive to rotate 180 degrees and operate its propeller in the most efficient direction instead of in the reverse direction and against the large appendage drag of the rudders.

The company said Z-drive towboats can therefore deliver a reduction in everyday fuel consumption, as has been observed with the initial Z-drive towboats now operating in the market. Operators may choose to simply accept these fuel savings to lower their operating costs, or put that more efficient power delivered by Z-drives to use in pushing larger tows faster to increase their revenue.

Besides studying the performance benefits of Z-drives, CFD provides an ideal tool to accurately analyze other items—such as nozzle configurations and different rudder types—to allow the company to better inform its clients of the merits of a potential modification prior to committing their hard-earned capital and vessel downtime on expensive modifications to their existing fleet.

For new construction, the hull form can be optimized and performance accurately estimated on the first design for an operator before drawings are delivered to a shipyard, thereby short-cutting the lessons-learned approach that doesn’t function well in the U.S. inland market where operators maintain their vessels well and keep them successfully operating for a long time.

Using CFD and other advanced technologies everyday allows Robert Allan to deliver to its clients’ innovative designs that are customized to their operations and the rivers they work on.

Robert Allan said its experience with the design of customized towboats and barges started in the 1970s, with the development of shallow, 5-foot-draft pushboats and barges for the Mackenzie River, North America’s second longest river, which flows into the Arctic. Using the experience gained from the continual support of those challenging operations, Robert Allan said that it has successfully won a number of high-profile inland design projects for the South American markets on both of the two major river systems.

These projects have showcased Robert Allan Ltd.’s wide-ranging capabilities from logistics analysis through to the construction and operational support of pushboats, barges and floating drydocks to operators where existing infrastructure is generally very limited. Most of this work has been focused on the design of modern Z-drive pushboats for independent operators or more commonly directly for the agricultural majors that also operate in the Mississippi River system.