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The Triple-Z-Drive Assist Tug Gets its U.S. Debut

RAL
The Trident, the first of two new Rotortug ship assist tugs for Seabulk Towing

Published Nov 17, 2016 3:27 PM by The Maritime Executive

Leading tug designer Robert Allan Ltd. has two new boats in the works featuring the triple-Z-drive Rotortug propulsion configuration. They are under construction at Master Boat Builders in Alabama for Seacor's Seabulk Towing division – and they will be the first of their kind to operate in the U.S.

“We have made a conscious decision to introduce the Advanced Rotortug in the United States because we believe they  . . . provide an ideal solution for LNG export terminals and other applications that demand safe moorings,” said Rick Groen, COO of Seabulk Towing.

The new boats will be just shy of 100 feet long, and will have two Z-drives drives forward and one drive aft, giving them an excellent combination of bollard pull and maneuverability – even relative to a Voith-Schneider or ASD system. They will be able to deliver up to 80 tons of bollard pull. 

Robert Allan Ltd. says that the design offers enhanced reliability, as it gets its power from three engines instead of two, and that it has better handling in the dynamic currents around a large moving vessel. The firm has been the sole designer for Rotortug applications since 2012; Damen built two of them for Elizabeth Limited and Kotug in 2013, and Singapore's ASL Shipyard is now building three larger support vessels with a Rotortug design for Shell's Prelude FLNG project off Australia. 

Robert G. Allan, the firm’s president, says that his company is able to design these high-performing propulsion configurations for tugs because it has its own computational fluid dynamics team – a costly investment that many naval architecture firms cannot afford for smaller vessel designs, but which Allan says is invaluable for pushing the envelope.

"There's no question it's expensive, and if you'd asked me about CFD about five years ago, I would have said 'no, it's not for us, we don't need it,'" Allan says. "But I'm not sure how we could actually work today without it. It's such a powerful tool if you're doing something completely unique.”