Lightweight carbon-fiber drive shafts becoming common on U.S. tractor tugs
As of this year, about a half dozen U.S. tug companies are using carbon-fiber shafts â€” each weighing about the same as the average tug crewmember â€” to connect engines with propulsion-drive units for z-drive tractor tugs. Four years ago, only one U.S. tug company, Western Towboat of Seattle, was using carbon-fiber shafts instead of steel shafts on its new tractor tugs. Now such well known companies as Foss Maritime, Harley Marine, Bay-Houston Towing, Wilmington Tug and Colle Towing have all chosen hollow-tube carbon fiber for shafting instead of solid stainless steel.
A regular solid steel shaft might weigh 1,000 pounds or more. These monoliths have to be lifted onboard with a crane, then lugged around with heavy equipment until they are finally in place in the confined space between engine and drive unit. The carbon-fiber shaft, however, can easily be lifted and carried by two people, as it weighs about 20 percent as much as its steel counterpart, according to Kurt Niederpruem, president of Centa Corp., of Westmont, Ill., which, so far, has been supplying most all of the carbon-fiber shafts for U.S. tugboats.
“The longer the shaft, the more carbon fiber makes sense,” Niederpruem said. “The lighter weight allows you to remove support bearings and pedestals from the system, and that’s where you save cost up front and lower your maintenance expense.”
A typical carbon-fiber shaft for a 100-foot z-drive tug might be 14 to 20 feet long and 10 to 12 inches in diameter. A steel shaft of that length might be supported by three or four support bearings mounted on pedestals. The carbon-fiber shaft requires no support bearings, so the cost of the complete system can compete with steel shafting.
Carbon fibers are made from organic polymers consisting of repeating carbon units. To make the fibers, the polymer is stretched into alignment, then oxidized and heated. Further refinement purifies the fiber until it is at least 92 percent carbon. Each carbon fiber is many times thinner than a human hair, but the fibers are twisted, braided, woven or otherwise made into workable shapes.
The end material often is used with resin in a manner similar to fiberglass strands, but carbon-fiber filaments are said to be stronger, more rigid and more durable. The resulting composite material is very strong in the direction in which the fibers run.
“Basically it’s a winding process,” Niederpruem said. “The manufacturer winds the carbon-fiber filaments around a mandrel, putting down layer after layer of material until a thickness is achieved that will carry the required torque and that will handle the unsupported shaft length.”
It’s unlikely that there would be even a single other piece of carbon fiber in the engine room of a typical tugboat. The material is high tech and expensive. A pair of carbon-fiber shafts of the dimensions described above might cost upwards of $50,000, including torsional couplings and required fittings at either end, which are typically installed at the manufacturer.
An active proponent of the use of carbon-fiber shafts for new tractor tugs is the Canadian design firm Robert Allan Ltd. Most of the half-dozen RAL-designed tractor tugs under construction this year are being equipped with carbon-fiber shafts.
“If the folks at Robert Allan recommend them, then it probably makes sense,” said John Colle, president of Colle Towing of Pascagoula, Miss. “But my primary interest in this new material is that it will reduce vibration.”
The family-owned Colle Towing is completing work this year on a new z-drive tractor in its own local shipyard.
Carbon fiber is indeed associated with lower levels of noise and vibration. That’s an important consideration because of the nature of tractor-tug propulsion. In a typical z-drive installation, the drive shaft is connected directly to the flywheel end of its engine. There is no reduction gear. Revolution speed from the engine is reduced, as needed, within the actual z-drive mechanism. Propulsion shafts typically turn at a higher speed than might be the case with a conventional power plant.
Although the carbon-fiber shaft typically is installed without support bearings, it is often fitted with vibration-reducing and alignment-correcting gear at each end.
In some installations, the shafts are fitted with Centa’s CentaLink flexible shaft elements on each end, thus allowing the shafts to be isolated as much as possible from movement by both the engine and the drive unit.
“You need to have those couplings at each end of the shaft to take up the transient, shifting misalignments that occur because of these movements,” said John Hyslop, project engineer for RAL on several new tractors being built in the United States.
It is no coincidence that most of the new tugs being fitted with carbon-fiber shafts are products of a single naval architecture firm. Short runs of carbon-fiber shafts generally are not considered to be cost-efficient, so the tug’s drive train needs to be designed for the longest straight-run possible, to take advantage of the new shafting material.
“Designs with short runs of Cardan shafts running at angles with universal joints are not conducive to carbon-fiber shafting,” Hyslop said. “We have more or less designed these tugs for straight runs of shafting with no joints and no bearings. That’s when carbon-fiber begins to make sense, and the thing you save by going with carbon is bearings.”
The Shrewsburys of Western Towboat in Seattle are long-time users of carbon fiber. Working with naval architects at Jensen Maritime Consultants, they pioneered the practice in the United States with installations, so far, on three large z-drive tugs used for ocean towing to Alaska.
“There are several advantages to carbon fiber for this purpose,” said Ric Shrewsbury, an operating partner at Western Towboat.
“We save on weight over a period of many years, which means fuel savings. We also have less vibration and noise; reduced maintenance, since we don’t have those bearings; and the shafts can be removed easily by just a couple of guys.”