Slow-to-arrive z-drive units create headaches for tug operators and builders

Ocean Henry Bain is a new tug employing controllable-pitch propellers. The vessel is the third in a series of 5,000-hp z-drive tugs built for Ocean Group, a Quebec company that operates on the St. Lawrence River. The tug has Rolls-Royce drives with CP props in nozzles. (Mac McKay)

What takes longer than ordering a new Toyota Prius, or, say, obtaining a U.S. passport for a first trip abroad? The answer: ordering an azimuthing stern drive (z-drive) for your new tugboat.

The wait for a new z-drive has lengthened to about two years, making the process of building a new ship-assist tug awkward and potentially more expensive for all but the largest tug companies.
Manufacturers of these complex propulsion units, mostly European, are reportedly overwhelmed by orders for new equipment — so much so that a system of advance options or “slots” has evolved, with customers paying years in advance for the right to purchase z-drive units as they become available. The system places a premium on the ability to make a financial commitment for expenditures two to three years in advance with detailed planning necessary — conditions that could make it difficult for a small company to venture into the world of z-drive technology.
Long delays in z-drive deliveries also create stress for shipyard managers who can’t afford to be stuck with half-built tugboats waiting month after month for the arrival of stern drives.
“It’s become quite an issue for shipyards,” said Chris Rowland, vice president of Wilmington Tug Inc., of Wilmington, Del. The company has a new tug under construction in Massachusetts.
“Most of them could build one of these tugs in about nine months,” Rowland said. “But as it is, they have to time their construction schedule based on the planned arrival of z-drives. That is the real schedule driver for them. They might take an order for a new tug, but not lay the keel until six months later.”
A pair of Rolls-Royce z-drives ordered recently comes with a price tag of about $875,000, plus ancillary costs, according to shipyard reports. Rolls-Royce (Finnish) and Schottel (German) are the two main venders of z-drive units in the United States. Other suppliers include HRP (Dutch), Niigata (Japanese), and Thrustmaster (American). Delivery times for Rolls-Royce z-drives were delayed by an additional five weeks during 2007, following a fire that damaged a factory in Europe that supplies gears for the units.
“I can’t really blame these companies for the long delays,” said Scott Slatten, president of Bisso Towboat of New Orleans, with its third z-drive tugboat under construction at Main Iron Works in Houma, La. “There are so many of these tugs under construction right now all over the world and everyone wants z-drives. Rolls-Royce is probably producing 80 percent of the z-drives, and they just don’t have the capacity to produce them any faster.”
Slatten, who manages one of three primary ship-assist companies on the lower Mississippi, said the z-drives and the engines for his next tractor-style tug have recently arrived at the shipyard, as they were ordered in 2006; but construction on the tug is still eight months behind schedule, and he fears that he has been “put on the back burner.” One reason for the delay, along with all the other recent shipyard difficulties on the Gulf Coast, is the long lead time required to obtain z-drives for other new tugs in line ahead of him.
“Rolls-Royce has been calling me asking if I would like to sell our ownership of those z-drives, but there’s no way I’m letting them go, even though they have promised to get us replacements when they are needed by the shipyard.” said Slatten.
Rowland, of Wilmington Tug, explained that his company has a tug under construction with planned delivery in February or March 2008. But as of September, he still did not have the z-drives that were ordered in June 2006. The new tug, the sixth z-drive in the Wilmington fleet, is under construction at Gladding-Hearn Shipbuilding, in Somerset, Mass. “The potential problem for the shipyard, no matter how well they try to plan around this delay, is that this boat could be in the way of other building projects that they’ve got going and coming along behind us,” said Rowland. “They don’t charge us for the delay, but they definitely feel the pain.”
“The situation does become costly,” said Rowland. “We have to order all kinds of equipment, including the engines, and pay for it right away. It costs us quite a bit to have all this equipment and a half-built boat just sitting around. Obviously we are building the boat for a reason, because we need it, so there is also a lot of lost revenue along with the expenses.”
Moran Towing Corp., based in Connecticut but with operations from Texas to New Hampshire, is one of the most active builders of new ship-assist tugs in the nation. Jim Coyne, a vice president in charge of new construction, said the company is always working a couple of years ahead when it comes to procuring new z-drive equipment. “It’s easy for us to look ahead and, since we know we’re going to be building new tugs, to make a commitment for the next available slots that might be available to us,” said Coyne. Moran has lately been ordering z-drives from both Rolls-Royce and Schottel.
“Right now the lead time is such that we are into 2009 and 2010, and as we use up the slots that we made a year or two in the past, we put money down for new ones as far ahead as necessary.”
The long lead time in ordering z-drive equipment also makes it difficult for companies to maintain a supply of replacement equipment for use in the event of damage or failure of existing drive units. Coyne said his company tries to keep at least one set of lower drive units for each type of z-drive in use, plus some gear assemblies and other parts for upper units.
“We have spare parts but we don’t have entire extra z-drive units,” he explained.
The cost of new equipment and the long lead time in deliveries make it almost impossible for a company to maintain entire z-drive units in the spare parts locker.
“It sure would be nice to have spares, but I can’t afford to keep a large piece of machinery worth about $450,000 sitting around gathering dust. Most tug companies are just not big enough to be able to afford that,” said Bisso’s Slatten.
CP props on z-drives for LNG terminal

Boston Towing & Transportation raised a few eyebrows recently with the announcement that a pair of new azimuthing-stern-drive (ASD) tugs to be built for service at an offshore liquefied natural gas (LNG) facility will include controllable-pitch propellers as part of their z-drive propulsion units.
Boston Towing, a unit of Reinauer Transportation, of New York, is in the midst of building a pair of tugs to service the Neptune Offshore LNG Terminal under construction off Massachusetts’ Cape Ann. Robert Allan Ltd. of Vancouver is designing the tugs.
Boston Towing’s 20-year contract with the owners of the LNG terminal calls for construction of one 128-foot, 5,400-hp tug and a smaller 101-foot, 5,400-hp tug. Both units will be rated FiFi-1 and will be equipped with controllable-pitch propellers, according to Vincent Tibbetts Jr., president of Boston Towing.
With the addition of the two tugs, the company’s fleet will include four tractor tugs and eight conventional tugs. The two new tractors are being built at the Derecktor Shipyard in Bridgeport, Conn.
In announcing the contract with Suez Energy N.A., Tibbetts claimed that these would be the first z-drive tractor tugs in the United States to be equipped with controllable-pitch (CP) propellers. That is not exactly true, as it turns out, but it is close. And who can say whether it might be the beginning of a trend?
CP props with z-drives are already in use in Europe and Canada (the most recent tugs for Ocean Group, Quebec) but their use is thus far limited in the United States. Crowley Marine Services operates three oil-spill response ASD tugs with CP props (Alert, Aware, Attentive) with its fleet of ship-assist tugs in Alaska, while Foss Maritime outfitted its cycloidal-drive tugs Weddell Foss and Henry Foss with auxiliary ASD-CP configurations in the past few years. McAllister Towing has a pair of converted Navy YTBs with single azimuthing stern drives with CP props supplied by Schottel (Donal G. McAllister and Kaleen McAllister). In addition Edison Chouest Offshore operates a fleet of CP z-drive true tractors (forward-mounted z-drives) for the U.S. Navy in Kings Bay, Ga., Mayport, Fla., and San Diego, Calif. Edison Chouest is also building four 6,000-plus horsepower scaled up versions of these boats for the Sabine Pass LNG terminal, located in Louisiana, with four more scheduled to serve the Golden Pass (Texas) LNG terminal.
And now comes this pair of new tugs for Boston Towing & Transportation.
Most propellers, whether with conventional propulsion or with azimuthing stern drives, incorporate three or four (and sometimes five) blades that are fixed in pitch. When such a tug is built, it is outfitted with propellers with pitch that will provide maximum power and efficiency at a certain speed. From then on, shaft rpm and direction of rotation may be changed, but the prop pitch is, as they say, fixed.
A CP propeller, however, features blades that can be feathered. The blades can be rotated around their long axis, enabling them to change their angle of attack. When feathered, with zero pitch, the propeller rotates without developing thrust. Give the blades a very slight angle of attack, and the vessel may move ahead slowly. Increase the angle and off she goes. When the pitch is rotated into a negative mode, the propeller will offer reverse propulsion, although this negative mode would not be necessary with z-drive propulsion, since the drive units can simply be rotated to thrust the tug backwards or in any other direction. Changes in the CP propeller’s pitch can be made without requiring a change in rotational direction or speed of the propeller shaft.
Such a refinement becomes complex when applied to z-drive units that operate by rotating 360° and thus exert thrust in any desired direction with any amount of engine power.
With a standard z-drive tug, an operator typically holds the tug in a stopped or motionless position by turning the z-drives to thrust toward each other. This has the effect of canceling out each one’s thrust, but there is still a fair amount of prop wash swirling about, depending on the amount of engine power being used.
It’s easy to imagine that with CP props, the operator could simply set his propeller blades to zero pitch, and everything would come to a stop with, in theory, minimal motion and no prop wash to be concerned about. Better yet, the tug’s engines could still be kept at a healthy rpm while the propeller blades are set to any pitch angle that is required to do the work at hand.
“With variable pitch props you can run your engines at optimal rpm as long as you want,” said Gregg Brooks, a tractor tug consultant involved with several new tractor projects in the United States, Canada and Mexico. “All across the boat speed range, you can keep your engines at maximum efficiency (where they are producing their maximum horsepower, minimum fuel burn rate, or minimum emissions, depending on the desires of the tug operator), and by varying the pitch in different situations, you can apply all of that horsepower to the water. It’s a very efficient system.”
Tibbetts, of Boston Towing, predicted that the controllable-pitch props on his new tugs will provide the vessels with “an infinite degree of maneuverability,” and he is probably right. Since their mission will include a fair amount of standing by at the offshore LNG facility, CP props might be a good way to keep the tug’s engines operating at a reasonably healthy speed while minimizing motion through the water.
Tibbets also reported that his new tug’s fire pumps would be driven off the main engines instead of by auxiliary fire pump engines. While this is only indirectly related to the installation of CP props, it may be part of a trend to do away with those costly and heavy auxiliary engines in tugs rated FiFi-1 for firefighting.
A typical FiFi-1 auxiliary firefighting engine might be in the 500-to-600-hp range, such as the Caterpillar 3412 diesel. These engines weigh about 5,000 pounds by themselves, and a pair of them (one for each fire pump) takes up a large amount of engine room space. And they are hardly ever used, except for practice, drills and photo ops. While use of a PTO from each main engine to power the fire pumps may not save a lot in outfitting costs, it certainly has other advantages.
Every engine is different of course, and the same goes for PTOs, gearing and other links between engine and fire pump. But with one of these pumps operating at full speed (say, 1,800 rpm) its powering engine also needs to be operating at a substantial rpm, which means the skipper has to deal with possibly unneeded thrust from the z-drives and related prop wash. Not so with CP props that can be set to zero pitch or angled slightly for minimal thrust.
Still, in a firefighting situation the two fire pumps are only going to take about 1,000 hp or so from the main engine power, which, with a 5,000-hp tug, leaves about 4,000 hp available (between two engines) for moving the boat. In most firefighting situations a tug is not going to be powering through the water at 10 or 12 knots, so there should be plenty of power available for maneuvering — something that is made all the easier and more efficient with those controllable-pitch propellers.
Categories: Maritime News, Tugboats & Towing