Tugboat trends and challenges: engines, hulls and z-drives

1 Nanuo 6

The U.S. tug industry is adapting to international emissions standards that affect how vessels and engines are designed and built. U.S. Coast Guard safety rules, expected to be released this summer, will prove to be yet another influence on future newbuilds as well. We asked companies in the United States and Canada about today’s design trends and challenges. While spills are an industry concern, double hulls aren’t practical for tugs, other than articulated tug-barges or ATBs, they say. With regard to propulsion, azimuthing stern drives, or z-drives, are valued and will be around for a while.

Raising the standards
Engines are central to the fight against pollution. U.S. inland and coastal waters are quickly becoming the world’s most regulated emissions zones, according to Stefan Wolczko, business development manager at naval architecture firm Guido Perla & Associates in Seattle. Vessels must comply with U.S. Environmental Protection Agency rules stemming from the International Maritime Organization’s MARPOL Annex VI to prevent pollution from ships. 

“With full implementation of these EPA rules, tugs in the United States will be certified to have the least-polluting marine engines commercially available,” Wolczko said. “Hybrid tugs, with multiple generator sets below 800 horsepower and batteries, also seem to be a logical step toward emission compliance without aftertreatment systems. And we’re seeing more requests for diesel-engine tugs here in the United States.”

With the EPA’s Tier 4 regulations going into effect, engines must meet stricter standards — including lower NOx emissions and less particulate matter, said Rob Van Solingen, marine products line manager at GE Transportation in Erie, Pa. NOx refers to mono-nitrogen oxides from the reaction of nitrogen and oxygen gases in the air during combustion.

“This is quite a big deal for tugs and the marine industry as a whole,” Van Solingen said. Vessel operators have a few options to meet these rules. “We’ve seen a very positive market response so far to our new, non-selective catalytic reduction, non-urea, diesel engines,” he said. “The tugboat industry response has been very good and includes a recent order from Reinauer Transportation.” 


The ATB Freedom, designed by Guido Perla. ATBs are largely an American phenomenon, says Stefan Wolczko, business development manager at Guido Perla.

Guido Perla

In 2007, the EPA Tier 4 emission standards slashed sulfur content in marine diesel fuel. Then, in 2008, the agency finalized a program that cut emissions from marine diesel engines with per-cylinder displacement below 30 liters. This included propulsion engines used on tugboats and towboats, and effectively lowered particulate matter and NOx emissions from these engines by up to 90 and 80 percent, respectively. The 2008 rule set standards for existing marine diesel engines larger than 600 kW when re-manufactured, and set Tier 3 standards for newly built engines phased in from 2009. It also established Tier 4 standards for new marine diesel engines above 600 kW, based on applying high-efficiency catalytic after-treatment technology phased in during 2014.

Crowley Maritime Corp. of Jacksonville, Fla., has re-powered six tugs and chartered in other newbuild tugs to meet the California Air Resources Board regulations affecting its fleet there, said Scott Hoggarth, Crowley’s vice president of ship assist and tanker escort services. In the last seven years, Crowley has invested heavily to meet these requirements. “Other tug providers will need to make similar investments to remain viable in California and elsewhere on the West Coast,” Hoggarth said.

The next generation of vessels will be equipped to meet new emission rules, but fitting the systems for these standards can be a big challenge — especially in smaller vessels, according to Robert Allan, executive chairman at naval architectural firm Robert Allan Ltd. in Vancouver, British Columbia. “We haven’t seen many owners looking at repowers for cleaner engines though they may be considering them on their own,” Allan said. “I actually think they’ll hang on to their older, grandfathered machinery as long as they can.”

Robert Hill, president and principal naval architect at Ocean Tug & Barge Engineering Corp. in Milford, Mass., suggested that the Tier 4 rules weren’t well thought-out, saying, “Few T4 engines are available now for newbuild projects. Urea-based exhaust treatment units are way too large for workboats to install easily.” His company had to develop a new tug concept design to meet current regulations. “I’ve yet to see a repower done simply to meet the T4 standards,” Hill said. “The new engine installations seem to be focused on newbuilds, where the engines must be installed.”

Van Solingen said GE Transportation’s L250- and V250-series diesel engines cut key emissions by over 70 percent and reflect GE’s eight-year effort to help businesses meet new rules. “These engines don’t need urea-based aftertreatment so urea storage is eliminated, freeing up cargo and tank space,” he said. “They don’t require supplemental equipment or fluids. Given space constraints on tugs, they reduce a vessel’s design complexity and cut shipyard installation time and costs.”

Scott Hoggarth, Crowley’s ship assist and tanker escort services vice president. He says the company has repowered six of its tugboats to meet California Air Resources Board regulations.

Crowley Maritime

“Moreover, these engines are supported by our global parts distribution and service network,” Van Solingen said. GE engines are installed in tugboats, offshore oil support vessels, ferries, dredges and fishing boats.

Double hulls
Industry members say double hulls won’t be required on conventional tugs any time soon. “We’ve seen several projects recently, especially related to sensitive tanker operations, in which owners requested that no fuel or oil be carried against the tug’s shell to provide the highest-possible safety standard,” Allan said. “While this is rather complicated and certainly expensive to do, it shows that many owners are taking their responsibility to safeguard the environment very seriously. 

“But I don’t see this as a future regulatory requirement for tugs, especially since the amount of oil carried is very small compared with other common carriers,” Allan said. “It would make much more sense to require general cargo ships to isolate their double-bottom fuel tanks than to target tugs.”

Hoggarth doubts tugs will be required to have double hulls, other than ATB fleets that require them for their barge units. “Many tugs are built with their fuel tanks protected by ballast tanks and voids,” he said. This shields tanks from direct exposure to the tug’s outer skin.

Wolczko said a tug’s mechanical arrangements make double hulls impracticable. “A tug’s engine room is a significant portion of its underdeck volume, and engine rooms typically have too little space to allow for a double bottom below and in the way of the primary propulsion machinery,” he said. “Excluding such a space would invalidate the spirit of a double-bottom requirement, since only a small percent of the hull would be protected by the double hull. The rules for this are generally applicable to vessels carrying more than 600 cubic meters of fuel for their own use, and this excludes a lot of average tugs.”

A RAmpage 4500-class offshore/terminal support tug designed by Robert Allan. The Vancouver naval architecture firm says that equipping vessels to meet new environmental standards can be a challenge, especially in smaller tugs.

Robert Allan

Some companies have required that fuel oil not be in contact with the shell, but that can make vessels difficult and expensive to build, Wolczko said. 

“We have two classes of ATB tugs out there with double-skin protected fuel tanks — the Ruth Reinauer-class FacetTugs, and the 12,000-hp OSG Vision-class boats,” Hill said. His firm has designed for Reinauer Transportation and New York-based Overseas Shipholding Group.

“Reinauer has built a number of the Ruth class in domestic and Safety of Life at Sea, or SOLAS, versions,” Hill said. “The current ATB tugs that we’re designing stay below the fuel-load threshold that triggers the need for double-skin tanks. They aren’t very easy to build, the small spaces created can be a bear to maintain and the benefit-payback is not glaringly good. One benefit, however, is the creation of more ballast tanks on a type (of) vessel that often doesn’t have 1-to-1 fuel/ballast replacement capability.”

Comparing domestic and foreign designs
How do new U.S. tugs stack up against foreign builds? Wolczko said technological advances in tugs have been incremental of late, and differences are mainly in the types of tugs used in various parts of the world. “The newest U.S.-designed and -built tugs are quite comparable to their foreign counterparts,” he said. “European operators, however, prefer medium-speed engines, rather than the lighter, high-speed engines favored by American operators and shipyards.”

ATB tugs are a largely American phenomenon not seen much beyond U.S. inland and coastal waterways. “They’re quite different in design from something like a ship-assist tug,” Wolczko said. “The elevated pilothouse and pin connections to the ATB contrast with a low pilothouse and cable connections to a conventional barge.”

U.S. providers are building higher-horsepower, higher-bollard-pull tugs for general harbor services use, Hoggarth said. But overall, U.S. and foreign tug designs have become more alike in recent years as technology requirements have grown similar and vessel design has followed. 

A GE V250 Tier 4 engine. The worldwide engineering group says the L250 and V250 series diesel engines cut key emissions by over 70 percent.


While tugs that are quite dated in style and performance are still being built in North America, several owners have opted for the latest generation of tug designs, and those are equal to any built worldwide, Allan said.

“Our boats are every bit as innovative and as good as their European counterparts,” said Hill. “I deal only in ATB tugs and can say their propulsion and systems technology is second to none. Their performance and safety records are as good as anything you’ll find anywhere overseas. The accommodations on our boats are very comfortable and equal in size to anything overseas.”

Embracing the z-drive
Industry members say use of z-drive, or azimuthing, technology will be long lasting. Azimuth stern drive tugs are equipped with two stern engines that can generate a 360-degree, all-directional propulsion force. Z-drives eliminate the need for a rudder.

“We see z-drives from almost every manufacturer entering what I might call their third or even fourth generation of technology,” Allan said. “The earliest z-drives had teething problems, and suppliers tried different methods of, for instance, driving their steering pumps.” The quality of these early drives varied. “Twenty years ago, it was easy to say ‘X is the best,’” he said. “Today there’s more similarity in quality, and at least a couple of major manufacturers are introducing new and better drives.”

Allan doesn’t anticipate a shift away from z-drives any time soon. “They’re efficient, reasonably robust and increasingly reliable,” he said.

A Robert Allan ART 80-32 rotor tug. It incorporates a patented triple z-drive rotor tug concept, featuring enhanced omnidirectional maneuverability, plus fully redundant and precise propulsion machinery configuration.

Robert Allan

Wolczko expects use of azimuthing thrusters to continue for a while. “Considering the applicability of directional thrust to tug operations — by use of z-drives, l-drives, cycloidal propulsion or the like — there’s no better technology in use or on the horizon, other than some possible adaptations of the same principle,” he said. “We could see new arrangements on the location of the propulsion units and changes in how these units are driven.”

Azimuthing technology should be around for many years because of its flexibility and high conveyance of horsepower to direct bollard pull, Hoggarth said. “Engine power will undergo changes with the development of LNG and hybrid technologies,” he added, “but I don’t see anything new on the horizon for tug propulsion.”

Hill’s firm recently designed a class of ATBs in which three different rudder types were tested. “We want to learn which type is truly best for this kind of boat in the service for which she’s intended,” Hill said. “These boats are now contracted to build, and I look forward to taking a trip on one of them to see the real-world results.” 

Designing safer tugs
Efforts to make tugs safer continue. “Subchapter M will raise the bar for U.S. tug operators as manning and training requirements increase,” Hoggarth said. The Coast Guard’s proposed Subchapter M sets safety rules for towing vessels. Its provisions include electrical and machinery requirements for new and existing tow vessels, use of third-party auditors and surveyors, and inspection qualifications. Public comments ended in 2011 and the final rules should be ready in August 2015.

Subchapter M will only go so far, however. “Tugs have become more powerful for their size, and the dynamics of their operations need to be assessed for impacts on overall safety,” Allan said. An evaluation of tug stability by classical, static methods is no longer enough. “We need to understand the overall stability of a tug under the influence of omnidirectional propulsion systems alone, as well as the significant towline forces,” he said. “These are the most significant challenges in tug design.”

One of the new concepts from Robert Allan is the RAstar 4000-class LNG tug. Three are under construction, featuring foil-shaped escort skews and the form's LNG design.

Robert Allan

Allan doesn’t see collision avoidance as a big issue for today’s tugs. “But I do recognize that, in the United States, towboats seem to have a propensity to tangle with bridges,” he said.

Tugs will continue using electronic equipment to detect incoming traffic, and some operators on their own are installing dead-man detection in their pilothouses, Wolczko said. Dead-man detection sends out an alert if a pilot becomes incapacitated.

Hill said ATB tugs are loaded with bridge electronics, rivaling those of large ships. “They’re like the bridge of the Spaceship Enterprise,” he said. “Our latest boats feature radars with the electronic chart display and information system chart superimposed on the screen. Radars are so sophisticated now that they can track multiple course-crossing threats and post warnings.” Hill worries that navigators will stare at the screen and not look out the window. “But in my opinion that’s less likely with tug crews than other crews,” he said.

“I think we’re on the verge of a whole new look at tug safety worldwide,” Allan said. “The recent rash of tug capsizings has many looking hard at the causes and what should be done. The newbuild tug tragedy in China, with the loss of 22 lives, is receiving critical attention on many fronts.” The capsizing occurred in mid-January when a Singapore-registered tug overturned in the Yangtze River during a test voyage.

“We’re thankful none of our firm’s tugs have been involved in these capsizings,” Allan said. “But we recognize that it’s impossible to rely on today’s regulations to ensure that a tug’s safe.”

When asked which country’s naval architects, builders and operators are most concerned about tug safety, Wolczko said all highly-developed nations care about safety of crews, operations and the environment.

Hill said U.S. and Canadian tug builders and operators are particularly safety conscious. “Our lead in the United States has to do with this nation’s large tug-and-barge, coastwise industry and infrastructure,” he said. “But I know of no nation that’s unconcerned.”

By Professional Mariner Staff