Bent for efficiency, unorthodox sterns promise energy savings


Since humans first took to the water, hull symmetry has been the rule almost without exception. For a while “steerboards” occupied what we now call the starboard side of a vessel, but that was just about the only variation. Now, researchers in Europe are promoting asymmetrical sterns as a way to improve fuel economy by better accounting for the hydrodynamic forces created by the rotation of the propeller on a single-screw vessel.

DNV GL-Maritime, the research arm of the international classification society headquartered in Norway, has found that an asymmetric shape — a stern with a “twist” — can account for the differing flow conditions on either side of the propeller. According to DNV GL, the concept surfaced first in the 1960s, but developing an appropriate shape that accounted for hull size, depth of water and propeller configuration was beyond the capabilities of engineers armed with slide rulers. Even computers of the day couldn’t handle the demands of computational fluid dynamics (CFD).

German designers during World War II conceptualized asymmetry in aircraft, and the innovative American aircraft designer Burt Rutan actually built planes reflecting these concepts starting in the 1980s. One of Rutan’s designs featured oblique wings that can change their angle relative to the fuselage (with one wing leading and the other trailing). Another aircraft featured asymmetric engine intakes to accommodate a weapons payload, and Rutan’s Boomerang plane even had one engine mounted ahead of the pilot in the fuselage and a second mounted in a single side pod on the aircraft’s left wing.

The hull optimization process involves selecting the best-performing solution out of thousands of virtual model variations. The colors in the graphic at right denote the pressure distribution on the hull.

Courtesy DNV GL

What has made it possible to consider more asymmetric design concepts is the advent of supercomputers and other very high-performance computing platforms able to perform CFD calculations. The challenge of CFD is that problems involving asymmetric hull and stern design involve simulating and calculating for many critical data points at once. This is because — roughly speaking — in the real world, all of the eddies, vortices and pressures influence each other simultaneously. Therefore, it isn’t enough to calculate for a few parts of the design; engineers need to calculate various solutions for thousands of design points simultaneously in order to come up with workable and accurate solutions. Those solutions then need to be compared to find optimal designs.

For instance, DNV GL-Maritime assessed hundreds of options on its way to finding one that struck the right balance between “pre-swirl and resistance” while meeting the ultimate design needs of the customer. Of course, water is an incompressible fluid, so the challenges are much different than those involving aircraft. The head of fluid engineering at DNV GL-Maritime noted that his organization’s in-house systems allow a designer to assess hundreds of options until they find the right one.

On a recent project, the organization used the techniques to optimize a 3,000-TEU containership so that it used 3 percent less power for the same performance. A traditional symmetrical design was the starting point and the improvements with the asymmetric design were proven through tank testing. In another example involving a 38,000-dwt tanker, optimization around an asymmetric stern design yielded a 3.5 percent decrease in power requirements compared to a traditional design. Improvements of up to 5 percent are deemed possible.

“Designers have tried similar things on twin-screw ships, such as some American destroyers, which use a twisted rudder to reduce cavitation due to the asymmetrical flows that the propeller generates,” said Kevin Maki, assistant professor of naval architecture and marine engineering at the University of Michigan.

A scale model of a bulk carrier with an asymmetric aftbody is tested at the Hamburg Ship Model Basin in Germany to confirm the result of computer research. In a recent DNV GL project, the stern of a 3,000-TEU containership was modified to reduce power consumption by 3 percent.

Courtesy DNV GL

Dr. Raju Datla, research associate professor at the Stevens Institute of Technology in Hoboken, N.J., also has worked with asymmetric hull forms. He said that with all of the new regulations pertaining to commercial vessels, there is interest in many emerging technologies. He said designers have tried and continue to try various means to reduce flow restrictions and turbulence. Datla said he was involved in a small project for the U.S. Navy as part of an effort to provide amphibious landing capabilities for the Lockheed C-130 military transport.

“It was found that spray from the floats on the wing tips was damaging the aircraft propellers, so the floats were redesigned to be asymmetric — with more vertical surfaces on the inboard side and a more traditional hull form on the outside,” he said. That small innovation solved the problem and made the system feasible, though the Navy ultimately abandoned the project, he said.

Datla said CFD is the enabling technology. “With more powerful computers and advanced software, designers can test lots of shapes very quickly and get to an optimal form,” he said.

Datla and his team at Stevens are working with the Office of Naval Research on multihull forms such as trimarans. Working with computer modeling teams from George Mason University, Datla said they have found that keeping a traditional hull form for the center unit in a trimaran is acceptable, but the whole system works best if the two outer hulls have a non-traditional form. “We have found we can significantly reduce drag that way,” he said.

Dr. Raju Datla, center, works with students in the Davidson Laboratory at the Stevens Institute of Technology in Hoboken, N.J. Datla and his team have studied asymmetric hull forms using advanced computational fluid dynamics (CFD).

Courtesy Stevens Institute of Technology

In terms of stern section optimization, Datla said chasing performance improvements of 3 percent to 5 percent is worthwhile, and asymmetric sterns will “likely become mainstream in the next 10 to 15 years” as the technology is proven in practice. He said there are likely to be few downsides, except perhaps small changes in maneuvering characteristics.

With drivers such as energy-efficient design standards promulgated by the International Maritime Organization (IMO) for new ships, as well as associated operational measures for existing vessels, the need for performance improvement is almost unavoidable. According to the IMO, nearly 2,500 new oceangoing ships already have been certified as complying with the energy-efficiency standards. This year, the IMO also adopted guidelines regarding the verification of fuel oil consumption to support the implementation of the mandatory MARPOL (International Convention for the Prevention of Pollution from Ships) requirements for ships of 5,000 gross tons and above.

“Ship operators need to come up with a plan for meeting these requirements which look at baseline data and set goals for reducing emissions by significant percentages,” Datla said. Some operators are simply reducing vessel speeds, which is a sure-fire way to achieve these goals, but that approach can have economic costs. “If this new design technology can improve performance without reducing speeds, it will be very appealing,” he said.

“I expect ship designers and owners to look for any type of advantage they can find, even if it is small,” Maki said.

By Professional Mariner Staff