Technical Director’s Innovation Challenge Stern Flap Integration Hull Form Selection
(Source: US Naval Sea Systems Command; issued Jan 23, 2019)
WEST BETHESDA, Md. --- What if the Navy could have more volume for their ships and maintain its power at the same time? A team of engineers at Naval Surface Warfare Center, Carderock Division contend that if designers change the way they approach their initial design, both feats can be achieved.

Michael Lacny and Dominic Cusanelli from Carderock’s Full Scale Trials Branch and Wesley Wilson in the Computational Analysis and Design Branch collaborated on a Technical Director’s Innovation Challenge (TDIC) project to prove why ship designers should start including stern flaps as essential pieces to a vessel’s build rather than an attachment later down the line.

The TDIC was created in 2014 to promote the research of innovative ideas amongst Carderock engineers related to the command’s mission. Their project, titled “Stern Flap Integration with Hull Form Selection,” aimed to show how adding stern flaps in the early stages would have multiple benefits that would interest the Navy in terms of both finances and performance.

The primary function of the stern flap is to reduce the drag on the hull, which leads to a handful of positive second- and third-order effects including saving fuel and decreasing the amount of power necessary to operate. Stern flaps have traditionally been placed at the transom of a ship after the design and build is complete, and often to enhance the performance of an existing hull. Because every inch of space is crucial on a watercraft, the group believes that integrating stern flaps in preliminary ship designs would additionally lead to more available volume or possibly a smaller engine.

Cusanelli, who has been at Carderock for 35 years, said that the idea of the stern flap as a post-production addition is a dated concept. “I have been involved in these designs since 1988 when I put the first one on a ship,” he said. “We’re trying to get rid of this perception and say ‘You can include it as part of your design spiral.’”

Actually testing their concept was a major challenge for the team as the cost to build physical, testable models of the hull forms and stern flaps far exceeded their limits. Lacny said that they used a combination of computer-aided design and computational fluid dynamics (CFD) software to run their experiments, the latter of which they trusted Wilson to complete. “We’ve known Wes and the capabilities of his office for quite some time and use him frequently for this type of stuff,” Cusanelli said.

For CFD to work the way they intended, the group had to find a way to incorporate real, full-scale data into the program. “The hardest part was getting all the inputs into CFD then finding out we don’t have acceptable data in real-world scenarios. We were doing bare hull in CFD; meanwhile, in the real world, every appendage is on the ship.”

Additionally, to fit within the scope of the TDIC the first attempt was to see if simpler and faster hydrodynamics analysis tools, that would be more appropriate in an early concept design study, could predict the performance changes of the stern flap. Analyses were performed using the CREATE-Ships Integrated Hydrodynamics Design Environment (IHDE), which is being developed at NSWC Carderock Division as part of the DOD High Performance Computing Modernization Program (HPCMP) CREATE software development program.

While the challenges made for a complex project, they still saw results that they knew were steering them in the right direction. Lacny displayed multiple designs of hull forms with a diverse set of stern sizes and their corresponding stern flaps that were in turn tested in CFD to explore the effects of the stern flaps on the ship performance. Some designs had a wider stern, others had a deeper stern, and a few more expanded both the width and depth. Doing the extreme of the third option proved to be the least effective in terms of their desired end goal, but the former options proved to be effective for maintaining power requirements while increasing volume.

“That was the most rewarding part, knowing that I am able to do this in software and it wasn’t breaking,” Lacny said. For Cusanelli, working on the project was just short of a dream come true. “I’ve sort of had this idea forever and seeing Mike take it, run with it and get some progress, it’s more rewarding than anything,” he said.

Although the TDIC is over for this year, their work has not gone unnoticed. Lacny said they were asked whether a stern flap would be good for the hull form of a ship being designed now but, “the hull form was shaped different, so at that time we said maybe it wouldn’t be beneficial.” More tests need to be done as they continue to try and sway others toward their unconventional idea.

According to Wilson, “To do so, however, will require some additional exploration in how to leverage more trusted physics predictions with higher fidelity CFD tools. With increasing computational power available via high performance computing (HPC) resources, this is becoming more tractable and could help to yield significant design improvements that incorporate stern flaps as part of the design.”

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