Stealthy ship hull cuts through waves like butter

Borrowing from drug-smuggling subs, Michigan engineers are helping the Navy design autonomous ships that blend in with the ocean surface.

The sharp bow of a nearly 10-foot-long model ship pointed ahead in the 360-foot towing tank in the Aaron Friedman Marine Hydrodynamics Laboratory. While the model’s sleek, yellow body was typical, its position in the water was unorthodox. The deck was nearly parallel with the water’s surface. Only a few inches of the hull were dry.

When the deck of a metal ship sits so low, it easily blends in with the surrounding water and literally hides behind waves. Such low-profile vessels have been used by drug smugglers to covertly move cocaine for more than 20 years.

On a platform a few feet above, engineers prepared the model for a test cruise down the tank. The Department of Defense has funded the Michigan engineers’ tests in the hope that they could help pluck a thorn from the Coast Guard’s and Navy’s sides, and turn it into a boon. The Coast Guard and Navy want to improve their methods for detecting low-profile vessels, and the Navy is hoping to design their own autonomous versions that could run resupply missions without putting sailors’ lives at risk. U-M is one of the few American universities with the facilities to assist.

“Drug smugglers are making these vessels for their own use. They aren’t concerned about modeling the hydrodynamics and making public engineering models,” said Eric Gimple, a recent master’s graduate in naval architecture and marine engineering and a research assistant working on the project.

“We really have no information about these vessels beyond a couple of small projects,” Gimple said. “We can set a baseline by building and testing a physical model.”

To test their model, which was about six times smaller than a real vessel, the engineers loaded it with varying amounts of cargo and pulled it through waves of various size and complexity. Their experiments are revealing how the vessel behaves in different kinds of waves, as well as the optimal locations for the sensors that an autonomous ship needs to pilot itself.

For Gimple, the work is not just about engineering, but honoring his father, Capt. Matthew Gimple. Before Capt. Gimple retired from the Coast Guard in 2019, he helped capture low-profile, narco subs—including the first that the Coast Guard ever detained at sea.

“My dad came back with all these wild stories about how he’s at sea, stopping drugs and helping people,” said Gimple, who also served in the Coast Guard for seven years before studying engineering at U-M. “I was really proud to follow him into the Coast Guard. So, I thought it was a rather serendipitous fit that the type of vessel that my dad told me about would be something that I would get to research.”

For their experiments, the engineers attached the model ship to the tank’s carriage—a large, wheeled platform that tows models down the tank on a set of steel rails that flank the tank. The model was connected to the carriage with a metal mast. Force sensors in the mast measured how strongly water resisted the ship’s forward motion. The engineers also switched on lasers, pointing them at the model’s center and stern to measure how the ship might rock and bob.

Once everything was in the proper position, Jason Bundoff, lead engineer at the Marine Hydrodynamics Laboratory, clapped his hands to mark the start of their test like a film crew using a clapperboard. A mechanical whirring sound grated against the engineers’ eardrums as a steel paddle at the far end of the tank started churning into the water. Waves roughened the glass-like surface, mimicking the ocean. A little over a minute later, the waves reached the model ship, and project engineer Jim Smith fired up the carriage’s electric motors.

The carriage pulled the model ship and the engineering team forward at nearly four miles per hour, matching the behavior of a full-sized vessel moving eight knots.

Conventional vessels would have plowed through the water, breaking it into tumbling waves and sea spray. But the model was so slender and close to the surface that it cut through the water like butter, creating a splash-free bow wave that looked like a smooth ripple near the front of the ship. As the ship cut through more waves, water spilled onto the foredeck, coating it in a thin layer that flowed down the length of the deck before parting at a wavebreak near the center mast.

“It’s fascinating how the waves are coming over the deck so smoothly,” said Matthew Collette, a professor of naval architecture and marine engineering and the project’s principal investigator. “The location of that wavebreak may turn out to be really important. It’s possible that if we pushed it forward, we could reduce the load on the deck by keeping it drier.”

Additional runs uncovered more surprises—the boat sometimes rocked harder than the waves rolled. The instability is likely caused by the small amount of watertight space above the waves, which normally creates reserve buoyancy that balances the ship. The next step is computer simulations that extrapolate their measurements to stormier seas and larger vessel sizes, which the Navy can use in future designs.

“It’s been interesting to take this thing that has only been a detriment to the country and try to find a positive use for it,” said Gimple.