A “serial inventor” builds a hybrid vessel to study the complexities of slamming.
The custom dates back to the Vikings and maybe the ancient Greeks: as a new ship embarks on its first voyage, members of the crew break a bottle of wine – more recently champagne – over its bow to ensure good fortune.
The venerable tradition came to Lehigh last fall when a 29-foot speedboat made of stainless steel and composite materials was unveiled and formally named The Numerette by Lehigh president Alice Gast.
Under a crystal blue sky, in front of the lawn and flagpole on Lehigh’s Asa Packer Campus, a crowd of 150 engineering students and faculty and staff members raised glasses of sparkling cider to toast the new craft.
The crowd soon swelled with passers-by – other Lehigh students, a few school children and landscape workers – who were attracted by the craft’s gleaming orange and white colors and its gently beveled exterior.
The Numerette – the english pronunciation of the name resembles how the words “number one” are said in Swedish – is the product of more than a decade of design, testing, mathematical modeling and construction.
Its inventor, Joachim Grenestedt, professor of mechanical engineering and mechanics and director of Lehigh’s Composites Lab, believes the craft is the largest to date made with panels of a composite sandwich material bonded to a frame of stainless steel.
The new vessel has embarked on a twofold mission. It will serve as a portable research lab to investigate the destructive but little-understood phenomenon of “slamming,” which occurs when a ship belly-flops rhythmically against oncoming waves. and it will seek to show that a steel-composite hybrid construction can be applied to large ships to make them lighter, stronger, stealthier and more fuel-efficient.
“We believe a steel-composite hybrid may be the optimal way to construct a larger vessel, such as a frigate or a destroyer,” says Grenestedt. “Cruise ships, lightweight blast-resistant buildings and bridges in earthquake-prone areas are other potential applications.
“The hybrid construction enables us to take advantage of the particular qualities of both the steel and the composites. The combination is greater than the sum of its parts.”
A strategic harmony
The properties of the carbon and glass fibers in the composite panels of The Numerette, says Grenestedt, complement those of stainless steel.
The materials are nonmagnetic, which enables a ship to pass over explosive mines without triggering them. Composites provide strength, resistance to fatigue (repeated stresses) and other environmental impacts, and insulation. They lack stiffness, but they are lightweight and can be molded easily into complex shapes or made very flat to help a ship avoid radar detection. The 50 sandwich panels on The Numerette have a lightweight foam core and two composite skins for strength and lightness.
Stainless steel is heavier and more ductile than composites, and it provides the stiffness necessary for a ship’s hull.
Grenestedt played a role in the structural design and materials testing of the Visby, a 239-foot- long Swedish stealth corvette (small warship) that fits 20 people and was the largest carbon-fiber vessel ever built when it was launched in 2000.
“For a ship like the Visby, carbon fiber is a great choice,” says Grenestedt. “But to get to the scale of, say, a destroyer, a steel-composite hybrid concept has a lot of advantages.” The main challenge in working with a steel-composite hybrid, he says, is joining the two materials.
“We’ve made every single panel on the bottom of the boat differently so we can determine which construction works best for slamming.” —Robert Thodal, graduate student
“It’s challenging to join steel and composites because each has a very different stiffness. Steel can be strained on the order of 0.2 percent, while a composite, at such low strains, is not making much of a contribution. composites need to be strained extensively. The challenge is to allow them to do this without over-straining the steel. We have developed very special joints in order to accomplish this.”
The group that designed and built The Numerette, which also included six graduate students as well as engineering technician William Maroun, performed finite-element analyses and conducted extensive stress and loading tests in Lehigh’s ATLSS (Advanced Technology for Large Structural Systems) Center on everything from small coupon specimens up to a 16-ton fatigue-loaded specimen representing a ship hull section. The large specimen survived 100,000 fatigue cycles at loads 20 percent greater than those for which they were designed without damage to the composite materials or the bonded joints.
“We learned very much from this test, in particular the importance of the joint details,” says Grenestedt.
Shining a light on slamming
Next to explosions and bomb attacks, slamming – the harsh and steady slapping of waves against a hull – imposes the highest forces on a water vessel. Slamming, along with associated vertical accelerations, is also one of the main causes of injury to sailors, especially those on high-speed boats.
The Numerette is equipped with 123 strain gages, and prepared for 160 pressure sensors, to measure the effects of slamming on its composite panels and steel skeleton. In an effort to gather more information, Grenestedt’s group has given each panel on the bottom of the boat a different construction, varying the strength, stiffness and mass.
“Slamming lasts a few milliseconds,” says Robert Thodal, a graduate student in the group. “It is very dynamic, very quick and quite hard to measure.
“Each of our sensors is sampled 50,000 times per second. When a wave hits the boat, we will be able to determine what happens to each panel transversely and longitudinally, how each panel bends, and how the wave moves across the hull.
“We’ve made all the bottom panels differently so that we can compare their performances to determine which construction works best for slamming.”
In addition to the sensors, The Numerette contains sophisticated instruments for data acquisition and analysis. Computers and electronic equipment are mounted in water- and shockproof boxes attached to the steel frame. (All the attached features – battery, engine, fire extinguisher and more – are fixed to the frame firmly enough to withstand 23 G’s of acceleration.) A video screen shows in real time how the bottom panels are deforming.
“Our goal,” says Grenestedt, “is to gain a better understanding of slamming and eventually work this into design codes that will lead to lighter and stronger boats and ships, both military and civil.”
A ho-hum on the first run
Besides Thodal, other graduate students who have worked on The Numerette include Jian Lv, Scott Shirey, Drew Truxel, Brett Snowden and Jack Reany. The Office of Naval Research has funded the project since Grenestedt began studying steel-composite hybrid hulls a decade ago.
The maiden voyage of The Numerette took place on Pennsylvania’s Lake Beltzville and lasted two hours. In subsequent tests, the boat reached speeds approaching 60 mph.
“We were surprised the boat performed so well,” says Grenestedt. “We started at a slow speed, did a lot of turns at different rates and then slowly increased speed. The testing of any new vehicle requires slow and careful envelope expansion.
“I had envisioned a white-knuckle experience. The adrenaline was pumping when we started out, but after an hour of intense testing, it became almost boring. We have so far found no bad behavior in the boat.”
Grenestedt calls the Composites Lab a “rare find” in a university setting.
“Seldom are large structures designed, analyzed, built and tested in a single lab. This gives students a broad educational experience while requiring ‘out-of-the-box thinking.’ In the end, students acquire solid engineering skills that enable them to hit the ground running.”
At the unveiling of The Numerette last fall, S. David Wu, dean of the engineering college, noted that Grenestedt incorporates composite materials into much of his life. He owns and flies a two-seat airplane made of composites. a beam supporting the deck of his house is made of carbon fibers. Last year, at Utah’s Bonneville Speedway, Grenestedt set the U.S. land speed record for 125-cc gasoline engines in an enclosed streamliner motorcycle that he designed and built from composite materials.
And Grenestedt’s students have caught the fever – they are building an all-composite land yacht and aiming to break the land-sailing speed record.
“You are a serial inventor,” Wu said to Grenestedt. “You are no stranger to new innovations, all of them offered at high speeds.”
