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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.
|Grenestedt and The Numerette (right, below). Alice Gast (right, above) unveils the new vessel while Bill Maroun (above, standing) meets with Brian Kaplun ’06. Maroun and Grenestedt designed a mono-ski and bucket seat to let Kaplun glide down the slopes.|
“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.”