# A 127-year-old physics riddle solved

phys.org | 1/27/2016 | Staff
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He solved a 127-year-old physics problem on paper and proved that off-centered boat wakes could exist. Five years later, practical experiments proved him right.

"Seeing the pictures appear on the computer screen was the best day at work I've ever had," says Simen Ådnøy Ellingsen, an associate professor at NTNU's Department of Energy and Process Engineering.

#### Day - PhD - Benjamin - Keeler - Smeltzer

That was the day that Ph.D. candidate Benjamin Keeler Smeltzer and master's student Eirik Æsøy had shown in the lab that Ellingsen was right and sent him the photos from the experiment. Five years ago, Ellingsen had challenged accepted knowledge from 1887, armed with a pen and paper, and won.

He solved a problem regarding the so-called Kelvinangle in boat wakes, which has been unchallenged for 127 years. The boat wake is the v-shaped pattern that a boat or canoe makes when moving through the water. You've undoubtedly seen one at some point.

#### Angle - Wake - Boat - Degrees - Water

It has long been assumed that the angle of the v-shaped wake behind a boat should always be just below 39 degrees, as long as the water isn't too shallow. Regardless whether it's behind a supertanker or a duck, this should always be true. Or not. For like so many accepted facts, this turns out to be wrong, or at least not always the case. Ellingsen showed this.

"For me, it was a totally new field, and nobody told me it was hard," Ellingsen explained when he first made his discovery.

#### Boat - Wakes - Angle - Circumstances - Respect

Boat wakes can actually have a completely different angle under certain circumstances, and can even be off-centered with respect to the direction of the boat. This can happen when there are different currents in different layers of water, known as shear flow. For shear flow, Kelvin's theory on boat wakes isn't applicable.

"It took the genius of people like Cauchy, Poisson and Kelvin to solve these wave...