Is synchronicity the best way to move a boat?

After more than a hundred years of theoretical arguments and on-water experiments by proponents of “asynchronous” rowing (where rowers’ oars enter and exit the water at different times relative to each other in an effort to reduce fluctuations in boat speed), the issue may finally have a definite answer.

PhD students Jean-Philippe Boucher and Romain Labbé are part of a group of researchers in sport physics at the laboratory of hydrodynamics at the École Polytechnique in Paris, France. The group used state-of-the-art robotic rowers, sculling together in a scaled down version of an elite racing shell. Their findings were published earlier this year in an article titled “Row Bots” in the journal Physics Today. Read it here.

Boucher and Labbé came to two important conclusions. First, that rowing out of synch is actually beneficial when it comes to reducing the ups and down of boat speed throughout the stroke cycle.

“Synchronisation is responsible for large fluctuations of boat speed. This in turn increases the friction on the boat hull.” Essentially, when everyone is pulling through the water together, that produces a jump in boat speed once the blades are removed and the shell runs out on the recovery. That speed jump doesn’t last long and the boat quickly begins to lose momentum as the rowers move up to the catch for another stroke.

Far from a surprise, this was what Boucher and Labbé expected to find. It also agrees with what is observed in nature when looking at how multi-legged animals use asynchronous leg movements to improve efficiency. Boucher uses the example of a shrimp, which appears to scramble through the water with a number of legs providing thrust. “Presumably,” he says, “this reduces fluctuations of speed and thus the energy needed for swimming.”

The second finding was more unexpected. Despite having a more stable velocity (i.e. fewer ups and downs in speed), the overall boat speed was slower when the oars were not entering and exiting the water together.

“We were a bit surprised because we expected the opposite result,” says Boucher. “Our initial hypothesis was that for a given power supplied by the rowers, the boat with the least velocity fluctuation (the asynchronous boat) should be the fastest.”

This initially presented Boucher and Labbé with a conundrum of how to account for the missing speed. They didn’t have to search for long, however, since the answer was in plain sight and something rowers seem to feel intuitively when they fall naturally into a synchronous pattern.

Feeling the swing

“We did not take into account the effect of synchronisation on the propulsion and especially on the ‘inertial boost’, which is lost when the rowers are not in-phase (timed together),” says Labbé.

“In the asynchronous configuration,” he explains, “the effect of the ‘swing’ (the motion of the rowers on the boat during the recovery stroke) is lost. This is due to the fact that, in this case, the mass motions of the rowers are counter-balancing each other.” In other words, it is only when the entire crew swings forward at the same time out of the finish once the oars are all extracted from the water, that the boat will gain an “inertial boost”.

This is good news for rowers and coaches, who can keep on trying to row in perfect unison and keep on searching for that feeling of perfect swing.

Experimental notes

While the conclusions of Boucher and Labbé ’s study may not turn the rowing world on its head, it does open up an interesting line of questions that modern research techniques and robotics can hope to answer. “Our study aims at understanding better the physics of this sport and particularly the key parameters on which to play to improve rowing performance,” Boucher says.

One question that this experiment raises is the reliability of taking results from a scaled down model in a lab and applying it to the real world. It really isn’t an issue say Boucher and Labbé. When the right procedures are followed, the results are just as applicable in the real world.

“We designed our boat and our row-bots in such a way as to keep the main physical features of real rowing, that is the same kind of rowing cycle (propulsive stroke and recovery stroke) for the oar, coupled with the motion of the rower on the boat and the same flow regime,” says Boucher. “Also, we worked on a theoretical model to account for these observations at small scale and extend them to the real scale. With this model we found that, at the full size, synchronous rowing was also the fastest.”

With more research turning to robotic experimentation for the mechanics, physics and hydrodynamics of rowing, it may be simply a matter of time before a breakthrough occurs that revolutionises the sport … then again, it may be that rowing will continue as it always has, changing little through the ages.