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posted by cmn32480 on Monday June 12 2017, @04:03PM   Printer-friendly
from the row-row-row-your-bot dept.

An article in Physics Today delved into the question of which would be faster: rowing in phase with all of your teammates, or rowing out-of-phase:

Rowing is a challenging sport, and not just for athletes. It mixes physiology, mechanics, and fluid dynamics, so from a physicist's perspective, the sport is much more complex than the elegant movement of a rowing shell might suggest.

Many scientists have tried to work out the details of rowing propulsion, often with a view to improving the performance of rowing crews. For example, in a 1971 Science paper (volume 173, page 349), Thomas McMahon showed that the speed of a racing boat scales as the number of rowers to the power 1/9. In our research, we have taken a closer look at the boat speed within one rowing cycle. In a single stroke, a propulsive phase is followed by a gliding phase. As the figure shows, for racing boats, the variation in speed during the stroke is typically around 20% of the mean speed of 5 m/s or so. Such a variation is a consequence of the synchronized rowing of the crew, a technique that seems to be essential for success in top-level rowing competitions. Consider, however, that for a boat moving through water, larger fluctuations about the boat's average speed imply increased friction on the hull. As a consequence, the mean power dissipated due to fluid friction for speed variations typical of a racing boat is about 5% higher than it would be if the boat could somehow be propelled steadily at the same mean speed.

The investigators suspected that reducing the variation in speed would result in less friction and a higher average speed. To test this, they created a 'bot boat' where they could programmatically control the phase at which each mechanical rower placed their oars into the water — anywhere from 0 to 45° out of phase. Initial results showed that out-of-phase rowing was smoother. Yay! But the boat was slower! Why?

Supplement: The great row bot race

In rowing races, a crew's synchronized motion forcefully propels the rowing shell forward. But the jerky motion of the boat creates a lot of friction on the hull. Can the crew gain an advantage by rowing out of sync? The video shows two trials conducted at the École Polytechnique in Paris using a 1/10-scale boat and robot rowers. In the top panel each robot rows 45° out of phase with its neighbor. In the bottom panel the row bots simulate conventional synchronized rowing. The asynchronous rowing is smoother, but in this case, slow and steady loses the race.

Download Original Video (7.1 MB)

An old adage humorously suggested that "Scientists frequently find that a week in the library can save an hour in the lab." What experiments have you been involved in where the results were counter to your expectations?


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  • (Score: 2) by tfried on Monday June 12 2017, @08:39PM (1 child)

    by tfried (5534) on Monday June 12 2017, @08:39PM (#524638)

    Initial results showed that out-of-phase rowing was smoother. Yay! But the boat was slower! Why?

    Why indeed. Am I the only dumb one, who does not see that question answered from the "spoiler" video?

    Apparently, the authors have this to offer:

    In our initial thinking, we failed to take into account that the rowers are not stationary. Indeed, if you return to the velocity profiles in figure panels b and d, you’ll see that the speed in the synchronized configuration keeps increasing at the beginning of the recovery stroke—that is, after the oars have been lifted from the water, as indicated by the red lines. If the velocity keeps increasing when the oars are out of the water, there must be an additional propulsive force that does not depend on oars. In fact, the force results from the motion of the rowers on the boat. When the rowers return together to the stern of the boat during the recovery stroke, they pull the hull beneath them and accelerate the boat. Since the crew of a coxed eight weighs several times what the boat does, the rowers generate a significant force. When they are desynchronized, that inertial boost is reduced.

    Not sure, I'm convinced, though: Why would 8 desynchronized inertias be worse than 8 synchronized ones?

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  • (Score: 2) by bob_super on Monday June 12 2017, @10:34PM

    by bob_super (1357) on Monday June 12 2017, @10:34PM (#524693)

    My uninformed pseudo-scientific guess is as follows:
    Replace water with concrete and paddles with super-grip.
    If synchronized, all 8 pull their weight plus 1/8th of the boat for the duration of the stroke, which is the fixed length based on the total angle of the stroke and length of the oar.
    If not synchronized, two may be pulling at an angle, two starting a stroke and two finishing it, while two are out of the water. pulling a number out of ass of 2/3rds strength for the starter/finishers, the total strength pulling is 2+4/3=3.3 guys pulling 100% of the boat+rowers.
    Sure, you say, but the lower pull you pulled out of your ass applies to the stroke on the in-sync boat, and that one has the return stroke dead time too.

    Which is where I point out that water isn't solid concrete where you push on fixed posts.
    If you use the same oars and stroke speed to pull 3.3/8ths of a boat+rowers as you do to pull yourself+1/8th of a boat, I'm willing to bet that the water doesn't behave the same way against the paddle.

    In a full-scale experiment, the rowers probably adjust their speed to deal with pulling more weight.
    In a small scale experiment like that video, the motors can pull that weight, but they are pushing a fluid which will not react linearly to the significantly higher push force.