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Long ago, an enthusiastic cyclist claimed to me that a human on a bike was more efficient than a fish in water. Is this true?

I would guess that the relevant measure of efficiency would be something like mD/E, where m=body mass, D=distance traveled, and E=energy consumed. Presumably we should compare a bike on level ground to a fish swimming horizontally.

closed as off-topic by Suma, Oddthinking Jan 7 '15 at 12:06

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  • I am not sure this is a notable claim as required by Skeptics. Do you know of people other than the cyclist you quote, and have you reason to suspect the claim was meant literally, rather than as a general statement on the efficiency of cycling? – P_S Jan 7 '15 at 8:46
  • We want to focus our attention on doubtful claims that are widely held or are made by notable people. Please provide some references to places where this claim is being made. – Oddthinking Jan 7 '15 at 12:06
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Short answer: Yes, but it depends on the size of the fish and the cyclist, and their speeds.

There is a nice published summary for energy cost (J/(kg * m)) for fish, and running or flying animals in this paper in PNAS which suggests that large fish are more efficient swimmers in terms of energy cost per unit of mass per unit of distance. According to this summary, typical values are ~1.5 J/(kg * m). Fish mass will relate more or less directly to their size and aerodynamic resistance. Bigger fish IE sharks will probably be more efficient still but I suppose it is hard to perform experiments on them.

Regarding cycling, the problem is that the mass actually has only a minor effect on the pedaling efficiency on flat ground. This study shows that energy cost for unit distance follows the relationship of (E = (0.606*V^2) + 29.56) where E is the energy cost in J/m and V is the air speed in m/s, on a racing bycicle with normal wheels. For an enthusiastic cyclists speed of 30 km/h = 8.33 m/s, we thus have an energy cost of 71.6 J/m which may be approx ~1 J/(kg*m) if our cyclist weighs ~72 kg.

Thus, a 72 kg cyclist on a race bike at 30km/h is about ~30 percent more efficient in terms of movement energy than a ~ 1 kg fish swimming at average speed.

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    Speed has a non-linear effect on wind resistance when cycling (and swimming), and wind resistance is the dominant force (more important than rolling resistance) at higher speeds such as 30 km/h, so I guess that the 'efficiency' numbers probably vary a lot depending on what speed[s] you choose for the comparison. – ChrisW Jan 7 '15 at 16:26
  • Correct. But this is also true for fish. I chose a speed an enthusiast cyclist might be at for extended tours. You can calculate it for whatever speed you want using the provided equations. The type of bike and tires will also make a huge difference. In the end this is an estimate after all. – Geochron Jan 7 '15 at 16:42
  • @Geochron even just considering small fish, eels are more efficient than cyclist. Eels are 0.42 J/(kg * m) jeb.biologists.org/content/208/7/1329.full#T1 – DavePhD Jun 1 '16 at 12:55

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