Energy balance of locomotion with pedal-driven watercrafts

In this study, we examined the mechanics and energetics of locomotion with a paddle-wheel boat and a water bike. Power output (W-tot) was measured directly on the water bike by means of an instrumented chain-ring. The simultaneous assessment of oxygen uptake (VO2) allowed the computation of the "overall" efficiency of locomotion (eta(o)=W-tot/VO2). Mean eta(o) was 0.27 (s=0.02), which was unaffected by the speed, and was assumed to be the same for the two boats as both are semi-recumbent bicycles. For the paddle-wheel boat, W-tot was then obtained from eta(o) and measures of VO2. The power to overcome (passive) drag was calculated as W-d=D center dot upsilon (where D is the force measured by means of a load cell when towing the boats at given speeds). Propelling efficiency was calculated as eta(p)=W-d/W-tot, which was lower with the paddle-wheel boat (mean 0.35, s=0.01) than with the water bike (mean 0.57, s=0.01). The observed differences in eta(p) and W-d explain why at the highest speed tested (similar to 3 m center dot s(-1)), the energy required to cover a unit distance with the water bike is similar to that required to move the paddle-wheel boat at 1.3 m center dot s(-1)