Energy balance of locomotion with pedal-driven watercraft

In this study, we examined the mechanics and energetics of locomotion with a paddle-wheel boat and a water bike. Power output (Ẇtot) was measured directly on the water bike by means of an instrumented chain-ring. The simultaneous assessment of oxygen uptake (V̇O2) allowed the computation of the "overall" efficiency of locomotion (ηo = Ẇtot/V̇O2). Mean ηo 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, Ẇtot was then obtained from ηo and measures of V̇O2. The power to overcome (passive) drag was calculated as Ẇd = D · v (where D is the force measured by means of a load cell when towing the boats at given speeds). Propelling efficiency was calculated as ηp = Ẇd/Ẇtot, 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 ηp and Ẇd explain why at the highest speed tested (∼3 m · 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 · s-1.