Archive for October, 2010

“It [the Froude propulsion efficiency] says that for highest efficiency, the velocity of the fluid issuing from the propulsive unit–paddle, propeller, jet, or whatever–should be as close as possible to the velocity of the craft…Clearly the way to maximize Froude propulsion efficiency consists of moving the largest possible mass-per-time (m/t) of fluid and giving it the least possible increase in speed (v2-v1). In practical terms that means maximizing S, the cross section of the propulsive flow stream.”

“The rear toes [of the platypus] are webbed like those of a duck; but the front feet are unique. The web extends along the sides of the feet and under, and beyond, the claws on all the toes (diagram a). This more than doubles the surface area, and thus the paddling power, of the foot.” (Foy and Oxford Scientific Films 1982:182)

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While the Froud efficiencies “vary in quality and involve differenty underlying assumptions and simplifications, the picture that emerges is satisfyingly consistent with our expectations.”

“* Moving water with undulating body, beating wing, or swinging tail beats squeezing water out of a jet, as anticipated. A squid may jet fast, but when it wants to go far, it’s more likely to use its fins.

“* The same undulating devices do better than systems that move water back-wards with a paddling system, with its alternating power and recovery strokes. We’ll return to this comparison between ‘lift-based’ and ‘drag-based’ propulsion in chapter 13.

“*Bigger (or at least moderate size) is better than smaller. Except for one questionable datum for a bacterial flagellum, no creature below about a centimeter in length does better than ηf = 0.5. The pernicious effects of low Reynolds number (chapter 11) cannot be denied.

“*The broad hydrozoan medusae (essentially small jellyfish) may use jet propulsion, but they do it by pushing out an especially large volume (relative to their own) through a wide aperture. So they have a much higher m and lower v2 than the other jetters, and thus evade most of the difficulty inherent in equations (7.5) and (7.6).” (Vogel 2003:142-143)”

Via AskNature.org

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Hydrophobic fabric

“Shark skin consists of tiny, tooth-like scales that prevent small eddys and vortexes from forming (which slow them down). Speedo has replicated this effect with its Fastskin body suits, which allow competitive swimmers to shave crucial seconds off their race times. Others have used this technology to create fast-traveling ship hulls that naturally deter the attachment of underwater organisms.”

Would this perhaps be something to explore (if we are not taking advantage of the vortices)? In terms of material use and another biomimetic element?

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