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Any thoughts on applying this principle to an AUV (autonomous underwater vehicle) ? if the power source is a battery, would its efficiency be beneficial to that of a propulsion system?

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  • $\begingroup$ There is nothing special about dyson air blades other than clever marketing and a nice mechanical package. $\endgroup$ – user6972 Apr 7 '14 at 9:25
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    $\begingroup$ This is more of an engineering question. It may be possible and the efficiency would depend heavily on the design so there is no straightforward, objective way to really answer this question. Primary losses in underwater propulsion come from friction losses (moving propellor through water) and rotation losses (swirling water does not contribute to propulsion). The Dyson configuration might eliminate the latter, but the friction losses incured to generate the laminar stream in this way may completely consume those profits. The only answer here can really be a full design and analysis. $\endgroup$ – J... Apr 7 '14 at 9:27
  • $\begingroup$ also : physics.stackexchange.com/q/82127/44080 $\endgroup$ – J... Apr 7 '14 at 9:31
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    $\begingroup$ Take note, also, that the link above talks about generating lift, while your question is more about thrust. Some of the arguments won't apply. The critical differences here are that : 1) air is compressible and water is not; pumping water through a tight, circuitous path is likely going to be very inefficient - and 2) water is dense and viscous; again, squeezing and squirting it around is not what you want to be doing. Look up, for example, ring thrusters as examples of propulsion optimized for water : tinyurl.com/k3t2ovf $\endgroup$ – J... Apr 7 '14 at 11:22
  • $\begingroup$ I suppose the final take is that anything you would end up deriving from this idea to adapt to water would end up needing so many modifications to work efficiently that you would end up with something that looked nothing like the Dyson fan you started out with. $\endgroup$ – J... Apr 7 '14 at 11:29
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Dyson fans are NOT efficient (despite claiming otherwise). They emit about 1 gram of high-speed air for every 10 grams of air accelerated. Sounds efficient, right? No!

The Dyson fan is analogous to throwing a 1kg dart at 10 m/s toward at a 9kg target sitting on ice. The resulting dart+target has a velocity of 1 m/s (momentum conservation). However, the dart contained 0.5*1*10^2 = 50J of kinetic energy, but the ensemble ends up with only 0.5*10*1^2 = 5J of energy. The "missing energy" went into making a hole in the board and heat.

Your boat have the same issue. In fact, using a "multiplier" does not increase the thrust over a simple jet, since the momentum is the same. Thus it's better to push more water at a lower speed. When a boat is traveling at a constant speed, the propellors don't actually push the water backward all that quickly. There is no wasteful high speed jet.

However, regular fans are probably inefficient also because they use low quality parts. At $200, Dyson makes up for the mechanical inefficiency with better motors, etc.

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