Submarines have propellors; whales have flukes; fish have tails. Which is the most efficient? And what about quietest and least-easy to detect?
(I’m not sure if there are any fish large enough to really count, even if you include the whale shark.)
I thought penguins were the most efficient.
I’d expect flukes and fishtails to be about equivalent, since they’re basically the same thing, just turned on its side.
As for either versus propellers, it depends on what the thing they’re driving is. A flexible-tailed submarine would be an engineering nightmare, and likewise a free-spinning propeller would be an evolutionary nightmare.
Are we not able to measure the noise, energy, and other efficiencies of a whale’s fluke?
That’s for another thread.
I’ve watched several PBS shows where some college puts on a challenge to design a better sub propulsion system. Invariably many entries feature some form of mechanical fin or flipper. At one of them a Navy egghead basically dismissed all of these entries as fancifully impractical, that it’s been proven long ago that a rotating propeller is by far the most efficient marine propulsion method.
It’s similar to the wheel. No life form has evolved one, but on a reasonably smooth surface its the best transportation system.
Maybe so, but they are absolutely the coolest things in the ocean.
Modern submarines, I have read, are dead silent. Often described as black holes in the water.
Mind you this is due to high tech design of propellers. I have a recollection of (I think Toshiba) getting into deep doo-doo in the 80’s because they sold hi-tech milling equipment to the Soviets which would allow them to manufacture silent propellers.
Most propellers cavitate when spinning (they form little bubbles along the edges of the propeller which implode and make sound). Flukes and tails do not do this…whether the design prevents it or merely because they do not churn water as fast as a propeller does I have no idea (I am guessing the latter). A spinning propeller moves much faster causing the bubbles (hence noise).
Super hi-tech submarine propellers these days do not do that. Or rather I should say are a lot less prone to it. IIRC if the sub goes to full power there is a point where there is no avoiding it. If you watch the movie Hunt for Red October there is a scene where the American captain tells the helmsman to go to full speed. The sonar operator complains they are cavitating (making noise). Of course the captain knows this and it was his intent to make the noise to let the other sub know they were there. Till that though the other sub had no clue the US sub was near them. Movie I know but accurate as far as it goes for that bit.
I have heard that when Tom Clancy was writing the book, ***The Hunt for Red October, ***he used publicly-known “Popular Science”-type information about state-of-the-art modern submarine design, and extrapolated and filled in the blanks some.
His information on top-secret propulsion technologies hit a little too close to home, some government agents came to see him, and he had a bit of 'splainin to do before they were satisfied that he had no access to restricted information.
I forget the exact story but supposedly during WWII some story was published in a sci-fi magazine describing a bomb remarkably similar to an atomic bomb.
As I recall the government freaked a bit and had people investigate. They realized it was just someone’s creative imagination and figured it would seem odd if the stories (the story was a serial) suddenly stopped for no good reason so allowed them to continue.
This was true for airplanes, too, until jet engines were developed. Could a marine jet be designed the same way, to be more efficient? I believe there are marine animals that move by jets rather than tails or flukes (squid, for example).
I believe there are microscopic life forms (protozoans?) that have evolved a wheel form of propulsion.
Perhaps you’re thinking of propeller-type propulsion? As I recall flagellates are known to whip their tails around in a circular motion, effecting a crude propeller.
Significantly different, I’d say. Aircraft jets provide constant thrust, with (if I’m not mistaken) the intake air providing oxygen for combustion. Doing that with water is a whole 'nother proposition. Squid do it one squirt at a time via muscular contraction; I suspect a mechanism to do that would be terribly unwieldy.
Dolphins have a propulsive efficiency of 0.81. Not sure what propulsive efficiency subs have.
If you want a detailed explanation of the differences between swimming modes in fish read this. Short version pertinent to the op:
They don’t just whip around in a circular motion–they actually rotate on an axle. Hard to imagine the same principle working on a large animal, though.
The Russians have a supercavitating rocket torpedo - it goes fast enough that the cavitation bubble from the (specially designed) nosecone extends over the whole torpedo (with some small control vanes extending out into the water for steering). There is minimal surface friction, so it more efficient, but it has to go 200+ knots to do it.
Si
Jet engines are generally less efficient than recip engines with props. Indeed, attempts to make jet engines more efficient often entail high bypass ratios (basically, the jet engine incorporates blades that move air, like propellors).
In many cases the blades are inside a cowling; in some, they are unducted.
A thumbnail description from Wikipedia:
There’s a lot more difference between a fish and a submarine than flukes and propellors. And efficiency has different definitions. Fish (and fish like mammals) are more maneuverable than submarines, and power themselves by consuming other fish and plants in the sea. They have flexible bodies made out of stuff like meat and bones, not rigid metal. The effectiveness of their propulsion is based on their entire body shape, not just the shape of their flukes and fins. Some studies indicate their skin contributes to laminar flow reducing drag. OTOH submarines have torpedos, which even way cool fish don’t have.
ETA: Also fish don’t implode if they spring a leak. Though they kind of explode if they’re rapidly depressurized.
Jet-type propulsion in boats:
Jets have been used to jet water to power boats and Waverunners, but be wary of the results, because jet boats will show worse fuel economy and top speeds, because the props on boats intended to plane also create lift, resulting in less wetted hull (less resistance).
Jet boats don’t get this lift, so to do an apples to apples test, you need to power some non-planing hulls (displacement hulls) with jets and compare to displacement hulls with props.
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