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Comment by winReInstall | Hacker News Reader

This parachute shape looks ineffective.. wouldnt a shape be ideal, that takes up stream direction of the water and redirects it partially into the exact oppossite direction, so the ideal shape would look somewhat like a jellyfish, converting the potential energy of the water streaming into directional energy countering the stream direction.

Or just a underwater kite going in circles? https://en.wikipedia.org/wiki/Kite

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> takes up stream direction of the water and redirects it partially into the exact oppossite direction

Well, the “outlet” is now also an inlet then, since you can’t position the outlet outside of the incoming stream. So you’ve got a U, where water slams into itself in the center. Add a second U rotated, so viewed from the current it’s a + with water meeting in the middle. Repeat this infinitely and you end up with a parachute shape with an extra blockage in the center. That extra blockage doesn’t add much extra resistance, so you remove it. Now we have our parachute shape.

Not if the outlet is smaller?

If the outlet is smaller, the current pushes it closed, unless it's rigid, in which case all the current getting pushed to the outside isn't being redirected in the opposite direction as you said. Instead, that current is just being directed aside, which does cause drag. Taking that design out to the logical conclusion, you end up with a drogue, as also mentioned on that page.

Note that a rigid design also has other constraints (storage) that may make it less desirable even if it is more efficient.

A jellyfish is a parachute that can pulse a little

Imagine a dragon like sail for water streams, it runs below the surface in the stream, and allows you to milk the potential energy of the water toward a direction change for your boat..

https://en.wikipedia.org/wiki/Sailing_into_the_wind

But with the gulf stream and automated underwater dragging dragons

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