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# divy-sub

buoyancy question about a shape changing submarine
 (+1) [vote for, against]

Image the spokes of a wheel from a central hub, say 5, with the imaginary wheel rim and hub in the horizontal. Spokes also come out in the up and down directions.

Each spoke is an arm with track and motor sled. On each sled is a buoyancy vessel. Each horizontal vessel is shaped like an orange segment with a truncated top and bottom. The vertical vessels are dome or cone shaped.

When all the vessels and their respective surface areas are at the ends of the arms, they are each obtaining enough buoyancy to counter the weight of their own arm.

What happens when the vessels are closed in with the new sphere shape (it could be a vertical torpedo) and new surface area ?

 — wjt, Aug 17 2017

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 The object will submerge until its weight is counterbalanced by the buoyancy.

Since you haven't changed the weight or the available buoyancy, if the object floated before, then it will still float after - just in a different position.
 — 8th of 7, Aug 17 2017

But 1....... +.........1 != ........ 2........ in this case. The method of the '+' looks like it matters. It looks changed so would this translate to some sort of vertical motion.
 — wjt, Aug 17 2017

Uh, why would rearranging bouyancy tanks change their bouyancy? That's like saying you can make your luggage lighter by moving things around.
 — MaxwellBuchanan, Aug 17 2017

Another way to look at it is to consider the total air- containing volume of the vessel. If that volume decreases after some sort of mechanical manipulation, then the average overall density of the vessel goes up, and a floater could become a sinker. If the volume does not change, its buoyancy won't change, either.
 — Vernon, Aug 17 2017

Protean deficiency
 — pertinax, Aug 18 2017

 Rearrange your luggage so it is easier to carry and you'll find it is lighter.

 The shape change is probably a red herring but there has to be a difference because the surface area facing the water changes. The difference may only be minuscule.

Can you have a change in surface area without a change in volume? Well obviously.
 — wjt, Aug 18 2017

 You can have surface area without volume, or rather a volume so small as to be negligible.

A sphere has the greatest volume for surface area. At the other extreme, a planar structure with thichness delta-t, where delta-t -> 0, can theoretically have a near - infinite surface area with near - zero volume.
 — 8th of 7, Aug 18 2017

 //there has to be a difference because the surface area facing the water changes.//

That statement represents a depth of misunderstanding which most people can only aspire to.
 — MaxwellBuchanan, Aug 18 2017

 // a depth of misunderstanding which most people can only aspire to. //

Well then, you should be honoured that he's chosen you as a rôle model, [MB].
 — 8th of 7, Aug 18 2017

I'm deeply flattened.
 — MaxwellBuchanan, Aug 18 2017

Are you ? Has your volume changed at all ? Are you still buoyant ?
 — 8th of 7, Aug 18 2017

I am as loud as always, and uncharacteristically cheerful.
 — MaxwellBuchanan, Aug 18 2017

 hmm ok, I get this. Yes,, if you make a spiky ball with air-filled bladders which can each approach center or extend outwards separately, you can make that spiky ball turn any way you want underwater....But unless you mean to compress gas by changing the shape of the containers then the buoyancy will remain the same no matter how you tweak the configuration. It will just raise and lower on its own like a Galileo thermometer bauble.

If you could however control the density of each pod and its position on the length of its arm then you could create some really funky squid-like propulsion systems. (+)
 — 2 fries shy of a happy meal, Aug 18 2017

 [8th] What I was trying to ask is, can a set volume (with set density) have a multitude of different surface areas or is it always a 1 to 1 relationship?

 The position of rest is the Bouyancy (the sum of all the pushes of all the water molecules) against the weight( the sum of all the masses being act on by gravity). An envelope calculation is using densities but that is not the 'actions' that are happening. A density doesn't move anything.

The two positions of the divy-sub have volume's with different surface areas facing the water's pushes and pulls.
 — wjt, Aug 19 2017

 //can a set volume (with set density) have a multitude of different surface areas//

 Yes, of course.

 Take two lumps of dough of equal weight. Roll one into a ball. Roll the other one out into a thin sheet, and it will be enough to wrap the first one several times over. You can make the surface area of the dough sheet arbitrarily large, depending on how thin you can roll it.

The point, though, is that surface area has nothing to do with bouyancy. The only exception is if you're talking about surface tension, which is irrelevant in this case since you're not at the surface.
 — MaxwellBuchanan, Aug 19 2017

 // You can make the surface area of the dough sheet arbitrarily large, //

 Let it go, [Max]. We said that already ...



He doesn't want to understand the math (actually, topology). Just offer him a pound for a cup of tea, and hope he goes away.
 — 8th of 7, Aug 19 2017

 just to confuse things consider a neutrally bouyant nuclear powered inside-out refrigerator.

For awhile you could coat the thing with exterior cold, causing it to sink, then flood the interior with cold water, permitting you to make the refrigerator sink again, or reverse the process and make it float.
 — beanangel, Aug 19 2017

 // you could coat the thing with exterior cold, causing it to sink //

Bad physics ... cool the water below 4 C and it expands - the buoyancy goes up.
 — 8th of 7, Aug 19 2017

 Ice cubes on the go, nice [beanangel]. Floatation though.

 Maybe it's the energy, lonely water can put into trying to be completely surrounded by the it's fellows.

If it's not the action of the water molecules, bouyancy's action must be a sub function of gravity, the graviton.
 — wjt, Aug 20 2017







<commences consumption of more ethanol, using [wjt]'s anno as justification>
 — 8th of 7, Aug 20 2017

[8th], I am well ahead of you on that one, and I can tell you it doesn't work.
 — MaxwellBuchanan, Aug 20 2017

Any shape change motion of a matter is going to be a density change. Whether that change is lasting is another matter. Admittedly, I am not really changing the matter of the divy-sub but I'm am changing the topology of the water a little bit.
 — wjt, Aug 24 2017

 //Any shape change motion of a matter is going to be a density change. // Ah, that's entirely true, but not really. Unless you mean the minuscule and brief compression of materials as you accelerate them.

//I am not really changing the matter of the divy-sub but I'm am changing the topology of the water a little bit.// which will have no impact on bouyance. You might as well argue that a brick will float if you turn it in a particular direction.
 — MaxwellBuchanan, Aug 24 2017

Really? After all that time on my houseboat project?
 — pertinax, Aug 24 2017

 [Max] Try and roll out your flour ball and not change the density. If a brick has two halves and they pulse together, a current of whatever medium it is in will be set up. Gap, No gap, gap ...

A wrinkle in the idea would be that the arms, at the flick of a switch, become rope like and therefore the sum volume would be different to the tethered volumes. With the tethers, the force vectors that make the divy-sub a complete whole wouldn't carry.
 — wjt, Aug 26 2017

[wjt], you seem to be posting from a universe where the laws of basic geometry are radically different from the ones we have here.
 — MaxwellBuchanan, Aug 26 2017

 How about tethering the inflatable/deflatable arms to a conveyor belt which moves around the hull of the craft?

 Each arm inflates as it passes the keel of the ship, and then deflates again as it reaches the highest point, remaining deflated as it moves back down towards the keel again.

 The increased drag on the inflated arms compared to the deflated ones produces a net downwards force on the vessel.

You could call it the "Hull o' Balloons".
 — Wrongfellow, Aug 26 2017

 [Wrongfellow] That would be density change via pressurisation. I trying to think about a rare exception case without presurisation. Water rearranges it's spacial orientation( loses some heat) to change a collective volume's density.

Aside: I hope no-one is voting for me because that would be wrong without my consent.
 — wjt, Aug 26 2017

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