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# self-raising prawn trap

 (+4) [vote for, against]

Prawns are trapped on the Pacific coast at between 500 and 1500ft depth. In this the depth is 1000ft where water pressure is approx 450psi. We have to assume two traps raised one at a time. The trap is dropped along with a cylinder contraption that is just below negative bouyancy. The cylinder is sealed at one end. Inside attached to the sealed end is a gas bag containing a spring, also attached to the sealed end. At the other end of the spring and gas bag is a disc that is water tight and will slide along the length on the cylinder. The other end of the cylinder has a cap with a valve that will open one way at 400psi. At full depth the cylinder valve opens and water enters the cylinder at 400+psi compressing the spring/gas bag. The disc has spring-loaded fingers that when full compression of the spring/bag is reached fit into holes in the side of the cylinder fixing the disc and spring/bag in place. At surface, the cylinder cap and valve are fixed to a fitting that opens the valve. A second device pushes the disc fingers back into the cylinder allowing the cap to slide freely. Once released the water in the cylinder is forced through the valve by the spring and gas bag pushing on the disc. This water turns a flywheel or a turbine or dynamo that runs a winch that raises the trap.
 — fried dwight, Sep 23 2011

procuct?
 — po, Sep 23 2011

self-raising crab traps sound great. this avoids the problem of lost crab traps after the rope to the buoy breaks (due to weather and/or rival crabbers).
 — sninctown, Jan 04 2020

 So storing water pressure energy from desired depth that can be utilized at surface?

Would the energy in the cylinder be enough to hall the drag of a full prawn trap and a, yet to be determined, cylinder volume containig 400+ psi of seawater up on to the deck?
 — wjt, Jan 04 2020

 No, because there are inevitable losses through friction and the non-adiabatic compression of the gas.

 There are two intertwined ideas; the self-recovering trap, and the recovery of lost potential energy. The second is dubious; since the trap is only just negatively buoyant, the energy to recover it is mostly going to be drag as it is raised. The faster it's pulled up, the more energy is consumed.

 Self-recovery involves engineering a change of buoyancy. The simplest scheme is probably the best - a container of rocks, sufficient to give the pot negative buoyancy, held closed by a catch with a water-soluble element. Tha makes it very failsafe. Best would be three containers; when one releases, the pot gains slight positive buoyancy, two gives it lots. The chances of all three mechanisms failing is proportionately small.

Put a lightweight mast with a passive radar reflector on the top to make the floating pot easy to spot.
 — 8th of 7, Jan 04 2020

 Simplicity is indeed best.

A general rule of thumb is that any mechanism more sophisticated than a brick will fail at sea.
 — MaxwellBuchanan, Jan 04 2020

So still a good idea then, but needs a totally different implementation to use the brick to get the pot up with that little extra jump from surface to deck.
 — wjt, Jan 05 2020

So, not "self-rising porn trap"?
 — xenzag, Jan 05 2020

 Only you* would think of that, [xen].

* and of course various members of [MB]'s family.
 — 8th of 7, Jan 05 2020

*sigh* This constant besmirching of our family names saddens us. Can I remind you that Vinnie is still on holiday from prison?
 — MaxwellBuchanan, Jan 05 2020

 // This constant besmirching of our family names saddens us. //

Then you should get your family to stop doing it.
 — 8th of 7, Jan 05 2020

An alternative to the self-raising prawn trap would be the self-raising prawn. Simply create a bicarbonate-based snack that is irresistable to prawns, and shovel it over the side of the boat. It sinks, the prawns eat it, and their digestive juices cause it to bubble up. After a couple of hours, you just have to skim the prawns off the surface of the water.
 — MaxwellBuchanan, Jan 05 2020

"Another pint of fizzy prawns, landlord, and put a good head on it ! "
 — 8th of 7, Jan 05 2020

 A general rule of thumb is that any mechanism more sophisticated than a brick will fail at sea. — MaxwellBuchanan, Jan 04 2020

This device sounds so reliable it brings to mind the tragic death of Audrey Mestre.
 — normzone, Jan 07 2020

I fell into the trap but shouldn't this have autoboner's warning.
 — wjt, Jan 08 2020

 // procuct? //

 Huh?

 // this avoids the problem of lost crab traps after the rope to the buoy breaks //

 No, because it's specified to use a winch and the rope to pull itself to the surface.

 // So, not "self-rising porn trap"? //

Aha!
 — notexactly, Jan 15 2020

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