am i missing something obvious?

[shipto]

My idea is a firm but flexible tube extruded to have air pockets. take a tank of water with a lip seal at the bottom to suit the tube. feed the tube into the tank pass it over pulleys to form a loop then fill the tank. the air pockets would float up hopefully with enough power to pull the tube through the lip seal.

To partly answer my own question I suppose that the pressure to seal the water around the tube may exceed the the force gained from those air pockets floating to the top?

[onthecuttingedge2005]

I assume you would being using a seal for the bottom of the container. this seal will cause vacuum issues and friction(drag) issues.

the weight of the boxes outside the container will out weight the bouyancy of the boxes in the container which is another issue.

just with these three issues it will not work. there are more issues when your done with those.

[Comassion]

The main reason that this won't work is because a vertical tube entering through the bottom of the container isn't actually buoyant.  In order to float upwards, water must be able to move underneath whatever is supposed to float.

[Cherryman]

The main reason that this won't work is because a vertical tube entering through the bottom of the container isn't actually buoyant.  In order to float upwards, water must be able to move underneath whatever is supposed to float.

So if i put a piece of wood (or any other floating stuff)  straight on the bottom of a water holding compartment it will not fload up? 

[Comassion]

So if i put a piece of wood (or any other floating stuff)  straight on the bottom of a water holding compartment it will not fload up?

Even then, water can still slip between the bottom of the tank and the wood surface.  However, if you put a vertical wood rod through a correspondingly sized hole in the bottom of a water tank with just a little bit sticking out of the bottom, you'll see the wood rod drop through the bottom rather than float upward.