[EMAIL PROTECTED] wrote:
> Excuse the crude graphics, but hopefully this will explain how Steve's
> idea would work. Best viewed in a fixed pitch font!
>
> | |
> | |
> /
> ___|__ /__|___
> | / |
> | __________/ |
> | |
> | |
> | |
> --------------------
> Caisson, sat on bottom of canal, water level in caisson same as outside.
> Boat in caisson and caisson doors shut.
> | | | |
> | | | |
> ___| / |___
> | / |
> | / |
> | __________/ |
> --------------------
>
> Pump water OUT of the caisson, caisson floats higher in the water, but
> the boat sinks to bottom of caisson, overall height above water level
> reduced.
Afraid not!
For every inch of water pumped out, the boat will drop one inch lower relative
to the caisson. But for every inch of water pumped out, the caisson will also
rise one inch higher in the canal. So the level of the top of the boat relative
to the tunnel stays constant.
This assumes that the sides of the caisson are thin enough that the outside and
inside dimensions can be considered to be equal.
If, however, you built it with very thick walls, such that the area of the
inside of the caisson was (say) only half of the area of the outside, then it
would only rise one inch for every two inches of water pumped out. But this
would require the outside dimensions of the caisson to be either much wider
than the boat (difficult in Froghall) or much longer (which might make the
bends at either end of the tunnel tricky).
Taking this one stage further (as mentioned in somebody else's posting), if you
make the outdside of the caisson either much longer or wider than the inside,
you can incorporate flotation chambers into the ends or sides (and ballast the
vessel as necessary to counter their effect), so that the water was pumped into
these rather then into the canal, and the caisson doesn't rise or sink at all.
That was the plan for a vessel of this type proposed for getting barges under
low bridges across the river in St Petersburg (probably still Leningrad in
those days) without holding up the road traffic by opening the bridges. I don't
think it was ever built.
A neat solution, but I'm not sure it's the right one for Froghall.
Martin L
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