Something on the radio just now pointed to the latest research into
the fluid dynamics of emptying drains. I couldn't find a link on the
beeb, no doubt it will turn up when they have finished broadcasting
it. (Sunday perhaps?)

Apparently the speed at the botton of the vortex is slower than at the
top. (Just as in a stream the top centre will be moving very fast but
the lower reaches relatively slowly.)

I didn't catch the rest but it seemed obvious what must happen next
even in a perfectly symmetrical vessel.