Brian and Ian will be glad to hear that their contributions to the
thread on the design of the extended range bilge pumping system were
not if vain.

I have experimented with both the siphon and checkvalve versions of
the cofferdam idea and it has been very instructive. Both seem like
simple ideas that just have to work. If fact, both will if you are
dealing with rapid water level changes such as would happen with a
broken stuffing box hose. Most of the time though, the bilge system
will be experiencing very slow accumulations and level changes. Things
happen very differently at this time scale.

Check valves that work reliably at very low heads and slow level
changes are not a trivial engineering proposition. The very slow rise
does not put any pressure in the flap so water seeps in. This
equalizes the pressure on each side and the cofferdam fills up. If you
pre-load the check valve, it will prevent the cofferdam filling but
then there won't be enough difference in pressure to allow it to
empty. It can be done but would be so delicate an adjustment that it
would never work reliably in the real word.

--

Roger Long