Dan,
If you have the room (Hull depth), the correct answer is to drain into a
grey water sump tank using a conventional water trap. The grey water tank
accepts all waste water from the showers and the sinks on board. The sump
tank has a float control where the pump turns on at the high level and off
at the low level. These levels should be adjustable. The output line should
also have an anti siphon valve attached. This arrangment in some cases saves
room and also saves unecessary tru hulls. It is also impervious to heel. An
additional input should be provided for salt water flushing.
Steve

"b393capt" wrote in message
oups.com...
Where could I find some good insight into designing the plumbing to get
water out of my shower drain to an underwater thru-hull, and staying
out of the shower? The plumbing in my Beneteau allows some water to
come back into the shower drain at the base of my head, enough that
when I heal the boat, it coats the bottom of the head floor.

Seems after the pump stops, the extra water in the hoses to the pump
slowly let out water, maybe only 4-6 ounces, but enough to fill the
bottom and come up a small amount above the drain.

I suspect its designed poorly and I could upgrade it myself. I noticed
for example the diaphram pump wasn't design to be connected to
underwater thru-hulls.

The current plumbing has a hose running up then down the wall to a thru
hull, in a loop,with an in-line filter and a diaphram pump at the top
of the loop. After I pump the water out of the head, even if I close
the thru hull, water runs back into the shower drain within a minute or
two. If I stick a cork in the bottom of the drain, the water gets
around it and forms a tiny pudle in the middle of the head anyhow.

Dan