"Jonathan Ganz" wrote

The discharge is well above the waterline - at least 18" or so,

But where longitudinally? It, or the top of the loop, has to be above the
waterline when the boat is heeled to the deckedge. You many not sail that
hard much, but you want the margin. If you are sailing that hard because of
trying to get off a lee shore or being unable to make sail reductions, the
last thing you need is some bilge flooding.

I definitely need to have something to prevent backflow. We typically get
following seas, even coming back from someplace local. Come back through
the the Gate, as I did last Sunday, and you definitely get a big following
sea.

The amount of water a following sea will push back down the hose isn't
likley to be significant. A plain loop up to deck level at the transom will
take care of that.

What do you think about a vented loop near the transom as you suggest,
plus a check valve near the pump?

Repeat after me, NO Check Valves, except in hose ends of hand bilge pumps to
speed priming. They will always have enough crud in them to let the water
flow back slowly and increase the chances of clogging in the other
direction. Standard electric bilge pumps don't have enough pressure head to
push crud through. They can barely clear air locks.

Check first that you really have a back flow problem. Just replace the
check valve with a nipple connector and run it. If it doesn't cycle when
you shut it off, don't worry about it.

It sounds like your discharge is in the transom, if so, just add enough hose
to create a loop right there. I wouldn't worry about the siphon break for
sailing around the bay. If you get in a situation where you are flooded
enough to bring the static waterline down 18", you'll be on the radio
anyway. The waves coming up astern are not going to push enough water into
the hose to get over a high (unvented) loop and start a siphon.

The water comes up the counter and actually immerses the bilge pump
discharges on "Strider" at full power. I once flooded the bilges a bit and
started the pumps to see if I could get a siphon going. Didn't happen.
Flow past the openings at 6.5 knots makes it pretty hard for water to flow
back into the hoses after the pumps shut off.

--
Roger Long