water pumps in sailboat engine usually are a rubber vaned pump mounted offset
in a housing. As the impeller turns the vanes open up, scoop up water and
move towards the restriction of the offset where the pump outlet is. It works
pretty well and has been used on outboards for at least 65 years that I am
aware of. The vanes seal against the housing but the seal is nowhere near
absolute. If the seal were absolute this total discussion would never have
taken place.

Jeff,
I've no stake in this thread but I am curious. The pumps I've worked with
(granted they are not for marine use) don't provide much backflow prevention
when not operating. The rotary vane pumps (we used them for moving gasoline)
depend on centrifugal force (and sometimes small springs) to maintain a
reduced clearance with the housing (and also allow for some wearing of the
vanes). What is the nature of the pump to which you refer?
Scout

"Jeff Morris" wrote in message
...
Why? The siphon break includes a loop that is about a foot above the
water.
Do you know what a siphon break is? It would appear not.

Actually, the waterpump impellor normally seals the path, but if a vane
were to
break off and go unnoticed, there could be a direct path - hence the need
for a
loop and "vacuum valve," commonly called a siphon break.


"JAXAshby" wrote in message
...
then the exhuast system floods when the engine is shut off if the intake
is
left open.

... if the exhaust
manifold ( at the water injection point) is below water at any angle
of
heel,

no ****, Sherlock. **That** is forbidden. And it only took you nine
days
to
figger it out.

It IS NOT forbidden. It is very common. The requirement is to have a
siphon
break. Wake up jaxie, the majority of sailboats are setup like this,
including
the boat in question. All your huffin' and puffin' doesn't change the
fact
that
you made a major blunder and now you're trying to save face.