T&T: water line to dripless shaft bearings

[Chester Brummett]

I have dripless shaft bearings on my boat, which require cooling water.  Each
engine has a feed point just after the water pump, a nipple on the input side
of the first heat exchanger.  Tube size is 3/8 inch ID from this heat
exchanger to the shaft bearing.

When changing the engine water pump impellor, no way to shut off the water
comming back through the tube feeding the dripless shaft bearing.

I would like to install either a bronze cut-off valve, or prefer a high vent
loop (siphon break) since this would eliminate the possiblilty of accidently
leaving the valve closed.

I have been unable to locate a vent loop or bronze valve and nipples for 3/8
inch tube.

Any recommendation?

Related question--the boat was built with no cross feed between the two
dripless shaft bearings.  Thus, if I run on a single engine, the "off side" is
not getting a water feed.

The boat is a "low power" boat, twin 135 HP Perkins.  If running on a single
engine, my speed would probably not exceed 6 knots.  Don't know if water feed
to the engine-off dripless bearing at this low speed is critical.

Thanks in advance for any advice.

Chet
DeFever 49, Cygnet
Daytona Beach, FL