T&T: water line to dripless shaft bearings

[LA Licata]

Chester,

I have done some research on this issue for my new trawler, and for  
the trawlers that the builder is constructing: At what boat / shaft  
speed is forced water cooling NOT required?

So far, I have checked with 3 or 4 manufacturers and the lowest speed  
was from a french company, which said "9 knots sustained." The other  
3 said 11+ knots.

Below that, all said that the bearing cavity only need to be full of  
water, which can be checked with vent line, and they said that if  
getting underway, one should check that the shaft volume is vented  
BEFORE getting underway.

If I am concerned if the cavity is not full while underway, I can  
fill it if I dip the vent valve into a pail or bucket of water before  
opening the vent.

The engineers also suggested that one "shoot" the face of the bearing  
while underway at "high speed" and note the temperatures before and  
after using the water from the pail trick. Eventually, one could tell  
just by the temps if the cavity is full of water or full of air.

If you can confirm the above is the same for your bearings, you may  
want to go "kiss."

Lee

On Feb 2, 2009, at 1751, Chester Brummett wrote:

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
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