Boat Forum / Cruising / March 2006

No, most water pumps are vane type impeller pumps. the vanes will block
the free passage of air.

I assume that this is a rdaw water cooled engine. When was the last
time you pickled your engine?  When you heat sea water above 145
degrees the carbonates (building blocks of rocks) will precipitate
(drop out) and form on the hot engine parts, especially the exhuast
manifold.  The carbonates are removed by 'pickling' using a boiler
descaling compound (doesnt hurt the metal) or muriatic acid.  Such
descalers/acids are pumped through the raw water side and allowed to
dissolve the 'salts'.  Another possibility of cooling water blockage is
degradation of the cast iron casting, especially in the exnaust
manifold.  Cast iron when it cools in the mold when manufactured cools
and stratifies like an onion and when the internals start to corrode
corrosion attacks one layer of the 'onion' at a time which will give
off huge 'slabs' of rust. If the slabs are free they will migrate and
block (usually) the outlet side of the exhaust manifold. When you shut
down the slabs will usually fall to the bottom of the manifold and the
engine will 'act normal' until either a slab gets caught in the flow or
another breaks off and blocks the outlet.  Typically one has to remove
the manifold, turn it upside down, reach inside it with a stiff wire
and break up all the slabs, then shake them out.  A radiator repair
shop that has ultrasonic cleaning means will be able to better clean
out such a manifold.   To prevent slab rust, never store the engine
'dry'.  Always long term store the engine filled with an antifreeze
that has corrosion inhibitors in it.  Cast iron is a good material vs.
water.  Hot water will promote a 'good' from of rust (black/blue) which
is ferrous oxide - a 'blueing'.  Cold water and or especially letting
the cast iron dry out will promote 'bad' destructive rust (red/brown)
ferric oxide.

Hope this helps.