4. QUICK ACCESS TO THE RAW-WATER PUMP.
This is to ensure that you can clear any clog
or breakage in the pump. Clogging is un-
likely with the sea chest and duplex sea
strainer. In fact, on my designs so equipped,
it’s never happened yet. It pays to be safe,
though.
REPLACEMENT IMPELLER ALWAYS READY;
CHANGE EVERY 6 MONTHS.
The most common cause of raw-water pump
failure is for one of its impeller vanes to
break. With a spare always ready, a new one
can be popped in quickly. This, too, is un-
likely if you simply make a point of putting
in a new impeller every 6 months. This also
means you’ll be familiar with the process of
changing out the impeller should you ever
need to do it in an emergency.
REGULAR CHANGE OF HEAT-EXCHANGER
ZINC.
Too often the zincs are forgotten or ignored.
They play a vital role in protecting against
corrosion. Be sure to have spares, check
them regularly, and change them per the
manufacturer’s recommended schedule.
REGULAR INSPECTION AND FLUSHING OF THE
HEAT EXCHANGER.
A standard schedule of flushing the raw-
water heat-exchanger circuit with fresh wa-
ter will greatly reduce the chance of corro-
sion and silting.
8. CUSTOM DRY EXHAUST RISER OFINCONEL OR
HASTELLOYC.
As we’ve seen, standard engine exhaust risers
are water jacketed. This means the raw water
from the exhaust manifold is injected into a
double wall or exterior pipe that surrounds the
exhaust pipe to cool it. Water-jacketed exhaust
risers are a known source of corrosion. In most
of the designs from my office we discard water-
jacketed exhaust risers in favor of custom dry
exhaust risers made of Inconel or Hastelloy C.
These are exceptionally corrosion-resistant al-
loys. With no water jacket, the dry riser is insu-
lated or lagged all around (it’s fire hot) but is
small enough not to put too much heat into the
boat. At the end of the riser, the raw water is
injected again to cool and quiet the exhaust,
which then goes into the exhaust hose.
9. PRECAUTIONS AGAINST BACKFLOW.
We’ve seen that these are simple and
common sense. You don’t want water back-
ing up the exhaust line into the turbo or the
manifold. A waterlift muffler is good protec-
tion in itself. Also use surge chambers and
hinged flappers. Standard stuff, but vital.
10. ALL COMPONENTS IN EXHAUST LINE
SHOULD BE HOSE ORFRP.
By using only hose and FRP, you avoid
corrosion problems in the exhaust line. No
metal—even the best stainless—is fully
corrosion resistant.Chapter 7: Wet Exhaust Systems