DECEMBER/JANUARY 2018 67BEST PRACTICES: Exhaust
less water is being pumped into the sys-
tem), the valve handle must be labeled
as well as locked or removed to prevent
inadvertent adjustment.Materials
Many injected elbows are made of
cast iron, which is prone to rust and
scaling. is leads to internal and exter-
nal leaks, and debris that can block the
injection water and cause the elbow,
engine, and even the entire exhaust sys-
tem to overheat. I’ve encountered cast-
iron elbows that failed a er just 300
hours, so frequent close inspections are
necessary to check for cracks, burns,
and weeping, as well as discoloration
or plasticization of the hose immedi-
ately downstream of the injection
elbow—the precursors of potentially
catastrophic failures.
Homemade risers and mixing elbows
frequently prove unreliable, primarilythrottle rpm, wasting horsepower. is
is o en mistakenly “corrected” with a
reduction in propeller pitch. Addition-
ally, the correct back-pressure is
required for warranty compliance on
new engines, as well as for ensuring
that engines operate within their man-
dated emissions limits.
It’s common for wet exhaust system
designs to include a “performance-
enhancing” raw-water valve system
that diverts some cooling water over-
board rather than into the exhaust sys-
tem. More water in the wet exhaust
system than is necessary for cooling
means more e ort/horsepower must be
expended to expel it. A er the valve is
adjusted to ensure that the exhaust sys-
tem will not overheat throughout the
rpm range (wet exhaust systems o en
overheat at a lower rpm range becauseroom ventilation issues (see sidebar,
above)—the alarm will be of little use,
because it will sound under normal
operating conditions.
e dry portion of an exhaust sys-
tem, whether a dry, wet, or hybrid
design, should be equipped with a port,
typically a threaded ¼" or^3 ⁄^8 " NPT
hole, to measure exhaust system back-
pressure and temperature. ( e plugs
that ll these holes are notorious for
seizing; be sure to periodically remove
them.) Without this port, it is impossi-
ble to con rm whether an exhaust sys-
tem meets an engine manufacturer’s
requirements for back-pressure, while
exhaust gas temperature readings
can determine proper loading. Many
engines are unknowingly operated
under excessive back-pressure condi-
tions, which can diminish wide-openLeft—Frequently, back-pressure measurements are dismissed as unimportant.
Because excessive back-pressure is simply wasted horsepower, it’s often incorrectly
compensated for with propeller adjustments. However, exceeding manufacturer guide-
lines can affect emissions and void warranties. Right—To measure back-pressure,
the exhaust system must be equipped with a test port in the dry section between the
turbo/exhaust manifold and riser.On a valve that diverts some cooling
water overboard, a placard should warn
against the adjustment being unknow-
ingly changed. The handle can also be
removed.W
hile high surface temperatures are usually the result
of insu cient insulation, they may also indicate
inadequate engineroom ventilation. A few years ago, I
determined during the sea trial of a newly repowered ves-
sel that the dry riser, which was insulated with a hard-
coated material, was reaching temperatures in excess of
280°F (138°C). e re t yard and exhaust system manufac-
turer’s engineers crunched some numbers and concluded
that the engineroom air-turnover rate was insu cient, eventhough the di erence between the combustion intake and
ambient air temperature was below the engine manufac-
turer’s threshold of 30°F (1°C). Consequently, once they
installed a higher-capacity engineroom extraction fan,
along with two smaller fans to more thoroughly distribute
air, the dry-exhaust surface temperatures dropped below
the 200°F threshold, and no further changes were required.
(See “Venting the Engineroom” on ProBoat.com.)
—S.D’A.Engineroom Ventilation
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