Nearlyeveryinboardandinboard/ outboard (I/O) engine has exhaust manifolds and risers. But ask most people about
the life span of these essential compo-
nents, and you’re likely to get responses
ranging from “a few years” to “forever.”
Ponder for a moment that something
made of iron on your boat is exposed
to hot, acidic exhaust gas and seawater,
and you begin to realize that even “a
few years” may be too long. In reality,
boats operating in Florida may get only
two or three years out of risers and not
much more from manifolds, while those
in Maine may get five or six years. Boats
in freshwater areas can get a few years
more – maybe.
It’s the “maybe” part that’s a problem because manifolds and risers don’t
have an expiration date or a warning
light to tell you when they’re failing.
And when they fail, they can destroy
the engine. Damage to gas engines is
far more common, so that’s the focus
of this article.
What they do
Exhaust manifolds and risers are large
metal castings that carry hot exhaust
gases away from the engine on inboard
engines. All V- 8 and V- 6 engines, for
instance, have a separate exhaust manifold along the side of each cylinder
bank. The riser, which is often shaped
like an inverted U, is located at the aft
end or centered on top of the manifold.
Sometimes the riser slopes down from
the end of the manifold, if the engine
sits high enough above the waterline,
in which case it’s often called an elbow.
The exhaust hose is then attached to the
aft end of the riser or elbow.
What makes these cast-iron parts
unique is that they’re pipes within another pipe. This double-wall arrangement
allows hot exhaust gases in the internal
pipe to be surrounded by water in the
external pipe, called a water jacket, which
should remain cool enough to touch. At
the aft end of the riser, water from the
water jacket combines with and cools
the hot exhaust gases before continuing
out the exhaust overboard discharge.
Without the cooling effect of the water,
the exhaust gas would overheat the manifold and risers, then burn through the
exhaust hose in short order.
Keeping the water and exhaust gases
separated until they exit the riser is
crucial. If water finds its way into the
gas-only chamber before the end of the
BY CHARLES FORT
Manifolds and risers are some
of the less-discussed “
consumables” on your boat. But when
they corrode internally, they
often destroy the engine
BLUE is cooling water
RED is exhaust gas
Corrosion between the water and gas
passages may let water into the engine,
which can destroy it.