Keep It Down!
As we’ve seen, simple wet exhaust systems
decrease engine exhaust noise considerately
even without a muffler. Clearly, adding a
muffler—if you don’t already have one—will
reduce noise still further. Most marine in-line
mufflers (as opposed to waterlift mufflers)
work by breaking the exhaust flow with baf-
fles. These reflect the pulses (bangs) of the
individual cylinders back and forth so they
tend to cancel each other out. If you can find
the space, a larger muffler for the same ex-
haust line will almost always give a quieter
exhaust—it allows for more refractions to
take place in the short time the exhaust travelsthrough it. A little-known, extra step worth
consideration for the very quietest boat is to
wrap the muffler in 1 to 2 inches (25 to 50
mm) of fiberglass sound-absorbing insula-
tion. In and of itself, it won’t make the differ-
ence between a loud and a quiet boat, but it
will reduce the racket one step more—and
every step helps.Make Room for Exhausts
An often-overlooked aspect of exhaust sys-
tem installations is the amount of volume
they take up inside the boat (see Figure 7-9).
A pair of 300 hp (224 kW) diesels will usually
require 6-inch (150 mm) diameter wet ex-
haust systems. Standard in-line mufflers will
be roughly 1.7 times this diameter, and that
extra diameter will extend roughly 5.4 times
the diameter. So for a 6-inch-diameter
exhaust tube, the muffler will be about 10 to
11 inches in diameter and about 32 to
34 inches long; or for a 150 mm diameter
exhaust tube, the muffler will be about 255
to 280 mm in diameter and about 810 to
870 mm long.
Add up the exhaust tube, surge chamber,
muffler, brackets, and required access, and
you realize just how much space you must al-
low. This can’t be an afterthought. Indeed, if
you’re contemplating a repowering project,PART THREE: EXHAUST SYSTEMS
Figure 7-8.
Malleable iron
hose clamp
(Courtesy Buck
Algonquin)
Figure 7-9. Note
how much space
the exhaust sys-
tem requires