Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

Chapter 9:Dry Exhaust Systems

6. The principal exit run of the exhaust
   should be as close to vertical as possi-
   ble, and the system should have as few
   bends as practical.

Figure 9-1 illustrates all the components
needed to meet the previous requirement
when the exhaust rises straight up from the
engine and exits the cabin roof immediately
above. This exhaust run is entirely in a ma-
chinery space, so only the bottom portion of
the exhaust and muffler is lagged (insulated),
while the upper portion is shielded with a
steel open-wire mesh or frame to protect the
crew from accidental burns. Then the exhaust
exits through a steel “funnel” (the bottom or
side portion of which may be aluminum). The
cap of this funnel has an air gap all around to
provide for a generous flow of hot air out.
Figure 9-2 shows how the air gap is cre-
ated all around. The exhaust outlet pipe is
welded to the steel (or stainless) cover plate.
The cover plate should be one standard plate

size thicker than the wall thickness of the

heaviest exhaust pipe. The lugs are the same

thickness as the cover plate. If they attach to

an aluminum funnel, they must be bent over

at an angle and bolted (rather than welded)

to the funnel. Since the pipe is rigidly at-

tached to the cover plate, it must be free to

expand below it, and this requires an expan-

sion slip joint with—2-inches (50 mm) mini-

mum travel. This may be fabricated as shown

in Figure 9-3.

Exhaust pipe for marine drystack appli-

cations is of standard weight (schedule 40 or

DIN 2448) in carbon steel and schedule 10 in

Figure 9-1. Basic

drystack exhaust

Figure 9-2.

Section A through

the funnel cover in

Figure 9-1