Once a keel-cooling system has been worked
out, you can turn your attention to configur-
ing a drystack exhaust to go with it. The fun-
damental concern is that a drystack gets ex-
ceptionally hot. As we saw in Chapter 7, the
exhaust gas temperatures can reach around
1,200°F (648°C). Temperatures on the outside
surface of uninsulated mufflers and exhaust
pipe routinely reach between 400° and 900°F
(200° and 480°C). Under insulation, these sur-
face temperatures can exceed 1,000°F
(540°C). Though it’s fairly common and ac-
ceptable to use carbon steel mufflers and
pipes, carbon steel starts to deteriorate and
scale when temperatures reach 1,000°F
(540°C). For this reason, stainless is superior
for insulated piping and mufflers. This is even
more important in the marine environment.
High temperatures create the two overrid-
ing considerations in drystack exhaust design:
- Protect the hull structure, machinery,
and crew from burns and heat damage.
- Allow for the expansion and contraction
of the exhaust piping to avoid cracking.
The other prime considerations—as with
any exhaust—are ensuring that back pres-
sure is not excessive and preventing noxious
fumes from entering the boat. Following the
engine manufacturer’s recommended ex-
haust diameter is usually sufficient. If the ex-
haust run is particularly long, though, it may
be necessary to increase diameter by one
standard pipe size.
The Basic Drystack Configuration
One of the common arguments in favor of dry-
stack exhausts is their simplicity. As we’ll see,
this “simplicity” may be an illusion in proper
installations. Nevertheless, the most basic
drystack exhaust installation runs directly up
from the engine and out the deck or cabin roof.
Straightforward as this appears, however, the
following standard components and consider-
ations must be included or allowed for.
Requirements for Drystack
Exhausts
- Flexible stainless bellows (or flexible
dry exhaust hose of stainless) connect-
ing the exhaust pipe to the exhaust-exit
flange on the engine to allow for expan-
sion and contraction and to prevent
cracking from engine vibration
- Insulation or adequate clearance from
the hull and other machinery
- Pipe supports or hangers that allow for
pipe expansion and that won’t transmit
heat to the boat’s structure
- Provisions to keep water out of the en-
gine, both at the exhaust outlet and
with a water trap at or near the bottom
of the vertical exhaust exit pipe
- Provisions for additional ventilation to
carry away the extra heat from the hot
exhaust piping and to cool the exhaust-
pipe or exhaust-trunk area
CHAPTER 9 Dry Exhaust Systems