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

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:

1. Protect the hull structure, machinery,
   and crew from burns and heat damage.


2. 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

1. 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


2. Insulation or adequate clearance from
   the hull and other machinery


3. Pipe supports or hangers that allow for
   pipe expansion and that won’t transmit
   heat to the boat’s structure


4. 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


5. 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