CHAPTER 8
In the previous chapter we discussed the
most common exhaust systems on boats: wet
exhausts. As we saw, these exhausts are
“wet” because engine cooling water is in-
jected into the gases exiting the engine both
to cool them and, through mixing and expan-
sion, to make the exhaust quieter. Such
exhaust systems have only a short dry por-
tion prior to water injection. You can, how-
ever, have an all-dry exhaust system with no
water injection at all. Such exhausts are, not
surprisingly, termed dry exhaust ordrystack
exhaust systems.
The advantage of a drystack exhaust is
that there is no seawater in the cooling sys-
tem; it’s 100 percent fresh water mixed with
antifreeze. This reduces the likelihood of cor-
rosion and eliminates any need for a seawa-
ter pump for cooling. We’ll make a detailed
comparison of the pros and cons of drystack
versus wet exhausts at the end of the next
chapter, but we’ll begin by examining the
engine cooling requirements particular to
drystack installations.
NOTE: Some references call the dry sec-
tion or dry line of a water-separator exhaust
system (see Chapter 7) a “drystack.” This is
not strictly accurate. It should be termed the
“dry leg of a water-separator exhaust system.”
When you see the term drystack exhaust,
double-check which type of exhaust is being
referred to.Engine Cooling
First, we need to take a step back and think
about engine cooling systems. All internal-
combustion engines produce large amounts
of waste heat that must be dissipated to avoid
overheating. Maximum combustion temper-
atures range between 3,000° and 5,000°F
(1,650° to 2,750°C). Very roughly, 30 to 35
percent of the total heat energy from the fuel
is removed through the exhaust; 34 to 39
percent actually goes to propel the boat; and
30 to 35 percent has to be removed by actively
cooling the engine itself. If engine cooling
fails, the lubricating oil begins to break down
at temperatures of 400°F (200°C); at around
500°F (260°C), the valve guides begin to
score and the valve seats start to fail; and
over 500°F (260°C), aluminum components
lose strength and break.
Lubricating oil, fuel evaporating in the
cylinders and manifold, and air around the en-
gine do carry away about 10 percent of the ex-
cess heat. Largely radiated into the air immedi-
ately around the engine, this heat is important,
and it is one reason for good ventilation. Re-
moving only about 10 percent of the excess
heat doesn’t do the job, however. Accordingly,
nearly all engines require a cooling system to
keep them from literally melting and seizing
up. Some small engines used outdoors—such
as in lawn mowers, chainsaws, some older