small cars (like the original VW Beetle), and
even some airplane engines—are, indeed,
simply air cooled. This means that the engine
is specifically designed to be cooled entirely
by air flowing over it. In fact, some small boats
have used low-powered, air-cooled engines.
This is simple and free from water-pump or
corrosion problems, but—deep in the bilge—
it’s impractical to air-cool anything but diminu-
tive engines, and it’s not as efficient as using
the much colder and denser seawater sur-
rounding the hull for cooling.
Keep in mind that engines shouldn’t run
too cold, though. That’s inefficient as well. In
theory, the hotter an engine can run, the more
efficient it is, but maximum possible operat-
ing temperatures are constrained by the lim-
its of the lubricant and the structural materi-
als we have to work with. Cooling-system
thermostats on typical diesels are set to start
to open (reduce coolant temperature) at
around 160°F to 180°F (70°C to 82°C) and
fully open at around 180°F to 200°F (82°C to
94°C), with maximum allowable water tem-
perature just under freshwater boil, around
210°F (98°C). These are the optimum running
temperatures for most standard-engine cool-
ing systems. High-performance engines, run-
ning a 50/50 mixture of antifreeze and water
under pressure, can run at coolant tempera-
tures up to around 260°F (125°C). (Antifreeze
is usually ethylene glycol or the newer pro-
pylene glycol.) The 50/50 mixture also protects
against freezing down to about −40°F (which
also happens to be −40°C).
There are three basic methods for using
seawater to cool engines: raw-water cooling,
heat-exchanger cooling, andkeel cooling.Raw-Water Cooling
The simplest water-cooled system ingests wa-
ter from outside through a seacock and then
a strainer to provide seawater directly to the
engine. The usual path is from the strainer
through the oil cooler, next through the ex-
haust manifold, then through the engine
block, and finally injected into the exhaust
and overboard. This is termed raw-water
coolingordirect cooling (Figure 8-1). The ad-
vantage is that it is simple and light. The dis-
advantage is that the raw salt water (with al-
gae, silt, pollutants, and other impurities)
runs through the engine itself, where it can
cause both corrosion and fouling (“silting”)
inside this most essential machine. A thermo-
stat can and should be fitted. This switches
some portion of the water flow from the wa-
ter injection into the exhaust to recirculate
through the raw-water pump and back into
the engine, until the engine reaches the
proper operating temperature.
Some raw-water systems aren’t fitted
with the thermostat and raw-water recircula-
tion and so tend always to run cold, which is
inefficient, as we’ve seen. Even with a ther-
mostat and recirculation, the fluctuations in
seawater temperature and the fact that this
isn’t a controlled closed circuit result in
poorer engine temperature control. Raw-waterChapter 8: Engine Cooling Systems and Their Exhausts
Central or Shared Cooling Systems
When two or more engines (or an engine and a generator)are installed, sharing a single
cooling system (cooling circuit) can appear to simplify things. The advantage of apparent
simplicity can be deceptive, however. Problems like the chance of unequal water flow are all
too likely. External head pressures may be too great, and if there’s a serious problem in the
single cooling system, then you lose all engines. Troubleshooting is also complex. With two
engines and a generator sharing a single, central cooling system, there are 162 possible
operational permutations!
On a shared system there must be independent flow control through each engine; the
heat exchanger or keel cooler must be able to deliver proper cooling at all engine loads.
Balancing coolant flow through engines operating at different loads is difficult. An engine
running at full throttle is pushing much more cooling water than one running at just over idle.
For these reasons—though it can be done with very careful engineering and intelligent crew
management—shared cooling systems aren’t recommended.