all the metal piping should be from schedule
80 (double-weight) pipe. The plate caps and fit-
tings should be as thick as or slightly thicker
than the pipe wall they’re fastened to.
THE MUFFLER OPTION By the way, some
folks—on low-powered displacement vessels—
don’t bother with a muffler on a standpipe ex-
haust system. Again, the process of mixing wa-
ter and exhaust gases in the standpipe—along
with the expansion and cooling that take place
there—makes it a pretty fair silencer in its own
right. Nevertheless, if you want a really quiet
boat, you won’t regret installing a separate in-
line muffler aft of the standpipe.
High-Powered Engines:
Excess Water
Some large high-powered diesels are engi-
neered to pump very large quantities of water
through the system. The more cooling water
available, the more you can use to cool in-
take air and fuel; the cooler the air and fuel
are kept, the more efficiently the engine
runs. This, however, poses a problem with
exhaust runs. Where most small to midsize
diesels can dump all their cooling water
into the exhaust, such large engines may
push so much raw water through the system
that dumping it all into the exhaust would
effectively clog it up, creating excess back
pressure.
The solution is to T off a branch of the
cooling water, directing, say, 60 percent into
the exhaust system and the remainder di-
rectly overboard though a bypass line. The
schematic in Figure 7-18 shows this installa-
tion on a design from my office with twin
MTU, V-12 4000 diesels, rated at 2,300 hp
(1,715 kW) each, and driving the propsChapter 7: Wet Exhaust Systems
Figure 7-17.
Details of a stand-
pipe exhaust