Practical Bilge Piping
Diameter
As a general rule, ABS recommendations for
bilge-system piping diameter work well; how-
ever, keep in mind that the standard inlet/
suction and outlet/discharge port on the pump
you install is optimized for best pump effi-
ciency. Unless you are building a classed ves-
sel, it’s better to use the inlet and outlet size on
the pump as your bilge-pipe diameter. The ex-
ception is a long or convoluted pipe-exit run.
When such a run can’t be avoided, it will re-
duce friction head in the line to increase the
bilge-pipe diameter with an expander after the
pump-discharge port. Don’t do this ahead of
the pump suction/inlet port, however, and
don’t reduce the line diameter on the outlet
line after expanding it. The larger diameter
must continue through the through-hull outlet.
Thoughts and Recommendations on Bilge Systems for Boats
CFR Commercial and ABS, in varying de-
grees, approach bilge systems as if they were
installed on ships. The assumption is that a
fairly large professional crew will be intelli-
gently operating the system. Boats are differ-
ent; most are pleasure craft. As such, they
have a small crew, which—by definition—are
not professional. Even small passenger ves-
sels and fishing vessels have relatively small
crews and tight spaces (at least compared
with ships). Though a small passenger vessel
must comply with CFR Commercial, my pref-
erence is to install automatic submersible
bilge pumps in each compartment to meet orChapter 18: Bilge Systems, Fire Mains, and Fire Extinguishers
Figure 18-3. Bilge-
system vented-loop
location