heavily insulated (lagged, see Chapter 9) and
firmly fastened—which will raise the exhaust
at least 18 inches (457 mm) above the water-
line and the water injection point at least
12 inches (305 mm) above the inlet into the
waterlift canister.Deceptively Light
Keep in mind that one of the nicest things
about waterlift mufflers is that they’re light and
easy to install. The waterlift canister is usually
of fiberglass (metal ones are heavier, more ex-
pensive, and subject to corrosion), and almostPART THREE: EXHAUST SYSTEMS
Vented Loops: Whys and Wherefores
Obviously, vented loops are important—but what exactly are they? Well, if you remember
your high school science, a tube that’s filled with a liquid will transfer that liquid from a higher-
level container to a lower-level container even though the top of the tube may be far above
the top of either container. Yes, a siphon. As long as the tube is closed to the atmosphere, the
liquid “thinks” that the whole assembly—filled tube and two containers—is one. In our case,
the inside of the bilge or engine is one container, seawater is the liquid, and the ocean is the
second container—albeit an awfully big one!To avoid siphoning, all you have to do is open the tube to the surrounding air (even a
pinhole will do) and you have two separate containers—again, no flow and no siphon. Pic-
tured here is a simple form of a vented loop. (There are other varieties, but they all work on
the same principle.) The flapper is pushed up, sealing the vent opening by the pressure of wa-
ter or gases flowing through the pipe. At engine shutdown, when flow and pressure stop, the
flap opens, opening the line to the air and eliminating any chance of siphon backflow.
Sometimes these little siphon-break valves get salt encrusted and either jam open or jam
closed. It pays to inspect them periodically and to wipe them clean with fresh water. On engine
and generator installations, another option is to do away with the flapper or other valve
mechanism and run a long tube vertically up from the break opening at the top of the U and into
the cockpit. Without the flap, there is virtually no chance of clogging open or closed. The
drawback is possibly an occasional slight weeping of sooty water into the cockpit.
Every through-hull that exits 6 inches (150 mm) above the water or lower and is attached to
a hose or pipe leading down into the bilge—whether for engines, generators, heads, bilge pumps,
or whatever—should be fitted with a metal or plastic pipe vented loop or siphon break to elimi-
nate the chance of flooding (and sinking!) by siphon. Heads, though, ought not to be fitted with the
open tube into the cockpit—unwanted odors would be interesting, to say the least!Vented loop or siphon break