through remote V-gears. The ratio of cooling
water in the exhaust to that in the bypass line
not only varies from engine to engine but
with the same engine in different installa-
tions. You must consult the manufacturer,
and even then it’s a good idea to provide
adjustment to the flow. A valve on the bypass
line does this nicely.
Close the valve off, and 100 percent of
the water will exit thorough the main exhaust
piping. Open it completely and, if properly
sized, 70 percent or more will exit via the by-
pass line. Somewhere in between is about
right. The correct setting is found during sea
trials with reference to the exhaust pyrome-
ter and manometer. Once set to the right mix,
the bypass-adjustment valve should be wired
in place. A less expensive option is to insert
variable-line reducers or plugs manually fit-
ted into the bypass pipe. This is more difficult
to adjust but is OK once the right combina-
tion is found, since the adjustment only
needs to be done initially.
Mixing and Matching
Following the principles for each type of
exhaust system, you can mix and match
exhaust system types to solve specific instal-
lation problems. Figures 7-19 and 7-20 show
profiles of the exhaust system of a 72-foot
(22 m) schooner from my office. As often
happens on sailboats, the engine installation
was in a restricted space, and in this case the
engine was reversed to drive a remote
V drive. There was no room to shoot straight
up with a conventional standpipe. Here I
used a dry exhaust system running up and
aft, with water injected at the end and then
into a standard waterlift. (See Chapter 9 for
details of dry exhaust installations.)
Going Underwater
So far, we’ve been discussing standard above-
water exhaust outlets. If you could exhaust
underwater, then you would have no smell
and minimal noise from the exhaust port.
Even better, if you install an underwater out-
let correctly, the motion of the water under
the hull and past the outlet can generate a
slight suction for a small additional “turbo”
boost.
Underwater exhaust outlets have many
potential pitfalls, however. There can be too
much back pressure; exhaust gases can find
their way into the propellers, causing venti-
lation and loss of thrust; exhaust gases can
cause the rudders to ventilate; or exhaust
gases can increase the likelihood of sucking
air down under the hull and, on some plan-
ing hulls, lead to dynamic instability.
All these problems can be avoided, and
underwater exhaust outlets can be quite suc-
cessful. Figure 7-21 shows the underwater
exhaust on the highly efficient, 40-knot,
116-foot (35 m) German Schnell boat from
World War II (known to the Allies as
“E-boats”). And Figure 7-22 shows the Cape
Dory 40, which was fitted with a very suc-
cessful underwater exhaust.Requirements for
Underwater Exhaust Systems
To avoid back pressure, there must be an idle
bypass line forked off the after third of the
main exhaust line. The rule of thumb is that
the bypass line should be one-third the diam-
eter of the standard wet exhaust line. At
speed, there is no problem with back pres-
sure, as the water rushing past the sub-
merged outlet creates a suction. At low speed
and at idle, however, the exhaust would have
to push out against the static head of water,
which would be far too much back pressure.
The idle bypass simply provides an exit
for exhaust gases at low speed. There is less
water pumped through at low speeds, and all
or virtually all of the water will exit satisfac-
torily through the underwater outlet. It’s best
to branch the idle bypass at a distinctly sharp
angle off the main exhaust line, and to run the
bypass line upward. In fact, the idle bypass
line on the Cape Dory 40 can be seen in
Figures 7-23, 7-24, and 7-25 (as it can on the
Schnell boat drawings). On the Cape Dory,
the sole problem with the arrangement as
drawn and installed was that at idle the by-
pass would sometimes spit a small trickle of
black, sooty water, which was unsightly. If I
did this again, I would probably raise the exit
of the idle bypass somewhat higher to reduce
this, even though the problem was minimal.
No valve is needed to control or close the
idle bypass. The motion of the boat andChapter 7: Wet Exhaust Systems