resulting suction, combined with gravity, will
ensure that all or almost all the exhaust exits
underwater at speed.
Underwater Nacelles
The suction effect is enhanced by surrounding
most of the main exhaust outlet’s underwater
exit with a fairing nacelle to generate suction
through hydrodynamic lift. Figure 7-26 shows
how this was done on the Schnell boats,
which I think is ideal. On the Cape Dory 40,
Figure 7-27 shows very large exhaust nacelles.
These worked fine, and the boat handled well
in all conditions, but I think I would cut these
very large projecting nacelles down by two-
thirds to more closely approximate the
smaller Schnell boat nacelles.
Underwater Location
The Schnell boat’s underwater exhausts
exit near midships and well ahead of the
propellers. This is a potential source of
trouble. It is possible that the exhaust gases
can get trapped under the hull and flow
back into the running gear. In the case of
the Schnell boat, this configuration worked
because the exhaust exits were well for-
ward of the running gear, and the exits—
though underwater—were up near the turn
of the bilge on a round-bilge hull. This al-
lowed the gases to escape up and outboard
without being directed aft and down. On a
hard-chine hull—with the exhaust exits
under the chine—this might not work and
could cause a problem.
The Cape Dory 40’s exhausts exit as far
aft and as far outboard as possible. All is aft
of the running gear. This ensures that there
is no possibility of the exhaust gases being
trapped under the hull or ventilating pro-
pellers or rudders.Chapter 7: Wet Exhaust Systems
Figure 7-23. Inside view of the Cape Dory’s
underwater exhaust
Figure 7-22. Cape Dory 40 with underwater
exhaust outlet
Figure 7-24. Plan
of the Cape Dory
40 ’s underwater
exhaust system.
Item #9 is the idle
bypass, and #10 is
the underwater
exhaust.