hull working—however slight—will be sure
to open a seam somewhere. Exhaust hose is,
again, the perfect solution, except where any
dry, hot exhaust gases may be present. Here,
you have to use flexible metal bellows of
Inconel and Hastelloy C or—not quite as
good, but the usually available standard—
type 316L stainless. Accept no substitutes;
get the alloy specs in writing before you
buy.No other metals will stand up to the ex-
haust’s high-temperature, corrosive, saltwater-
hydrocarbon-sulfur-sulfuric-acid mix.
A related consideration here is that on
FRP boats, it’s tempting to glass a long length
of rigid fiberglass exhaust tube right to the
transom. Don’t do it; it’s likely to crack. One
builder I worked with had been fabricating
their own glass exhaust piping for sailboats
and installing them this way for years. They’d
lucked out and had no trouble. I specified
that the exhaust tubing for the 420 hp
(313 kW) diesel on a new powerboat be con-
nected with flexible hose, but they persisted
in doing it their old way. Well, after just a few
hours of operation, the durn thing cracked
big time, both at the transom and at a bulk-
head. Luckily, someone noticed the water in
the bilge immediately and they were able to
limp home and retrofit with hose. I know
there are successful boats out there with long
fiberglass exhaust tubes glassed rigidly to the
transom, but any boat fitted this way is play-
ing a game of Russian roulette.
Metal boats with rigid metal exhaust
pipes welded to bulkheads and the transom
are OK, but they still are not ideal. The com-
bination of corrosion and thermal expansion
can cause cracking as well. Even on metal
boats, FRP exhaust tube and exhaust hose
are the best exhaust piping, and FRP piping
must be flexibly connected at all rigid points.
This, unfortunately, is not allowed under the
CFR for metal T-boats (inspected passenger
vessels, under 100 gross tons, and carrying
less than 150 passengers). I distrust metal in
wet exhaust systems, and I think the CFR re-
quiring it in metal hulls is a shame.
Fiberglass exhaust tube through water-
tight bulkheads should consist of short FRP
pipe fittings with flanges fastenerd to the
bulkhead and the tube projecting on either
side of the bulkhead. The flanges are bolted
to the bulkhead and caulked. Exhaust hose
connects to the pipe ends on both sides.
Some class or government regulations may
require a metal shutoff valve right at the bulk-
head. In this case, all-metal piping is required.Hose Clamps
As we’ve just seen, a leak into the boat from
the wet exhaust piping can sink a boat fast.
It can also spew deadly carbon monoxide
into the accommodations. You must use the
very finest quality all-316-stainless, T-bar
hose clamps. Use double hose clamps, and be
sure you can get at allof them for inspection
and tightening. I once surveyed a 36-foot
(11 m) lobster yacht that had its large aft
cockpit sole glassed in permanently. Under-
neath was the exhaust run, with clamps but
with no access at all. When I insisted that at
least a cover plate be cut and installed over
the exhaust hose where I could see (but not
reach) a single hose clamp, there was con-
siderable resistance. It was finally done, how-
ever, and I found that the single hose clamp
had never been tightened!
Note that traditional exhaust hose clamps
are of malleable iron (Figure 7-8). These usu-
ally were used singly, not in pairs. Malleable
iron hose clamps are still available and still
acceptable. I prefer double stainless steel, as
they seem more secure and longer lasting.Chapter 7: Wet Exhaust Systems
Figure 7-7. Exhaust tube glassed to tran-
som––not a good idea