Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

and outer pipe. The inner pipe carries the dry

exhaust gas from the manifold. The outer

pipe (the water jacket) is filled with engine

cooling water injected through an outside

tap. The advantage here is that the entire

riser exterior is relatively cool to the touch.

Also, it keeps less heat from radiating into the

engine compartment. Cooling water sprays

out into the exhaust gas proper through small

holes at the aft lower end of the edge of the

riser’s water jacket, where the water mixes

with and cools the hot gases directly.

There’s only one drawback—a potential

for corrosion inside the riser’s jacket itself.

For this reason, many commercial boats—

and hard-worked sportfishermen—are fitted

with dry exhaust risers. Here, the riser is

simply a pipe elbow with a tap on top aft to

inject cooling water directly into the gas flow

(Figure 7-5).(The injection should angle

down and aft, not at right angles to the riser

pipe, which could lead to splashback into the

turbo.) Even better, some manufacturers offer

water-injection rings that fit on the end of the

exhaust riser and disburse the injected water

evenly around the periphery of the exhaust

pipes.

Naturally, the hot riser has to be well

insulated (lagged) all around the outside to

prevent burns or chance of fire and to

reduce radiated heat. (See Chapter 9 for

more information on lagging.) Both water-

jacketed and dry risers work fine, and both

have their advocates. I lean toward the dry

riser—carefully insulated—as it seems to

have the least potential for long-range corro-

sion trouble.

Pyrometer Equals

Thermometer

Just aft of the water injection point is where

you should install an exhaust temperature

pyrometer.(Pyrometeris a fancy term for a

high-temperature thermometer.) This can be

rigged to a simple idiot light and alarm—or

better—-to a temperature indicator dial with

a programmable high-temperature alarm. In

either case, the idea is to give ample warning

if there is a reduction in cooling water. (A

pyrometer in the dry exhaust rise ahead of

the water injection is useful for tracking

engine load. It’s the best indicator of how

hard an engine is working.)

A water-flow sensor can be installed in

the injection line, just before entry into the ex-

haust riser. With the ignition on, it will sound

an alarm if the flow to the injection point

should stop. This is somewhat redundant with

the pyrometer, and the alarm inserted into the

exhaust system as described earlier; however,

it may be used in lieu of the pyrometer.

Metal’s Not So Hot

The metal parts of an exhaust system are

attacked by acids created in the fuel-water

mixture. Most stainless steels corrode in wet

exhausts. Inconel and Hastelloy C (nickel-

chrome-molybdenum alloys) are excellent,

though costly. Type 316L stainless (the L

stands for “low carbon”), which has some

molybdenum, is acceptable; 18-8, 302, and

304 stainless alloys should be avoided. Cop-

per, copper-nickel, nickel-copper, and Monel

can be used if high temperatures and stand-

ing water can be prevented, but copper and

copper alloys are for gasoline only—wet

exhaust diesel by-products (among them sul-

furic acid) attack the copper.

MANIFOLDCORROSIONDry exhaust risers, as

we’ve seen, can virtually eliminate corrosion

problems in the riser. One place where you

really can’t do much about corrosion in wet

exhaust systems is the water-jacketed ex-

haust manifold. Just like a water-jacketed

exhaust riser, the exhaust manifold is a

PART THREE: EXHAUST SYSTEMS

Figure 7-5. Water
injection
(Courtesy Vetus)