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

or

LPM=L× rpm ÷ 285

Where

GPM=flow in gallons per minute

LPM=flow in liters per minute

cu. in. =engine displacement in cubic

inches

L=engine displacement in liters

rpm=maximum engine rpm

Keeping the Water Where

It Belongs

Although water injected into the exhaust cools
it (and quiets it), the water also creates a new
set of potential problems. Perhaps the most
important of these is that you absolutely must
prevent water—that is, anywater at all—from
working back up into the engine. Even small
quantities of saltwater vapor in the turbo can
seize it up fast. Water will also deteriorate the
exhaust and valve mechanism of even a natu-
rally aspirated engine. The consequences of a
solid slug of water working back up past the
manifold don’t bear thinking about.

SLOPE ANDRISE Figure 7-1 shows the prin-
cipal features that should be built into an ex-
haust run to keep water out. The exhaust line
should slope downward aft from the engine,

ideally not less than^1 / 4 inch for every foot of

length (20.8 mm for every meter of length)—

more is definitely better, and the bottom of

the outlet pipe at the transom should be no

more than 1 inch (25 mm) below the static

waterline. (True underwater exhaust systems

are viable options. We’ll examine these later.)

The bottom inside surface of the highest

bend in the exhaust riser should be at least

22 inches (55 cm) above the highest waterline

on which the boat is likely to float. Indeed, it’s so

important to keep water out of the engine that

some references call for the bottom inside wall

of the exhaust riser pipe to be at least 24 inches

(60 cm) above the highest expected waterline.

This makes good sense. If you have an expen-

sive turbocharged diesel engine sitting in a

boat, and the boat’s exhaust riser is less than

18 inches (45 cm) above the real flotation water-

line, it would be a good investment to retrofit a

higher riser in the off-season. This is assuming

other internal-geometry constraints permit.

Risers: Water Jacketed

or Dry?

The exhaust riser itself poses an interesting

question: water jacketed or dry? A water-

jacketed riser (Figure 7-4) consists of an inner

Chapter 7: Wet Exhaust Systems

Figure 7-4. Water-

jacketed exhaust

riser