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

There’s nothing better than a large

diameter and the fewest possible large-

radius bends in an exhaust run to keep back

pressure down. The exhaust line diameter

chart in Figure 7-3 gives recommended pre-

liminary design diameters for wet exhaust

systems for engines of various horsepower,

but you should confirm these dimensions

with your manufacturer. Greater diameter

doesn’t hurt, but it is more expensive and

more difficult to fit in the boat. A rough guide

is that for every 12 feet (3.6 m) over the first

12 feet (3.6 m) of exhaust run, the exhaust

diameter should be increased by^1 / 2 inch

(12 mm).

Watering the Exhaust

Of course, almost all yachts (and many small

commercial vessels) have wet exhaust sys-

tems, which is our focus here. The engine cool-

ing water is injected into the exhaust line just

after the exhaust riser begins to drop. This

serves two important functions. First, it cools

the exhaust run tremendously. The dry sec-

tions of, say, a diesel exhaust reach 1,200°F

(648°C). That’s hot enough to start wood, oil,

or cloth burning pretty quick! What’s more, this

heat is radiated into the engine compartment,

reducing power potential. Cooling the exhaust

with water eliminates these difficulties. The

expansion of the cooling water into steam and

the process of mixing with the exhaust also

quiets the exhaust appreciably.

Formula 7-1 is a good guide to the volume

of water needed for adequate cooling.

Formula 7-1. Water Volumes for

Adequate Cooling

GPM=cu. in.× rpm ÷66,000

PART THREE: EXHAUST SYSTEMS

Figure 7-3.
Exhaust line
diameter as a
function of engine
power for gas and
diesel engines

The Meaning of Pressure

Atmospheric pressure, roughly 14.7 lb./sq. in. (psi) at sea level (101.3 kPa), envelops us all

constantly. When engine pressures (or vacuums) are measured, they are measured relativeto

this surrounding air pressure, not added to it. This is known as gauge pressure.The instrument

used is essentially a barometer, though in engineering work it’s termed a manometer.Manome-

ter and barometer readings are given in a number of different units, of which psig(pounds

per square inch gauge),in. H 2 Oormm H 2 O(inches or mm of water), and in. Hgormm Hg

(inches or mm of mercury) are three of the most common.

Roughly

2.5 in. Hg = 34 in. H 2 O = 1.2 psig = 63 mm Hg = 864 mm H 2 O = 8.2 kPa

2.0 in. Hg = 27 in. H 2 O = 1.0 psig = 51 mm Hg = 686 mm H 2 O = 6.9 kPa

Althoughpsigis the technically correct terminology, many references drop the gforgauge

as it is taken for granted. In metric measure, pressure is given in kilopascals (kPa), which also

should be kPag,but again the gforgaugeis almost always omitted.

Formula 7-1.