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

all the plumbing can be of exhaust hose. Light
as the muffler and hose components seem,
they’ll weigh more when filled with water. All
these components must be very firmly fas-
tened in place. As always, use only true
exhaust hose and double stainless steel hose
clamps at all hose connections. Be certain the
hose is protected from chafing on other ma-
chinery, on the hull, or where it passes through
bulkheads and panels. To keep the vertical
clearances constant when a sailboat heels, you
should try to mount the muffler on or close to
the centerline. You mustn’t have any sags or
dips in the line where water can collect.

Canister Versus Lift-Line Size

When you shut off the engine or gen set, all the
water in the exhaust lift will run back down into
the muffler. Clearly, if there is too long a run
and too small a waterlift canister, the water will
overflow the canister and back up into the
engine. This is a frequently overlooked pitfall.
The safe rule of thumb is that the muffler can-
ister should have 130 percent more volume
than the entire lift line, from the canister to the
highest point. (Aft of that point, the water will
run out the transom.) How do you check this?
Simple, as the following formula shows.

Formula 7-2. Volume in Canisters or
Exhaust Lift Lines

Canister Volume (cu. in.) =canister

radius (in.)× canister radius (in.)×

3. 14 × canister height (in.)

Exhaust Lift-Line Volume (cu. in.) =

hose radius (in.)× hose radius (in.)×

3. 14 × length of lift run from muffler to

highest point (in.)—both ways, from

the manifold to the canister and from

the canister out and up to the highest

point aft of the canister

Or

Canister Volume (cu. cm) =canister

radius (cm)× canister radius (cm)×

3. 14 × canister height (cm)

Exhaust Lift-Line Volume (cu. cm) =

hose radius (cm)× hose radius (cm)×

3. 14 × length of lift run from muffler to

highest point (cm)—both ways, from

the manifold to the canister and from

the canister out and up to the highest

point aft of the canister

Chapter 7: Wet Exhaust Systems

Formula 7-2.

Formula 7-3.

Divide the muffler canister volume by the

exhaust lift-line volume. If the answer is 1. 3

or larger, everything’s OK. If the answer is

less than 1.3, you need a larger-capacity wa-

terlift muffler.

Standard Waterlift Muffler

Dimensions

The standard minimum diameter for a water-

lift muffler is 4 times the square root of the

inlet pipe area. Height is usually equal to di-

ameter or a bit more. Remember, this is the

standard minimum. A larger muffler may be

required as described above. Using this rule,

a 3-inch (75 cm) wet exhaust inlet would re-

quire a 10.6-inch (266 mm) diameter canister

about 11 to 12 inches (270–300 mm) high.

Formula 7-3. Standard Minimum Diame-

ter for a Waterlift Muffler with 3-Inch

(75 mm) Inlet

or

Canister Dia.

in

=× 10.6 in.

⎛

⎝⎜

⎞

(^4) ⎠⎟ =

3

2

2

π

.

Canister Dia.

mm

=× use

⎛

⎝⎜

⎞

(^4) ⎠⎟ =

75

2

265 8 2

2

π .; 6 66 mm

Greater diameter and/or height (more volume)

never hurts.

It is also possible to flood the waterlift

back up into the engine if an engine is hard

starting. Repeated cranking will fill the

waterlift canister with water, with no

exhaust-gas pressure to push the water out

the tail end. This is another good reason to

fit the largest waterlift canister you can. It is

also often worthwhile to have a drain cock on

the bottom of the canister to draw off water

should there ever be an engine problem with

multiple crankings and no start.

Use Fat Hose in the

Long Run

Very long lift runs increase back pressure. You

should try to avoid lift-line runs greater than

30 times the exhaust-line diameter from the

engine. If a boat has a 2^1 / 2 - inch (65 mm)

exhaust-line diameter, the run shouldn’t be

over 75 inches, or 6 feet 3 inches (1.95 m),