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

create a dangerous situation that doesn’t

exist when you’re underway. You should

study both ABYC TH-22, Educational Infor-

mation about Carbon Monoxide, and A-24,

Carbon Monoxide Detection Systems. Use

the information both to help work out your

exhaust installation to minimize CO on board

and to give early warning of a CO problem.

Summary of Requirements for a Reliable Wet Exhaust System

We can summarize the requirements for a re-

liable wet exhaust system, both for installa-

tion and for operation and maintenance:

1 .ideally, raw-water intake from sea chest

(best two-sided), see Chapter 17

2 .seacock at water intakes, see Chapter 17

3 .duplex sea strainer, see Chapter 17

4 .quick access to raw-water pump

5 .replacement impeller always ready;

change every 6 months

6 .regular change of heat-exchanger zincs

7 .regular inspection and flushing of the

heat exchanger

8 .custom dry exhaust riser of Inconel or

Hastelloy C

9 .precautions against backflow of any

water into the engine

10 .all components in exhaust line should

be hose or FRP

Review of Requirements

We should go down the list to see what each

item means.

1. WATER INTAKE FROM A SEA CHEST(BEST

TWO-SIDED).

One potential problem with wet exhaust is

sucking something into the raw-water intake

and starving the engine and exhaust for wa-

ter. Usually this is a plastic bag. Sometimes

it can be seaweed or kelp. The optimal solu-

tion is a sea chest that opens on both sides of

the keel with a flush grate on either side.

With such a sea chest, you would have to

have a plastic bag stuck against both sides

of the keel to block water flow. This is virtu-

ally impossible. What’s more—with the in-

take grates flush—the forward motion of the

boat will sweep away any obstruction. I’ve

never had such a sea chest blocked—ever.

Not all boats have keel configurations that

will allow such a two-sided sea chest, how-

ever. In this case even a single-sided sea

chest with a flush grate is extremely unlikely

to be clogged, because water flow will still

sweep away any obstruction. There are

many very successful vessels, though, with-

out sea chests. Sea chests are not a require-

ment, but they are a plus when it’s possible

to install them. (See also Chapter 17.)

2. SEACOCK AT RAW-WATER INTAKE.

This is plain enough. It’s a standard basic

safety requirement. See Chapter 17.

3. DUPLEX SEA STRAINER.

These ensure that sand or mud—which can

get past the sea chest grates—can’t clog the

raw-water intake lines or pump. The sea

strainers should each be as large as possi-

ble and be switchable. This way, if one

clogs up with silt at sea, you can switch

over to the other without missing a stroke,

and then clean out the gunk from the now-

disconnected intake. See Chapter 17.

PART THREE: EXHAUST SYSTEMS

Figure 7-30. The Centek VernaSep muffler