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

bare lickety-split, 1,200°F (648°C) exhaust

gases will momentarily be sprayed onto the

hose and FRP tube. For this reason, I believe

all fiberglass exhaust-line components (pip-

ing, mufflers, check valves, etc.) are best

made of fire-retardant isopolyester resin to

Milspec Mil-R-21607. Check with your manu-

facturer to see what they use. Built with this

goop, the fiberglass exhaust tube or hose

won’t smoke or burst into flames should the

water pump fail. And more than likely, the

crew will have enough problems to worry

about in such circumstances.

Support Your Local Exhaust

It’s also important that the entire exhaust line

be well supported with strong clamps and

brackets. When filled with water, the exhaust

piping can weigh a fair bit. When the vessel

is jumping from wave crest to wave crest,

resulting g-forces will break loose a flimsily

supported line. Naturally, a cracked exhaust

will admit not only noxious gases, but also all

that water you’ve been pumping through the

engine to keep it running cool. As many an in-

surance surveyor can attest, cracked exhaust

lines are quite an effective way to sink a boat!

Where brackets support hot, dry portions

of the exhaust system, the bracket must not

only be able to withstand the heat, but also be

insulated against transmitting the heat to the

hull structure, and the bracket must allow for

the expansion and contraction of the pipe from

full-throttle hot to full-off cold (see Chapter 9).

Penetrating Bulkheads

Bulkheads frequently need to be penetrated by

the exhaust line. This can actually be useful,

and the bulkhead makes a fine support. You do

have to be careful that there’s no chafe and

that there’s room for expansion. For wet ex-

haust systems in wood and FRP bulkheads,

the minimum clearance around the bulkhead

is 2 inches (50 mm). On a number of boats,

we’ve supported the exhaust pipe by cutting

the hole in the bulkhead 2inches (50 mm)

oversize and then wrapping 2 inches (50 mm)

of fiberglass sound insulation around the ex-

haust pipe where it went through. Stuffing this

in and closing it off with an aluminum finish

ring sealed the bulkhead for sound, eliminated

transfer of vibration, and held the exhaust in

place while allowing enough movement for

expansion and contraction.

Dry exhaust pipes need much more

clearance. Ten inches (250 mm) is the bare

minimum. There should be insulation around

the pipe (lagging) and around the bulkhead.

Tender Turbos

Turbochargers run at such high speeds

and temperatures that it’s critical not to place

any external strain on their housings. Such

strains can be enough to throw bearings out

of whack and create nasty headaches. Make

sure your exhaust piping—and anything else

for that matter—isn’t putting weight, a side

load, or other strain on your turbo’s case. Any

piping here must be firmly supported by and

fastened to the engine block itself, not by any

portion of the turbo.

Supporting piping on the engine block at

the turbo is OK, but common practice with

larger boats is to isolate the dry riser pipe

with the metal bellows, and the riser is sup-

ported by brackets to the vessel’s structure,

generally to the overhead (see Chapter 9).

Flexible Joints

To avoid cracks due to flexing, you must con-

nect all the individual piping components

with flexible connectors (Figure 7-6). If this

isn’t done, engine vibration combined with

PART THREE: EXHAUST SYSTEMS

Figure 7-6. Exhaust hose clamps