Inboard Hardware: Exhaust Systems
Dry Systems
Continued
Plate 55 — One method of insulating a dry exhaust
line.
A common insulation method is the use of various
types of lagging. One form of lagging uses asbestos rope or thick tape wrapped
around the pipe and then wrapped with fiberglass tape or screen to prevent the
asbestos from flaking off. An air space between the asbestos and the pipe can
be used for additional insulative capacity. This can be accomplished by
wrapping with heavy screen or expanded wire mesh prior to wrapping with
asbestos (see Plate 55). Spacers can be used on the pipe between it and the
mesh to create a "chimney" effect between the pipe and the insulation
for the circulation of air as long as the ends are open. Another insulation
method is similar to that used for steam pipes in shoreside systems using a
lagging adhesive or compound. Lagging compound can be made by using a teaspoon
of plaster-of-Paris to a pound of ground asbestos made into a heavy paste by
mixing with water. This mixture is reinforced over the pipe with screen,
expanded mesh, or wire. Another solution to the hot pipe situation is to use a
pipe within a pipe to form an insulative flue. The outer pipe should allow at
least 1" of space around the exhaust pipe at all points, and must be open
at both ends. Where the pipe passes to the outside, however, there must be some
means of keeping water from entering. In addition to insulation, protective
guards may be necessary for the safety of crew members.
On vertical dry stacks, a funnel or "dummy
stack" is often used to conceal the exhaust outlet. Regardless of this,
rain and other water must not be allowed to enter the dry stack. The stack can
be angled so that the exhaust exits in a more horizontal direction, and a flap
over the opening is recommended. Nevertheless, a drain cock and condensation
trap should be installed at the lowest portion of the exhaust system to get rid
of any condensation which could freeze and cause damage during lay-ups. The
outlet of the dry stack should also be fitted with a suitable spark
arrestor.
A dry exhaust system should not be rigidly mounted
to the engine, especially if the engine uses flexible mounts. A typical method
is to join the exhaust to the engine by means of a flexible metal bellows
section (see Plate 53). If these are
used, they should be installed to be free from strain, and the stainless steel
type is recommended for durability. Where a long horizontal run is necessary, a
water jacketed pipe is recommended (see Plate 54). Cooling water can be
directed to the pipe jacket and then to the muffler or other outlets where it
can be ejected overboard.
Another type of dry exhaust system is often used on
competition high speed racing boats. Basically, these boats used header-type
exhaust pipes or manifolds which expel the exhaust nearly adjacent to the
engine. Since these are often in open cockpit boats with only the driver in the
boat, and are used only for regulated competition, their use is tolerated. This
is one case where a hot exhaust system without any form of auxiliary cooling is
used aboard a boat. Obviously such an installation offers no noise reduction
whatsoever.
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