|
ABYC P-l states that exhaust-system piping must be "independently supported to
minimize failure from vibration, shock, expansion and contraction," with "all
fittings, joints, clamps and supports... accessible for inspection and repair" NFPA
302 contains similar specifications.
Each engine must have its own exhaust. It is impermissible to have any other line tee'd
into an exhaust other than the water injection line on a water-cooled exhaust.
On occasion, boat builders have tee'd cock pit, sink, and other drains into exhausts, but
every one of these then becomes a potential avenue for exhaust gases to enter
accommodation spaces. To further ensure that these gases do not make their way into living
quarters, any above-the-waterline through-hull, such as a sink drain that leads into an
accommodation space, should have a built-in U-trap that seals the line.
Under certain conditions, exhaust gases can be drawn back into the boat after they've left
the exhaust outlet. This is called the "station wagon effect." In order to
disperse the exhaust gases before this phenomenon can occur, the ABYC standard specifies
that an exhaust should exit a boat close to the junction of the side of the boat and the
transom, either in the side of the boat or the transom. From the point of view of gas
dispersal, it's advantageous to place the exhaust outlet underwater, although as explained
before, this is not a desirable location for other reasons. Finally, to ensure the
integrity of gasoline exhaust systems, which pose a greater threat of carbon monoxide
poisoning than diesel systems, the ABYC standard requires the gasoline exhaust system to
be able to hold a test pressure of 4 psi for 5 minutes with no loss of pressure.
PREVENTING Fires
The other principal safety concern in an exhaust installation is minimizing the risk of
fire. A dry exhaust can get exceedingly hot. ABYC P-l calls for protective guards and
jacketing at any spot where the exhaust temperature may exceed 200oF, and where
people or gear might come into contact with the exhaust. All hangers, brackets, and other
exhaust system supports on a dry exhaust must be non-combustible and 'constructed so that
the temperatures transmitted to the supporting materials will not cause combustion."
NFPA 302 requires installation such that no adjacent combustible surfaces will exceed 250oF.
Although it is not written into any standards, exhaust jacketing should be so designed
that it can be readily removed and replaced without damage. Otherwise, sooner or later it
will be taken off during engine servicing, and then quite likely not properly reinstalled.
The result will be hot spots that may char or ignite surrounding surfaces.
In a water-cooled exhaust, good quality exhaust hose can be used to advantage beyond the
water-injection point. The most recent edition of the ABYC standard states that this hose
should comply with SAE J2006 or UL 1129. These standards contain a number of performance
tests that the hose must pass, including the ability to withstand shock, vibration, and
various corrosive substances such as oil, diesel, and salt water. It must also be able to
withstand two minutes of dry operation, with the engine at full output. The hose must
maintain its watertight integrity when cooling water is restored. SAEJ2006 quantifies this
requirement as follows: The minimum temperature during the dry heat test must be 1,100oF,
with a minimum airflow of 4,900 cubic feet per minute. After this portion of the test,
there must be no delamination of the hose, and the hose must be able to hold a pressure of
10 psi without leakage.
If a hose is not stamped "SAEJ2006" or "UL 1129, " it does not comply
with the current ABYC standard. As far as I know, no exhaust hose on the market meets UL
1129, because none of them can satisfy a requirement to be able to withstand a prolonged
immersion in hot gasoline. There are, however, some hoses now coming on the market that do
meet SAE J2006. Those designated SAE J2006-R1 are "soft-wall hoses" (without
wire reinforcement); those designated SAEJ2006R2 have wire reinforcement. Trident Rubber
(Canonsburg, Pennsylvania) produces three grades of SAEJ200 compliant hose: The standard
rubber hose will tolerate continuous temperatures of 250oF; the blue silicone
hose is good to 350oF; while the top of the line red silicone hose will
withstand temperatures to 500oF.
A note for those manufacturers gearing up for ISO certification for export purposes: There
are currently no ISO standards specifically governing engine exhaust systems. There is,
however, talk of incorporating SAEJ2006 specifications for marine exhaust hoses into
future standards.
In addition to requiring UL- or SAE compliant exhaust hose, the most recent version of the
ABYC's P-l standard, written in June 1993, requires all wet-exhaust systems to have
"an indicator...that is effective at all helm positions to indicate loss of exhaust
system cooling-water supply. This indicator shall be independent of the engine block
temperature indicator(s)." In practice, almost no boat engines are installed with
such an indicator, although a good number of generator sets do have one. Many marine
industry professionals feel the indicator is unnecessary. They reason that: If installers
use marine-grade exhaust hose (SAE J2006 or UL 1129 compliant) in the system, then in the
event of a loss of cooling water that goes undetected by the operator, the engine-mounted
high-temperature alarm will sound before the hose burns through. In recognition of this
point of view, there has been some discussion within the ABYC about removing the indicator
requirement in the next review of P-l.
Other Considerations
Water-lift boxes and in-line silencers in a wet-exhaust system can be made of either
metal, or fire-retardant fiberglass or plastic. The metal boxes are prone to corrosion,
particularly around the welds, while the FRP and plastic boxes are not. The fiberglass
boxes sometimes suffer damage from vibration, probably as a result of the constant pulsing
that occurs as each exhaust valve opens and closes, and slugs of water are driven up the
hose. The joint between the pipe stubs and the box is the weakest point, and the point
where damage is most likely to occur.
To reduce stresses, firmly support the hose on either side of the box. Properly installed
this way, a good-quality fiberglass or plastic water-lift silencer has an almost
indefinite life span.
If you're fabricating a metal box, use welding wire that's the same material as the rest
of the box. If you make the box from stainless steel, use a low-carbon content
steel--designated by the letter "L"--preferably 316L.
The water injection point, or mixing elbow, is a frequent source of difficulties. The
combination of very hot gases, high exhaust gas velocity a curve in the exhaust pipe, and
salt water is a potent troublemaker. If you're fabricating an elbow, you should weld,
rather than braze it, with all the components, including the welding wire, once again of a
similar metal, The tendency now is to cast these elbows and injection nipples in one
piece, which result in a part far less prone to corrosion. Nevertheless, once a year you
should remove the exhaust hose from the elbow and inspect it thoroughly, particularly on
the outer curve of the bend. At the same time, remove the water injection hose from its
injection nipple, since this is a potential spot for scale and debris to gather and form a
plug. (Note that if the siphon break is replaced with a hose that vents into the cockpit,
water weeping from the vent will give early warning of such a plug.)
A Dry Engine
And that's about all there is to a proper water-cooled exhaust installation: a
siphon-break on the supply side, a decent drop from the exhaust manifold to the water-lift
box, an adequate loop in the exhaust hose above the waterline. along with keeping the
exhaust gas from leaking and at a safe temperature. Getting these things right is neither
difficult nor complicated, and in most instances is no more expensive than getting them
wrong. It's simply a matter of paying attention to sound, well recognized principles.
There is just no reason, barring some specific equipment failure, why engines should drown
because of an improperly installed exhaust system.
by: Nigel Calder published by International Marine.
If you like this type of in-depth article, and want to go to the source of some of
the best technical information about boats, their design, and construction, get yourself a
subscription to Professional BoatBuilder. It's worth
every penny. ( No, I don't get a kickback)
Or better yet, you can buy Nigel's
book
|
|
