BYC 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.

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.

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