Fresh water circulated through the engine jacket is cooled by seawater pumped through a heat exchanger. The seawater is discharged overboard either through the exhaust or directly over the side of the vessel. If discharged through the exhaust, it is known as a wet marine exhaust system.

The water is injected into the exhaust system, usually in the form of an injection bend fitted immediately after the manifold. In most cases all the raw water flow is taken by the exhaust but in cases where this results in excessive back pressure, the amount of water injected into the exhaust can be reduced and the remainder piped overboard.

A wet marine exhaust system has the following advantages:

  • The exhaust pipe is cool enough to be made of uninsulated fiberglass reinforced plastic (FRP) or rubber.
  • The use of a flexible exhaust pipe more easily accommodates the movement of the engine when running.
  • the wet exhaust is quieter than the dry marine exhaust due to the silencing effect of the water.
  • The wet marine exhaust line is particularly suitable with a flexible mounted engine because it can mostly be made of oil-and-heat resistant rubber exhaust hose.

Many types of mixers, silencers and mufflers are available, which should be fitted only in consultation with the marine engine manufacturer. The design of the wet exhaust system must be such that water cannot run back or siphon into the engine when it is at rest (even when subjected to violent motion such as at an exposed anchorage).

Engines installed with cylinder heads below the waterline are more vulnerable. Therefore, an anti-siphon valve (vacuum breaking device) is fitted on their exhaust cooling line, which helps prevent water siphoning back into the engine.

… from Australian Boating Manual

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