As opposed to an automotive application where the heat of the engine’s exhaust is lost to the atmosphere around the exhaust pipe, marine engines live in a closed engine room so exhaust heat has to be dealt with in other ways. Marine inboard gas engines, in most cases, are equipped with wet exhaust systems. This means that rather than just a simple pipe, the exhaust is jacketed and the jackets have cooling fluid running through them to remove the heat.
There are three main components to these systems. The manifolds, risers and mixing elbows.
Manifolds: a manifold is bolted to the cylinder head and collects the exhaust from multiple cylinders and concentrates it down to a single passage, directed upwards. The hot exhaust gases are separate from the cooling fluid in the surrounding jackets.
Mixing Elbows: hot exhaust gas flow and cooling water flow enter the mixing elbow separately and travel through the elbow separately. The direction of flow is changed from upwards to aft/downslope, and at the end of the mixing elbow the cooling water is injected into the exhaust gas and the mix is eventually directed overboard.
Risers: essentially jacketed extensions that fit in between the manifold and the mixing elbow to put the exit point of the exhaust higher. Not all systems have risers. This is something the boat manufacturer will determine based on the height of the engine relative to the waterline of the vessel.
Wet exhaust systems are very effective. Exhaust gas enters the system at the manifold with temperatures around 800*F and leaves the mixing elbow cool enough to touch. You can imagine that these exhaust system components are leading a very hard life, which leads us to the main point of this page, which is that these exhaust systems have a limited lifespan and need to be replaced as part of regular preventative maintenance, just like changing your oil and filters.
Mercury recommends that exhaust components that have saltwater running through them be replaced every 5 years, regardless of engine hours. If you are running in freshwater or have a full-loop closed cooling system then the service life will be longer, but we are located on the bay in San Diego so the vast majority of our customers have at least the mixing elbows exposed to saltwater.
Here is the problem: wet exhaust systems will work right up until the point where a component fails. There is no way to predict the failure. Wet exhaust system failures will result in expensive damage, including the possibility of catastrophic damage, meaning the entire engine has to be replaced.
Yes, wet exhaust system parts are expensive, but using cheap parts or pushing them beyond their expected service life is false economy. If you use cheap aftermarket exhaust parts or keep running your engine with an old exhaust system you are gambling with the life of your engine.