The wet exhaust systems found aboard recreational vessels use water to cool exhaust gases prior to discharging them overboard. The water also serves as a muffler of sorts to quiet engine noise.
Water-cooled exhaust systems inject cooling water into the exhaust via an exhaust riser or mixing elbow, where the exhaust then pushes the water out of the exhaust outlet. Exhaust cooling water is typically provided by the engine’s raw-water pump, although a second pump may also be used (depending on the system).
Most installations also include a muffler between the riser and the discharge outlet, there to reduce engine noise and — in the case of sailboats underway with the engine off — provide a place for water that enters the exhaust outlet a place to collect.
A great familiarization and maintenance strategy is to conduct an inspection of your exhaust system at regular intervals, as part of a fall layup or spring commissioning routine, for example. All materials used in your exhaust system should ideally be labeled as to their suitability. Hoses, for example, should be labeled “marine wet exhaust” and be able to withstand heat, water weight, pressure, oil and acids.
Anything other than short hose runs (i.e., four times the inside diameter of the hose) should feature wire-reinforced hose. Additionally, all hose connections should be made to a ridged component: pipe, muffler, exhaust outlet. Hose-to-hose joints are a definite no-no.
Vessels with a transom exit exhaust should also have an exhaust valve, which is simply a rubber flapper valve that attaches to the outside of the exhaust and creates a one-way check valve to help prevent water from entering the exhaust (and possibly the engine).
During an exhaust system inspection, check hoses for leaks, kinks, chafing, bulging (at hose clamps) and other signs of deterioration. Verify that each hose is double clamped where possible — where there is sufficient hose barb to allow it — and that hose clamps are tight and free of corrosion. Hose clamps should be installed no closer than one-quarter of an inch to the end of the hose and must fully engage the barb or fitting; otherwise, damage to the hose can occur.
Heat-resistant lagging (insulation) that covers “hot” sections of the exhaust should be removed periodically to allow for a full inspection of the components beneath.
Keep a lookout for leaks at other system components as well, paying special attention to engine manifold and exhaust elbow joints (prime places for leaks and corrosion).
Engine manifolds and exhaust risers should be periodically removed, pressure tested and inspected for internal corrosion and clogging, any of which can result in catastrophic engine failure. This should be considered standard maintenance, particularly with systems that operate in salt water. How often depends on vessel location (fresh or salt water) and use; however, at a minimum they should be removed and inspected every four to five years (more frequently depending on age). As labor is typically the biggest cost associated with doing this, most boat owners will forego testing and simply replace old manifolds and risers once they are pulled, regardless of condition, particularly if the vessel is used in salt water.
Finally, check the exhaust system with the engine off and while in operation, as some leaks will only occur with the engine running.
Aside from maintenance, finding and correcting exhaust leaks are also important from a safety standpoint, as they can introduce carbon monoxide (CO) into a vessel’s interior.