If you own a larger boat, it is probably equipped with a fully inboard engine. Unlike outboard or inboard-outboard systems, all parts of an inboard engine are mounted within the boat, with only a propeller shaft and propeller passing through the hull into the water. An inboard engine exhaust system is more complex than others and requires special care and maintenance.
Even though an exhaust system may be mounted in a hard-to-reach area, regular inspections are critical to engine performance. Carbon build-up in the exhaust piping can cause increased back pressure, which will in turn cause the engine to produce less power and overheat more easily. Open and inspect your exhaust system once a year, clean if necessary. Corrosion is another inspection item; many inboard engines dump water used for engine cooling into the exhaust, and the combination of hot exhaust and salt water is particularly good at eating away exhaust piping. Replace any corroded parts during your annual inspection, and carry spares in case something fails while at sea.
Install a Siphon Break
Because inboard engines are sometimes mounted below a boat's waterline, and the exhaust outlet is always mounted through a hole in the hull above the waterline, it's possible to create a condition where water can siphon from outside the boat, down the exhaust piping and into the engine, likely destroying it. A siphon break is a large vertical loop in the exhaust line. Routing exhaust line into a vertical loop that extends well above the waterline will prevent water from siphoning back through the exhaust. On sailboats, a shutoff valve should be installed at the very top of the loop to keep water from splashing in under certain sailing conditions. The top of the siphon break loop should be no higher than 40 inches from the exhaust ports on the engine, to prevent excessive back-pressure on the engine. Change this measurement to 20 inches if your engine has a turbocharger.
Use High Quality Materials
Because the combination of hot exhaust and seawater is so corrosive, you should only use high quality, corrosion-resistant materials in your through-hull exhaust system. Use fittings recommended by your engine manufacturer to connect the engine exhaust manifolds to the exhaust system and downstream of the point engine cooling water is injected into the system, wire-reinforced steam hose is a good choice. Use stainless steel exhaust tips if you choose to use exhaust tips at all; they're not necessary but some boats look good with them installed.
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