Human resuscitation has a mantra: “Out goes the bad air, in comes the good.” That applies to a boat’s engine too. A properly sized and well-designed marine exhaust system is the best way to get rid of the bad air and optimize an engine’s performance.
Anything that inhibits the fl ow of exhaust gases creates excessive back pressure, which decreases the amount of horsepower delivered. Back pressure is created by the length of the exhaust tube, its diameter, the bends along the way, and the restrictions imposed by silencers, couplings, valves and water injectors. Short, fat and straight is better than long, skinny and winding.
Marine exhaust systems for inboard engines are divided into two basic types: dry and wet. Most yachts have a wet exhaust system, generally injecting water from the engine’s raw-water discharge into the exhaust stream, downstream of the exhaust manifold or turbocharger, to cool the gases and quiet the engine noise. The cooled gases also allow the use of fiberglass, rubber and silicone components. Such systems can safely discharge the cooled exhaust through the hull near or below the waterline.
Dry exhaust systems have been around the longest, but they’re primarily used on commercial vessels or other specialty applications, such as high-performance craft. Dry exhausts require all-steel/iron construction, so they can withstand higher temperatures. The discharge must be topside, clear of guest areas, but in unfavorable wind conditions, such a location may deposit soot on the deck and guests.
Wet systems commonly use a perforated ring — it may be a separate component, an integral part of the risers when they’re needed to get a downward slope on the exhaust line, or a part of the silencer — to inject cooling water into the exhaust. Boat owners have many stock silencers to choose from, or they can contract with a specialty fabricator for custom units in fiberglass, stainless steel or aluminum. They’re available in both vertical and axial configurations, to suit the space available in any machinery area.