Every Engine Must Exhale

A properly maintained exhaust system is vital to an engine’s performance.

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.

In addition to incorporating the water injector, risers protect against water flooding back into the engine. Yachts that routinely back down, such as sportfishing vessels, and have exhaust outlets in the transom will benefit from having surge tubes at the forward end of the line to prevent back-flooding.


Underwater Exhaust

To circumvent the back-flooding noise, soot and fumes of transom exhausts, many yachts have underwater exhaust. A bypass must be fitted above the waterline to prevent excessive back pressure at idle, and a solenoid-operated valve is sometimes included to close the bypass once the yacht is underway.

If the underwater exhaust is not of continuous, substantial construction to well above the waterline, it should include a shutoff valve. The outlets of such exhausts shouldn’t lead the gases into the props, rudders, trim tabs or bottom strakes, because the aeration and turbulence undermine the efficiency of the props and the hydrodynamic effectiveness of the appendages. All components of the system should be suitable for marine-exhaust temperatures and chemical exposure. Look for ABYC, NMMA and/or SAE approval stamps on hoses and fl ex bellows, and keep in mind that type 316 stainless steel is preferred to the more common types 302 or 304 for hose clamps.

Every section of the system should be insulated where appropriate, to protect crewmembers and to keep heat from escaping into the engine room. All components should be well supported by resilient mounts or hangers. Sections upstream of the flex bellows should be supported from the engine, to move with it, and sections downstream should be supported from the yacht’s structure, independent of the engine’s movement.

If you monitor only an engine temperature gauge, an exhaust system may be damaged long before you notice overheating. And never ignore overheating: “It must be a problem with the gauge” isn’t the correct response.

Exhaust systems, like all boat systems, need regular inspection to ensure they continue to function properly. Check hoses for leaking, kinking, chafing, bulging and other signs of deterioration. Verify each hose is double clamped and that hose clamps are tight and free of corrosion. Also look for leaks at other system components, paying special attention to engine exhaust manifolds and elbow joints. Manifolds and risers should be periodically removed, cleaned and pressure-tested. Heat-resistant exhaust insulation covering should also be removed periodically to allow a full inspection.


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