Marine Exhaust System
High-performance diesel engines pump their own weight in seawater through the cooling system and out the exhausts about every three minutes. And those exhausts empty through two very large holes near the waterline. “If you don’t pay attention to your exhausts, you could have a lot of water going into your bilge,” says Craig McLeod, who handles product development and sales for Marine Exhaust Systems in Riviera Beach, Florida (www.marine-exhaust.com). And that’s just one of many bad days at sea that can come from neglected exhausts, ranging from seized turbochargers to bent piston rods, or lethal carbon monoxide lurking inside your boat. Exhausts are one of the most critical systems aboard, yet they’re often overlooked.
To keep heat out of the engine room, wet exhaust systems pass 800-degree exhaust gas through an inner pipe, and that inner pipe passes through a slightly larger outer pipe that carries seawater from the engine. Wet systems were popular from the 1970s through the mid-1990s, largely for looks, because those outer stainless-steel pipes can be polished to a mirror finish. But McLeod says a sparkling engine room isn’t worth the liability. “The pipe is double-walled starting right at the turbocharger,” McLeod says. “The day will come when that pipe will fail, and it will cost a lot of money in engine repairs.” In hours, or even a handful of minutes, leaking seawater can trickle back through the turbocharger, corroding it beyond repair, or into the engine’s cylinders, necessitating an overhaul.
McLeod says the key to making wet systems last is proper drainage. When the engines are running, flowing water keeps everything cool, but as soon as you shut them down, the water trapped anywhere within the system boils. This erodes the stainless-steel pipe and welded joints in the same spot again and again. McLeod says proper design eliminates this problem — at least, until a simple ⅜-inch drain hose valve is mistakenly closed!
It’s impossible to inspect the water passage between the inner and outer pipes. Pressure-testing is unreliable, since tiny cracks may open only when the metal expands with heat. “We have no way of pushing 800- or 900-degree gas through to check,” McLeod explains. The bottom line: “If a wet system is over 10 years old, I’d get it out of there.”
Most exhaust systems built since the mid-1990s are hybrids. Insulated dry pipes rise up from the engines and then turn down toward the waterline. Water is injected after that turn to cool exhaust gas and carry it overboard. From where water enters the system, leaks can flow only out of the exhaust, not back into the engine. Two decades ago dry exhaust sections were insulated with expensive but problematic removable blankets that stained, shed their silver coating or came loose. Now, maintenance-free, hard, black fiberglass insulation looks good for the life of the exhaust system.
Hybrid systems do have their problems. “Diesel exhaust contains sulfur,” McLeod says. “When we inject water at the spray rings, we make sulfuric acid.” That acid slowly erodes metal, and the difference in expansion of the hot inner exhaust pipe and cooler outer water jacket stresses welds, giving that water injection section roughly a seven-year life. “When [a mechanic] has exhausts apart, look for signs of water,” he says. “There should be undisturbed soot right up to the spray ring.” If exhausts are leaking water inside, McLeod says, it isn’t long before they’ll leak outside, potentially ruining electronic components on transmissions and other nearby equipment.
Electrolysis can also rapidly deteriorate seawater-filled exhaust components on either wet or hybrid systems. “Orange rust inside the spray rings is an indication that you’ve got an electrolysis problem,” McLeod says. The good news is that spray rings can be cut out and replaced, leaving most of the exhaust system undisturbed — but only once. “The sulfur works on that stainless,” McLeod says. “After 10 or 12 years, repairs may still work and look good, but you can’t weld that pipe and know it’s going to last another five years.”
In either wet or hybrid exhausts, water cools the exhaust gas so it can be carried through a rubber hose or a fiberglass tube to a muffler and then overboard. These components typically last for many years, but considering that there is no shutoff valve on exhausts, you should inspect clamps, hoses, mufflers and pipes thoroughly. Heat is the biggest factor in their lifespans. “Those components can withstand 250 degrees,” McLeod says, “but if they’re getting up around 200 degrees there’s a problem.” Deteriorated spray rings or modified cooling system plumbing are common culprits.
Underwater exhausts sweep away exhaust gas and engine noise, but McLeod says most systems become effective at only around 14 knots. Bypass systems for slow speeds carry the same concerns as other exhaust systems. In addition, valves that distribute water between bypass and underwater exhausts can be misadjusted, possibly overheating the components or causing engine-damaging exhaust back pressure. McLeod suggests periodically checking temperature and back pressure.
Dry exhaust systems carry exhaust gas through insulated pipes to stacks atop the vessel. This avoids all seawater problems, but dry systems are not often used aboard yachts, since exhaust particulates released into the air settle on the boat. McLeod says dry systems’ biggest problem is exhaust gas leaks caused when metal expands with heat and works on gasketed flanges. Insulating blankets are a cosmetic problem. While Marine Exhaust coats many dry exhaust pipe sections with hard fiberglass insulation, McLeod says, logistics make it impractical for boats not located near his south Florida facility.
On all exhaust systems, leaking gas is a major concern. “Exhaust gas is highly corrosive and abrasive. If it gets into engine air intakes it wipes out cylinders,” McLeod says. “By the time it affects performance, it’s already too late.” Most new engines have air filters that can handle short-term leaks, but older engines might not. Exhaust leaks can carry carbon monoxide into engine rooms or staterooms. McLeod says leaks are usually easy to find by looking or feeling for telltale soot, or feeling for hot gas escaping around suspected joints while the boat’s under way.
Catastrophic exhaust problems are rare, accounting for only 2 percent of BoatU.S. insurance claims and just five at-sea rescues for the Coast Guard in 2009. But things can quickly go very wrong when exhausts leak. Give them the attention they deserve.