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.
