How to Maintain the Exhaust System on Your Boat

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Engine room

Breathing Easier

Inspecting your boat’s exhaust system now might save your engine later. Or your life.

When was the last time you inspected your boat’s exhaust system? I’m guessing never: Most of us take our manifolds, elbows, risers, and mufflers for granted, and in most cases our neglect is rewarded by years of trouble-free service. The exhaust keeps exhaling through the exhaust ports, the carbon-monoxide alarm doesn’t go off, the pyrometer needle stays where it ought to be, and the engine runs cool and happy. Everything’s copacetic, so why worry?

Here’s why: If the exhaust system springs a leak, it can fill your bilge with water or, even worse, your cabin with lethal carbon monoxide. (Your CO detectors will warn you of this, but they don’t last forever: Some have lifespans as short as five years. Check yours to ensure they’re still protecting you.) Less catastrophic than asphyxiation, a corroded exhaust riser can still open a path for cooling water to seep back into your exhaust manifold, and maybe into a combustion chamber via an open exhaust valve. If that happens, the next time you crank the engine, you might get a lesson in hydraulics, as the incompressible water meets the compression-stroking piston. The winner? Your mechanic.

The good news is, exhaust-system failures are easy to prevent, about as easy as anything gets on your boat. All you need is a flashlight, the willingness to crawl into the remotest parts of your bilge, and common sense.

It’s Hot and It’s Wet

Most yachts have wet exhausts; each engine has its own separate system, so with twin engines and a generator, you have three to inspect. Although the specific design and components vary widely (the height of the engine relative to the waterline is the primary determinant), the basics of all wet exhausts are the same: Seawater from the engine’s cooling system is injected into the exhaust plumbing downstream of the exhaust manifold or turbocharger. (“Downstream” means toward the overboard exhaust port.) The water cools the very hot exhaust gases, so the exhaust lines can be made of rubber or fiberglass rather than heat-resistant steel, making them easy to maintain and repair. The cooling water muffles the engine racket, too, and an inline muffler knocks down the decibels even more. Eventually, the gas/water mix flows into the sea. 

Properly engineered wet exhausts work fine almost all the time. Since the purpose of this column isn’t to redesign your system, but to maintain it, let’s assume yours is done right. That’s not always the case, especially if your boat’s been repowered, or is a one-off from a custom builder: Some of these guys aren’t as scrupulous as they should be. If you have any doubts, call in a qualified surveyor or mechanic to check things out. (Northern Lights has a good, downloadable intro to exhaust system design. You can get more info at www.northern-lights.com. It’s written for generators, but the principles work for propulsion engines, too.)

Start your inspection at the exhaust manifold, which might have either a “riser” (it sort of resembles a sink trap turned upside-down) or a simple exhaust elbow bolted to it. The loop in the riser keeps raw water from backflowing into the manifold or turbo. (Some engines have custom stainless steel exhaust plumbing that does the same thing, insulated rather than water-jacketed.) Risers are usually cast iron, and typically water-jacketed to keep their surfaces cool, or at least not blistering hot; if not jacketed, they’re wrapped with insulation. Raw water is introduced into the exhaust at the downstream end of the riser.

Risers are the Achilles’ heel of any wet exhaust system: If the water jacket corrodes away inside, cooling water can leak into the exhaust far enough upstream so it finds its way into the manifold. How long does it take for this to happen? BoatUS data suggests saltwater-cooled risers have a 50-percent failure rate after roughly five years, but your life expectancy may vary. Power & Motoryacht’s engine guru, Capt. Richard Thiel, suggests removing the rubber exhaust hose from the riser every year and checking the innards for signs of rust, water seepage, and other nasties. I agree. New risers don’t cost much, relative to the potential damage caused by one going bad, so replace them sooner rather than later.

Some Like It Dry

When an engine is mounted deep in a full-bodied hull, e.g., a tugboat or long-range trawler, it takes creativity, and often lots of money, to make a water-cooled exhaust system work without risk of back-flooding. So dry (or dry-stack) exhaust has the advantage here. It doesn’t use water for cooling, so there’s no flooding risk. Hot exhaust is ducted up and away from the engine through a simple stack, like that on a semi-truck. The only problem: arranging heavily insulated pipes so there’s enough cool airflow around them. Dry-stack commercial boats I’ve worked on used conventional heat exchangers; raw water came in via a seacock and went out via a through-hull. Checking the flow meant sticking your head out the pilothouse window and listening for the splashing. Some boats use keel coolers, an arrangement of pipes outside a hull’s bottom. Hot cooling water runs into the keel cooler, through the pipes, where seawater cuts its temperature, and then back to the engine. When I was a kid hanging around the local shipyard, all the fishing vessels used keel cooling. Those guys couldn’t afford downtime, but when it came to maintenance they were as cheap as Ebenezer Scrooge. Keel cooling worked great for them. If you’re spec’ing out a new long-range trawler, give it a look.

Be A Pyro-Maniac

Look for signs of water leaks around the cooling-water injection nipple, seeping out from under the hose, etc. If a hose clamp is corroded (the screw usually goes first), replace it with a 316 stainless clamp intended for marine service—that means the screw is also 316 stainless. Most mechanics use T-bolt hose clamps on exhaust hoses rather than the typical worm-screw clamps, and you should, too. You get a more thorough fit, at least in my opinion. Every hose should be double-clamped. 

Downstream of the water-injection point you’ll find the pyrometer sending unit that measures exhaust-gas temperature; if you don’t have a pyrometer, have one installed. It’s as important as your engine-temp gauge. If cooling water is cut off—because the raw-water impeller packs it in, for example, or a plastic bag is sucked into the intake—the temperature inside the exhaust hose will skyrocket quickly, faster than your engine temperature. The hoses approved for wet exhausts aren’t rated for uncooled exhaust gas: Even a top-quality hose like Trident’s Blue Corra-sil (www.tridentmarine.com) is typically rated for just 350 degrees F. But exhaust gases can be twice that hot. If you just monitor an engine-temp gauge, your 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.

Last thing you should do before you leave this area: Some boats have an anti-siphon loop in the cooling water line, often between the water pump and the heat exchanger, or just ahead of the injection point into the exhaust. Why in Neptune’s name would they do that? Anti-siphon loops are installed when the injection point for the cooling water is below, or even near, the boat’s waterline. Siphoning can develop if the raw-water pump lets water run through it, e.g., if the impeller is worn. In the worst case, siphoned water can fill the waterlift muffler (typically used when the engine is lower than or at the waterline), then fill the exhaust hoses, and finally back up into the manifold. Again, not a good situation for you, but excellent for your mechanic. (And another reason to replace your raw-water impeller every season.) Unscrew the siphon-break valve on the top of the loop and clean the salt or other gunk out of it. Instead of a loop, some installations use a tube that vents overboard; check its fastenings at both ends, and make sure the tube is clear.

Plug That Muffler!

Finally, we can move away from the engine and follow the plumbing toward the exhaust port, usually in the transom. Along the way, look for leaks where the hose connects to elbows or solid sections of pipe (usually fiberglass); discolored sections, which can indicate overheating; corroded hose clamps; cracks in solid pipe; worn spots in a hose from chafing against hull structure, or where it passes through bulkheads; broken hangers or support brackets—basically, keep an eye out for anything that doesn’t look right. 

Some boats have inline mufflers, similar to the one in your car, but more often you’ll find a waterlift muffler, essentially a canister of water bolted securely to the hull structure. There’s a pipe in, and a pipe out; exhaust flows into the muffler, filling the canister with water and gas until the pressure is great enough to force some of the mixture up the “out” pipe and then through the exhaust port. The vertical exit pipe rises above the level of the entry port to an elbow that serves as a secondary riser, preventing seawater from flowing back into the exhaust plumbing. The water in the canister also muffles exhaust noise.

All waterlift and most inline mufflers have drain plugs that a mechanic will remove when winterizing the boat. Make sure the plugs are in place and securely tightened; if one backs out, water and exhaust gas will escape into the boat. The water can fill the bilge, and the gas can fill your lungs. Neither one is a good thing, but both are easy to prevent by spending a little quality time with your exhaust system.

This article originally appeared in the June 2016 issue of Power & Motoryacht magazine.

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