Wet Exhaust and Waterlift Mufflers: Stop Water Flooding the Engine and Catch Leaks Early

Trace the system from the raw-water seacock to the exhaust outlet: seacock, strainer, raw-water pump, heat exchanger (on most diesels), vented loop if fitted, mixing elbow, exhaust hose, waterlift muffler, the hose up to a high loop or gooseneck, and the outlet through the hull. Take photos and note where each piece is. Mark the seacock handle so anyone aboard can find and close it quickly.

If the engine doesn't start after a couple of tries, SAIL Magazine's advice is to close the raw-water intake seacock before cranking any more. Boating Magazine adds: don't just keep cranking, because the engine can't make enough exhaust pressure to clear the water. BoatUS gives the same instruction for bleeding a diesel: if you have to crank the engine to push fuel through the lines, close the engine's cooling-water seacock so water can't back up into the muffler, and open it again before the engine is started. This matters most when you'll be cranking for a long time, such as while bleeding a diesel's fuel system (see the fuel system bleeding guide), or after running out of fuel. Many engine manuals give the same instruction; follow yours.

Cranking with the seacock closed runs the raw-water pump impeller dry for that time. For a long bleeding session, BoatUS also describes two alternatives: take the impeller out until the job is done, or leave the seacock open but disconnect the water-injection hose at the exhaust elbow and let the water run into the bilge (then pump the bilge). Whichever you choose, put the impeller or hose back and reopen the seacock before the engine runs.

A closed seacock with a running engine destroys the impeller in seconds and lets the exhaust run dry, which can melt hose and muffler. As soon as the engine fires, reopen the seacock and check that water comes out of the exhaust. West Marine says you should see cooling water at the exhaust outlet within 30 seconds of starting. Hang a tag on the ignition or the throttle whenever the seacock is closed so nobody forgets.

Signs include a starter that clicks but won't turn the engine, a sudden clunk or bang, or the engine stopping partway through a turn. In the SAIL Magazine account, the owner's mechanic said that stopping at the first click would have saved the engine; the starter alone rarely bends anything, but if the engine fires on the other cylinders, it can. Do not try again. Close the seacock and call a diesel mechanic, who will remove the injectors (or spark plugs) and clear the water before turning it over. Milky oil afterward also needs a mechanic (see the water in engine oil guide).

With the engine off and cool, work along every inch of exhaust hose. West Marine's checks: squeeze it along its length, and replace any section that feels spongy, brittle, swollen or cracked. It warns that exhaust hose gives little outside warning because the inside wears faster than the outside, and suggests replacing hose older than about 5 to 7 years regardless of looks. Use only hose rated for wet marine exhaust.

Hard, shiny, blistered, cracked or discolored hose near the elbow, or a muffler that is warped, discolored or has soft spots (West Marine lists heat discoloration and soft spots as signs the material is breaking down), tells you the exhaust has run hot without enough water at some point. That usually means a closed seacock, a clogged strainer or a failed impeller. Replace the damaged parts and find the cause before running again (see the raw-water strainer and impeller guides). Westerbeke notes that steam at the exhaust also points to too little seawater flow.

West Marine says every exhaust connection at the manifold, elbow and muffler should have two stainless clamps. Look for rust stains, cracked bands and loose screws, and check that clamps sit on the fitting, not over its end. Snug loose ones with the engine cool; replace rusty ones with all-stainless clamps (see the hose clamp guide).

Look the canister over with a light and mirror: cracks, weeping at the seams or fittings, salt crust, soot stains, or a muffler that has moved on its mounts or is rubbing against something. West Marine says it should be securely mounted and not vibrating against structure. A plastic or fiberglass muffler with any crack needs replacing; don't try to patch it.

West Marine says exhaust hose runs must avoid low points where water can pool and should rise steadily to the muffler inlet and from the muffler outlet to the overboard fitting. A sagging hose holds water the exhaust can't push clear. Re-support any section that has dropped since it was installed.

If your cooling system has a vented loop between the heat exchanger and the mixing elbow, it is what stops seawater from siphoning into the exhaust after you shut down. BoatUS explains how it works, and West Marine says the loop must sit at least 12 inches above the waterline at all angles of heel and trim, should be taken apart and inspected annually, and replaced every 3 to 5 years. A stuck or salted-up valve fails silently (see the vented loop guide).

At the hull outlet, look for a cracked fitting, a loose hose, or a flapper valve that has gone stiff. West Marine says the rubber flap should flex freely and seal when pushed closed. On sailboats, SAIL Magazine suggests that routing the exhaust hose high near the middle of the boat, and crossing it to the side opposite the manifold, reduces the chance of seawater getting back in when heeled or rolling (see the exhaust flapper guide). Those are installation changes for a pro.

Boating Magazine lists everyday habits that push water up the exhaust: chopping the throttle so the wake hits the transom, too much weight aft (crew on the swim platform, full rear ballast), and backing hard into waves. Slow down gradually, let the wake pass, and let the engine idle at least 30 seconds before shutting down. In a following sea or a rolly anchorage, keep an eye on the engine for signs of water.

If your muffler has a drain plug, drain it during winterizing even after running antifreeze through, as the diesel winterizing guide describes, then reinstall the plug right away.

Record hose dates, clamp replacements, siphon break service and any scorch marks in BoatMend. A muffler or hose replaced after a dry-running event should go in the log with the cause.