Marine exhaust systems are an important and often overlooked part of a marine engine. Most boats have a “wet” exhaust system. The salt water is injected into the riser, the exhaust outlet of the exhaust manifold. This mixture of exhaust gases and water then passes through a series of bends until it exits the ship, preferably through the stern.
Exhaust gases mixed with salt water create a highly corrosive compound. This is why exhaust systems are commonly made with non-corrosive components, such as hoses reinforced with nitrile rubber (brown stripes, not green), galvanized steel, fiberglass, or plastic. The purpose of these components, which vary in size and shape depending on engine size and engine room layout, is to bypass engine hydraulics. This occurs when an engine has been filled with salt water that has entered through the exhaust and can cause extensive damage if left for more than two hours. Water enters the exhaust from wave action at the stern and poor exhaust design. In some ocean conditions, such as rough seas, water can back up the exhaust when the engine is not running. Poorly designed exhausts allow water to flow back and fill the muffler/waterlock box and then up the hose into the exhaust manifold, through the exhaust valves and into the combustion chamber. With the engine full of water, the engine cannot start because it cannot compress water.
You’ll know your motor is hydraulic if it won’t turn over, usually after a long period of cruising, and you’ve removed the starter motor and made sure it works. To remove water from inside the combustion chambers, first remove all injectors, crank the engine to expel the water, reinstall the injectors, bleed the injector lines, and start. Then let the engine run until the leak is fixed.
Check that the riser is not cracked or corroded, a common problem. To check the exhaust riser, remove the exhaust hose from the riser (often a difficult procedure) and look up to see if it is clogged by salt or exhaust buildup. If the buildup is excessive, it will be necessary to remove the lifter to check the end of the motor tube. The coke can be scraped off to provide a short-term solution, although the riser will often need to be replaced. There are aftermarket systems that vary in quality. Be sure to set the correct layout for the application.
Make sure your waterlock is low enough and big enough to contain all the water in the exhaust system. Is there a gooseneck or center vertical loop in the exhaust hose at the transom outlet? Is there a siphon break and is it working properly? No leaking valves?
Exhaust fumes are poisonous and can cause dizziness and headaches. Replace any defective part immediately. Use double hose clamps at each joint or preferably super clamps, bolt style and cement exhaust if necessary. Hot sections should be covered with fiberglass tape to prevent burns.
Beware of asbestos linings. Many older boats and marine engine installations had exhaust systems lined with asbestos tape and rope. Asbestos soundproofing was also common on older ships.