Marine exhaust systems are an important and often overlooked part of a marine engine. Most boats have a “wet” exhaust system. Salt water is injected into the riser, the exhaust outlet of the exhaust manifold. This mixture of exhaust gas and water is then passed through a series of curves until it exits the ship, preferably at 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 nitrile rubber reinforced hoses (brown stripe, not green), galvanized steel, fiberglass, or plastic. The purpose of these components, which vary in size and shape depending on the size of the engine and the layout of the engine room, is to avoid engine hydraulics. This occurs when an engine has been filled with salt water that has entered through the exhaust and can cause significant damage if left for more than two hours. Water enters the exhaust due to wave action at the stern and poor exhaust design. In some ocean conditions, such as a tracking sea, water can back up through the exhaust when the engine is not running. Poorly designed exhausts allow water to flow backward and fill the muffler / sluice box with water and then up the hose to 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 the water.

You will know that your engine is hydraulic if it does not turn, usually after a long period of boating, and if you removed the starter and made sure it was working. To remove water from inside the combustion chambers, first remove all injectors, crank the engine to expel the water, replace the injectors, purge the injector lines, and start. Then, let the engine run until the leak is fixed.

Check that the riser is not cooked or corroded, a common problem. To check the exhaust riser pipe, remove the exhaust hose from the riser pipe (often a difficult procedure) and look up the pipe to see if it is restricted by exhaust / salt build-up. If build-up is excessive, the riser tube will need to be removed to check the end of the motor tube. The coke can be removed to provide a short-term solution, although the riser will often need to be replaced. There are aftermarket systems that vary in quality. Make sure to set the correct layout for the application.

Make sure your water lock is low and large enough to hold all the water in the exhaust system. Is there a gooseneck or central vertical loop in the exhaust hose at the transom outlet? Is there a siphon break and is it working properly? No leaking valves?

Exhaust gases 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 exhaust cement if necessary. Hot sections should be covered with fiberglass tape to avoid burns.

Watch out for lagging asbestos. Many older vessels and marine engine installations had exhaust systems that were set back with asbestos tape and rope. Asbestos soundproofing was also common on older ships.