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Marine Exhaust Systems - How They Work

Dec 17, 2007
Marine exhausts systems are an important and often overlooked part of a marine engine. Most vessels have a 'wet' exhaust system. Salt water is injected at the riser - the outlet for exhaust from the exhaust manifold. This mixture of exhaust gas and water is then passed through a series of bends until it exits the boat, preferably at the stern.

Exhaust gases mixed with salt water create a highly corrosive compound. This is why exhaust systems are commonly made using non-corrosive components such as nitrile rubber reinforced hose (brown - not green - stripe), galvanized steel, fiberglass or plastic. The purpose of these components, which vary in size and shape depending on the engine size and layout of the engine room, is to prevent hydraulicing of the engine. This is caused when an engine has filled with salt water which has entered via the exhaust and can cause extensive damage if left for more than two hours. Water enters the exhaust from wave action at stern and poor exhaust design. In some ocean conditions, such as a following sea, water can be forced back up the exhaust when the engine is not running. Poorly designed exhausts allow water to flow back and fill the waterlock/muffler box 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 crank over as it is cannot compress water.

You will know your engine is hydrauliced if it will not turn over - generally after a long sailing period - and you have removed the starter motor and ensured it works. To remove water from inside the combustion chambers, first remove all injectors, crank engine over to blow water out, refit injectors, bleed injector lines, and start. Then leave motor running until exhaust sorted out.

Check to see that the riser is not coked up or corroded - a common problem. To check the exhaust riser remove the exhaust hose from the riser (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 will have to be removed to check the engine end of the pipe. Coke can be scraped out to provide a short term fix although often the riser will have to be replaced. There are aftermarket systems which vary in quality. Make sure you fit the right design for the application.

Ensure your water-lock is low enough and big enough to hold all the water in exhaust system. Is there a gooseneck or central vertical loop in the exhaust hose at the transom exit? Is there a siphon break and is it functioning properly - no leaking valves?

Exhaust gas is poisonous and can cause sea sickness and headaches. Replace any faulty parts immediately. Use double hose clamps on each joint or, preferably, super clamps, bolt style, and exhaust cement if need be. Hot sections should be lagged with fiberglass tape to prevent burns.

Beware of asbestos lagging. Many older vessels and marine engine installations had exhaust systems that were lagged with asbestos tape and rope. Asbestos sound-proofing was also common in older boats.
About the Author
Chris Miley is an online author and a keen boater. He maintains the website Marine Diesel Secrets and the Pontoon Boat Site.
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