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The exhaust system back pressure must not exceed 10.2 kPa (40 inch of H2O) when measured within 305 mm (12 inch) of the exhaust outlet from the engine. This includes wet exhaust systems, dry exhaust systems, part dry part wet exhaust systems, and water lift exhaust systems. It is important that the exhaust system is designed so that water does not enter the engine under any condition. Serious engine damage can occur. |
Wet Exhaust System
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| Illustration 1 | g00821819 |
Typical wet exhaust system installation (1) 5 degree minimum average slope (2) Exhaust piping (3) Centerline of the crankshaft (4) The point of water injection should be a minimum of 200 mm (8 inch) above the level of the sea water. (5) Level of the sea water (6) Marine engine | |
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| Illustration 2 | g00801181 |
Exhaust elbow that is water injected for the naturally aspirated 3034 Marine engine (7) Connection for temperature or pressure measurements (8) Water feed connection | |
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| Illustration 3 | g00801182 |
Typical exhaust elbow that is water injected for the turbocharged 3034 Marine engine (9) Connection for temperature or pressure measurements (10) Water feed connection | |
Wet exhaust systems are the most common type of exhaust system for small vessels. Wet exhaust systems use the sea water that is used for the cooling system. The sea water is injected into the exhaust piping in order to cool the exhaust gases. The sea water and the exhaust gases are then exited from the vessel. Rubber exhaust hose may be used. This is due to low surface temperature.
Illustration 1 represents a typical installation of a wet exhaust system. In most cases, the exhaust outlet will exit just above the level of the sea water. A minimum of 5 degrees slope is required. The point of water injection must be at least 200 mm (8 inch) above the level of the sea water. The actual height will depend on the type of service.
The diameter of the exhaust piping (2) for a wet exhaust system is 63.5 mm (2.50 inch) for the naturally aspirated 3034 Marine engine. The diameter of the exhaust piping (2) for a wet exhaust system is 76 mm (3 inch) for the turbocharged 3034 Marine engine.
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| Illustration 4 | g00745344 |
Typical arrangement for an exhaust bellows joint with a double hump (7) When there is restricted space, use an exhaust bellows joint with a double hump. (8) Engine (9) Marine transmission | |
Consideration must be given to flexibility in the exhaust piping. This is true especially if the engine is mounted with flexible engine mounts. If the exhaust piping must pass through a bulkhead that is immediately behind the engine, an exhaust bellows joint with a double hump should be installed. This will provide flexibility. The exhaust bellows joint should not be compressed after final installation.
Dry Exhaust System
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| Illustration 5 | g00806863 |
Typical example of a dry exhaust system for the naturally aspirated 3034 Marine engine (1) Insulating blanket (2) Stainless steel bellows | |
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| Illustration 6 | g00806864 |
Typical example of a dry exhaust system for the turbocharged 3034 Marine engine (1) Insulating blanket (2) Stainless steel bellows | |
A dry exhaust system for a marine installation needs to be designed carefully. Enclosing exhaust components that are high temperature in an enclosed space should be minimized.
The dry exhaust system should include a flexible connection. An exhaust bellows joint that is made of stainless steel is suitable for a flexible connection. A flexible connection is required to accommodate movements that do not involve twisting. There should be no load on the exhaust bellows joint. The weight of the exhaust system should be supported by brackets. The exhaust bellows joint will absorb the expansion and the movement of the engine. Insulate the rest of the exhaust system.
A condensate drain may be necessary if there is a long run of exhaust piping. Drain outside of the vessel.
The diameter of the exhaust piping for a dry exhaust system depends on the engine arrangement. Naturally aspirated engines require a 40.0 mm (1.60 inch) bore. Turbocharged engines require a 63.5 mm (2.50 inch) bore.
Part Wet, Part Dry Exhaust System
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| Illustration 7 | g00806873 |
Typical arrangement (1) Flexible hanger (2) Water injection elbow (3) 90 degree elbow (4) Riser (5) Stainless steel bellows (6) 90 degree elbow | |
If the engine is mounted well below the level of the sea water (3), then the advantages of a wet exhaust system can still be gained. However, the water injection must take place at a point sufficiently above the level of the sea water.
Refer to Illustration 7 for a typical arrangement of a part wet, part dry exhaust system. The modular exhaust components allow different arrangements. The part wet, part dry exhaust system can be used on turbocharged engines and on naturally aspirated engines.
The point of water injection should be at least 200 mm (8.00 inch) above the water line. The wet exhaust should have an average fall of at least 5 degrees.
The minimum diameter for the dry part of the system should be 40 mm (1.6 inch) for the naturally aspirated engine. The minimum diameter for the dry part of the system should be 63.5 mm (2.5 inch) for the turbocharged engine.
The minimum diameter for the wet part of the system should be 63.5 mm (2.50 inch) for the naturally aspirated engine. The minimum diameter for the wet part of the system should be 76 mm (3 inch) for the turbocharged engine.
Note: The bellows should not be restrained. This will allow full movement of the bellows. The weight of the exhaust system should be supported by the flexible hangers and brackets.
Water Lift Exhaust System
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| Illustration 8 | g00745335 |
Typical arrangement (1) The top of the exhaust riser must be above the level of the sea water. The connection for the siphon break to the exhaust pipe must be above the level of the sea water. The minimum distance is 450 mm (17.72 inch). (2) 12.7 mm (0.50 inch) bore for the siphon break (3) Overboard outlet (4) Water injection elbow (5) Exhaust riser (6) Exhaust tank | |
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| Illustration 9 | g00806868 |
Typical siphon break (7) Hull of the vessel (8) Siphon break (9) Inverted "U" hose (10) Skin fitting that is above the water line | |
The dry exhaust system or the part dry, part wet exhaust system should be considered when the engine is mounted at the level of the sea water or below the level of the sea water. If neither systems are suitable, then a water lift exhaust system may be required. However, a water lift exhaust system will make starting the engine more difficult. A water lift exhaust system is primarily used in sailing yachts and deep draught displacement vessels.
Refer to Illustration 8 for the features of the water lift exhaust system. The pressure that is developed by the exhaust gases will force a mixture of gas and water to a height which may be above the engine. The water from the exhaust riser falls back into the exhaust tank when the engine is stopped.
The top of the exhaust riser (5) should be 450 mm (18 inch) above the static water line. The exhaust outlet should be well above the level of the water.
The system must meet the requirements for exhaust back pressure. The back pressure must not be greater than 10.2 kPa (40 inch of H2O). The back pressure must be measured within 305 mm (12 inch) of the turbocharger outlet or the exhaust outlet. The minimum volume of the exhaust tank should be 3 times the volume of the water in the exhaust riser. The exhaust tank should be installed near the centerline of the vessel.
The siphon break admits air into the top of the inverted U bend. The siphon break prevents unwanted syphoning action. The siphon break should vent through a skin fitting that is above the level of the water.
A siphon break may be substituted with an anti-siphon valve. The anti-siphon valve is shut when the engine is running. The valve opens automatically when the engine is stopped in order to allow air to enter the inverted U bend. This also prevents unwanted syphoning action from being created.
Note: If a proprietary unit is installed, then follow the instructions from the OEM for installation.
Exhaust Systems for Operation at High Angles of Heel
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| Illustration 10 | g00806886 |
(1) Dry exhaust riser (2) Water injection elbow (3) Cross pipe | |
Racing yachts may require a special exhaust system. Refer to Illustration 10.
A dry exhaust riser (1) takes the exhaust gas up to a safe height above the water line. This is usually on the centerline of the hull. A water injection elbow (2) injects water that mixes with the exhaust gas. The mixture of gas and water exits the hull by a hose or a cross pipe (3). A cross pipe is used so that the exhaust can exit from the outlet that is not covered by water. The water exits through the submerged outlet.
The minimum diameter of the dry part of the exhaust system should be 40 mm (1.6 inch) for the naturally aspirated engine. The minimum diameter of the dry part of the exhaust system should be 76 mm (3.0 inch) for the turbocharged engine.
The minimum diameter of the wet part of the exhaust system should be 63.5 mm (2.50 inch) for the naturally aspirated engine. The minimum diameter of the wet part of the exhaust system should be 76 mm (3.0 inch) for the turbocharged engine.