40021 4 Mercruiser NUT, (M10) Stainless Steel

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
Illustration 1 g00745325
Typical wet exhaust system installation (1) 5 degree minimum average slope (2) Exhaust piping (3) Silicone rubber bellows (4) Centerline of the crankshaft (5) The point of water injection should be a minimum of 200 mm (8 inch) above the level of the sea water. (6) Level of the sea waterWet 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) will depend on the engine arrangement. Refer to Table for the required sizes of the exhaust piping.
Table 1
Required Bore for the Exhaust Piping    
Engine Model     Bore    
Naturally Aspirated 3054B Marine engine    
76.0 mm (3.00 inch)    
Turbocharged 3054 Marine engine    
90.0 mm (3.50 inch)    
Naturally Aspirated 3056 Marine engine    
76.0 mm (3.00 inch)    
Turbocharged Aftercooled 3056 Marine engine    
125.0 mm (5.00 inch)    
Illustration 2 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 transmissionConsideration 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.
Illustration 3 g00745326
Typical arrangement (10) Water injection elbow (11) Insulating blanket (12) Direction of possible rotation for the specific installation (13) Dry elbowUse the system that is shown in Illustration 3 in order to achieve sufficient height above the level of the sea water. The two 90 degree elbows can be rotated in order to fit the specific installation.Dry Exhaust System
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 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. Turbocharged engines require a 76 mm (3 inch) bore. Naturally aspirated engines require a 63 mm (2.5 inch) bore.
Illustration 4 g00786398
Typical arrangement for the naturally aspirated 3054B Marine engine (1) The exhaust system is allowed to move because of the bracket with the link. (2) Insulating lagging (3) The bracket that is rigid should be able to support the weight of the exhaust system. (4) Flange (5) Insulating blanket (6) Stainless steel bellows (7) Insulating blanket (8) 45 degree elbow (9) 90 degree elbow (10) Insulating blanket
Illustration 5 g00786396
Typical arrangement for the turbocharged 3054 Marine engine (11) The exhaust system is allowed to move because of the bracket with the link. (12) Insulating lagging (13) The bracket that is rigid should be able to support the weight of the exhaust system. (14) Flange (15) Insulating blanket (16) Stainless steel bellows (17) Insulating blanket (18) 90 degree elbow (19) 90 degree elbow (20) Insulating blanket
Illustration 6 g00786498
Typical arrangement for the naturally aspirated 3056 Marine engine (21) The exhaust system is allowed to move because of the bracket with the link. (22) Insulating lagging (23) The bracket that is rigid should be able to support the weight of the exhaust system. (24) Flange (25) Insulating blanket (26) Stainless steel bellows (27) Insulating blanket (28) Adapter for the exhaust manifold (29) 90 degree elbow (30) Insulating blanket