Chapter 2 N38143 Part wet/part dry systems In some installations this arrangement may be chosen in place of a water lift. The part wet/part dry system allows the engine exhaust manifold outlet to be near or below the waterline, and provides protection against sea water back-flooding into the engine exhaust.
The general arrangement (D) shows that the dry part of the system extends to a safe height above the water line, using an elbow (D1), a stainless steel bellows (D2), a riser pipe to give the required height (D3), and a further elbow (D4), which is supported from the deck head by a flexible hanger (D5), which supports the weight of the system but permits movement side to side and fore and aft. At this point water is injected into the exhaust through the injection elbow (D6), and the remainder of the system follows usual wet exhaust practice, with a rubber hose falling to a transom fitting.
The point of water injection should be at least 200mm (8”) above the water-line (D7), and the wet exhaust should have an average fall of at least 10 o . The table below shows the recommended minimum sizes. Note: The bellows should be in an unstrained condition when installed, so that the full bellows movement is available to absorb expansion and engine movement. The weight of the exhaust system should be supported by brackets, and not carried by the bellows.
Page Model Minimum diameter of dry part of exhaust system Minimum diameter of wet part of exhaust system 415GM 40mm (1.6ins) 51mm (2ins) 422GM 63.5mm (2.5ins) 422TGM 700GM 4GM 60mm (2.36ins) 76mm (3ins) 4TGM 4.4GM 4.4TGM 4.4TWGM 76mm (3ins) 102mm (4ins) 4.4TW2GM 6TG2AM 6TWGM 127mm (5ins) The dry part of the system should be insulated to avoid excessive heat loss to the engine compartment.