Chapter 3 TPD1317e 3.3 Part Dry, Part Wet Systems Even where the engine is mounted well below the waterline the advantages of a wet system can still be gained, providing that water injection takes place at a point sufficiently above the waterline. In these circumstances the part dry, part wet system shown in Figure 9 can be utilised. The modular exhaust components allow a system to be readily constructed, utilising a tall dry riser, followed by a water injection elbow.
Point of water injection to be 200mm (8 inches) minimum height above water line 5 O minimum Flexible hanger average fall Optional high rise extension Water line Stainless steel bellows Figure 9 - Part Wet, Part Dry Exhaust System - The illustration shows a turbocharged engine, but the principle also applies to naturally aspirated engines. 3.4 Water Lift Systems Whenever an engine is mounted with the exhaust outlet at or below the waterline, the exhaust systems shown in 3.2 and 3.3 should first be considered. If the layout of the boat is such that neither are suitable then a water lift system may be appropriate, but note that these systems make engine starting more difficult.
(This should only be noticeable at low temperatures.) Figure 10 shows the main features of such a system, which utilises pressure developed by the exhaust gases to force a mixture of gas and water to a height which may be considerably above the engine. When the engine is stopped the exhaust tank contains the water which falls back from the exhaust riser. If a proprietary unit is used the manufacturers instructions should be carefully followed, but figure 10 identifies the key features.
Note that the system must meet the requirement for the maximum exhaust back pressure to be not greater than 1 0 kPa, (except the M92B which is 15kPa), of mercury, measured within 305mm(12 inches) of the turbocharger/ exhaust outlet. Page 20