Chapter 3 TPD1317e
Page 20
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 sufciently 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.
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 rst 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 difcult. (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 gure 10 identies
the key features.
Note that the system must meet the requirement for the maximum exhaust back pressure to be not greater
than 10 kPa, (except the M92B which is 15kPa), of mercury, measured within 305mm(12 inches) of the
turbocharger/ exhaust outlet.
Figure 9 - Part Wet, Part Dry Exhaust System - The illustration shows a turbocharged engine, but the
principle also applies to naturally aspirated engines.
Point of water
injection to be
200mm (8 inches)
minimum height
above water line
5
O
minimum
average fall
Water line
Stainless
steel bellows
Optional high rise extension
Flexible hanger
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