Chapter 4 N40475 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 5 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. (1) Stainless steel bellows. (2) Optional high rise extension - not factory supplied. (3) Flexible hanger. Figure 5 (4) Point of water injection to be 200mm (8 inches) minimum height above water line. (5) 5 O minimum average fall.
(6) Water line. Water lift systems Figure 6 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 5 identifies the key features. (1) Exhaust tank (water lock). (2) Water injection elbow. (3) To overboard outlet. (4) 1/2” bore siphon break.
(5) Top of exhaust riser and point at which the siphon break is connected to the engine pipe work must be above the water line under the worst possible conditions (normally a distance of 450mm (18”) under static conditions will be sufficient) Figure 6 (6) Exhaust riser. Note: that the system must meet the requirement for the maximum exhaust back pressure to be not greater than 15kPa , measured within 305mm (12 inches) of the turbocharger/ exhaust outlet. Minimum volume of exhaust tank should be 3 times the volume of the water in the riser. The tank should be installed near the centre-line of sailing craft. Page 12