TPD 1399E Chapter 3
Page 11
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-ooding into the engine exhaust.
The general arrangement (A) shows that the dry part
of the system extends to a safe height above the
water line, using an elbow (A1), a stainless steel
bellows (A2), a riser pipe to give the required height
(A3), and a further elbow (A4), which is supported
from the deck head by a exible hanger, 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 (A6), and the remainder of the
system follows usual wet exhaust practice, with a
rubber hose falling to a transom tting.
The point of water injection should be at least 200mm
(8”) above the water-line, and the wet exhaust should
have an average fall of at least 5
O
. The minimum
diameter of the dry part of the system should be 40mm
(1.6”) for the M65 and 63.5 (2.5”) for the M85T,and the
wet part of the system should be not less than 63.5mm
(2. 1/2”) for the M65 and 76mm (3”) for the M85T. The
dry part of the system should be insulated to avoid
excessive heat loss to the engine compartment.
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.
A form of the part wet / part dry system shown on
the next page may be used in racing yachts, which
operate with high angles of heel.
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