0307560 JOHNSON SCREW,Rod cap

In all types of exhaust system the exhaust back pressure must not exceed 76mm of mercury (41in H2o) when measured within 305mm (12 inches) of the exhaust outlet from the engine.
Wet Systems
Wet exhaust systems, where the raw water used to circulate through the heat exchangers on the engine is finally dumped into the exhaust pipe to cool the exhaust gases, are the most common choice for small craft. Their principal advantage is that a rubber exhaust hose may be used, with a fairly low surface temperature, which presents no risk of fire.A general arrangement for such a system is shown on (A). In many cases the exhaust outlet passes through the transom, just above the waterline (A1). It will be seen that a minimum fall of 10 degrees is required (A2), and that the point of water injection must be at least 200mm (8 inches) above the waterline (A3), although the actual height necessary for a particular boat can only be decided in the light of the exhaust system design, and the pitch and roll which may be encountered in service.
It is essential that the exhaust system is designed so that water from the exhaust does not enter the engine under any conceivable operation condition.
Water Lift Systems
For applications where the engine is installed below the water line, and in any installations where it is possible for water from the exhaust pipe to run back into the engine there are a number of solutions which can be considered.In sailing yachts and deep draught displacement boats it may be found that the engine exhaust outlet is near or below the water line, and a water lift exhaust system is then an option that may be considered.The main features of such a system are shown in (B). Pressure developed by the exhaust gases 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 (B1) contains the water which falls back from the exhaust riser (B2).If a commercially available unit is used the manufacturers instructions should be carefully followed, but the key features are shown in (B).The exhaust muffler/water lift (B1) should have a minimum volume equal to 3 times the volume of the water that could be contained in the exhaust riser (B2). On sailing craft the muffler/water lift should be installed near the centre-line.Particular care should be taken to prevent water from siphoning into the system and then flooding the engine. There are two routes by which this could happen, from the exhaust outlet or by the sea water feed to the engine.The top of the exhaust riser (B3) should be a minimum of 450mm (18 inches) above the static water-line and the exhaust outlet (B4) should either be well above the static water level, or a siphon break should be fitted at (B5).The exhaust system may also become flooded by water entering through the sea water system on the engine, as the sea water pump impeller may be damaged, and can not be relied upon to seal when stationary. To prevent such flooding the sea water pipework should reach a high point (B6) of 450mm (18 inches) above static water level prior to the point of water injection, and a siphon break should be fitted at that point. A syphon break admits air to the top of any inverted U bend and prevents unwanted syphonic action.The syphon break should vent through a skin fitting well above the water level.Some commercially available syphon break units contain a valve to prevent water loss through the vent pipe and vent within the vessel. Syphon breaks with a valve requires periodic maintenance, checks should be carried out every time the engine oil is changed.
Do not overcrank your engine when it will not start and do not crank the engine to bleed the fuel system, as there will not be the exhaust gas present to empty the exhaust riser. Water fills the riser and can back up the exhaust and fill the engine. The seacock must be closed and raw water pump impeller removed if the engine is to be cranked for more than one minute. Remember to replace the impeller and open the seacock when finished.
Top of the exhaust riser and the point at which the syphon break (C1) is connected to the engine pipework must be above the water line under the worst possible conditions (normally a distance of 450mm (18 inches) (C2) under static conditions will be sufficient). There should be a minimum distance of 300mm (12 inches) (C3) between the water line and the exhaust tank inlet (C4). Minimum volume of exhaust muffler/water lift should be three times the volume of the water in the riser. The tank should be installed near the centre-line of sailing craft.
Sea water inlet fittings with a scoop must not be used. The pressure generated when the vessel is under way can force the water past the raw water pump when the generator is not in operation, thus allowing water to fill up the water lift. With no exhaust pressure to force the water out of the system, water can enter the engine via the exhaust manifold.
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 (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 flexible hanger (A5), 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