WHERE’S THE WATERLINE?
Marine exhausts are cooled by sea water, so it’s essential that the water cannot run back up the exhaust and into the engine’s cylinders. The resulting damage can be catastrophic. A few simple rules need to be followed, and engine installers should seek expert advice if in any doubt.
ENGINE WELL ABOVE WATERLINE
This engine is sited well above the waterline, and the exhaust system has a downhill gradient of 1 in 8, or steeper, as it scope to the stern. The water will naturally run down the slope and away from the
engine. A standard in-line silencer will achieve a noise reduction of approximately 40%. A dual chamber silencer will achieve up to 80%. (Sketch A)
ENGINE JUST ABOVE WATERLINE
Water could run back into the engine if the vessel pitches on its mooring – or a wave could do the same. There are two solutions:
1. Fit a "high riser". This is an extension to the exhaust outlet on the engine taking it well above water-line to create a 1 in 8 gradient from the water injection on the engine down to the stern. An inline silencer can then be used with a noise reduction of around 40%. Alternatively you might get the necessary height by using a Dual Chamber silencer, with a noise reduction of around 80%. (Sketch B)
2. Fit a dual chamber silencer. The dual chamber silencer means the hose scope steeply down from
engine to silencer, and again from silencer to stern. The geometry needs careful checking, but it can solve the problem and it offers a noise reduction up to 80%. (Sketch C)
ENGINE NEAR OR BELOW WATERLINE
Most displacement craft have their engines low down. The sketch shows a typical water-trap silencer positioned below and behind the engine. The hose from this silencer runs gradually down to the stern.
Water in the system drains into the silencer when the engine is stopped. A wave coming up the stern is unlikely to climb over the gooseneck. (Sketch D)
Siphon breakers should always be used in the cooling water supply to installations of this type. Normal lift silencers reduce noise by 45% or so. You can add a water separator to increase this around 90%.
NOTE: Dimensions on Lift Silencing systems vary widely, but a 12m yacht normally needs a drop of 300mm (12") from the water injection point on the engine exhaust outlet to the top of the lift silencer. The gooseneck should then be 450mm (18") above waterline – or more if the exhaust is to one side. The lift silencer should not fill with water by more than 30% when the engine is stopped, and it must be large enough for this. These are very general guidelines and will change with angles of heel, size of vessel, engine choice etc. Individual installations must be checked with qualified designers or marine engineers.