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Yanmar Pleasure Boat Marine Engine Help

Exhaust System Diagram

if you have water in the engine, go here

Component Descriptions

Anti-siphon valve, also known as a siphon breaker, vented loop and vacuum valve. This valve is important for all systems that have a raw water inlet and outlet. e.g. the toilet, the engine, a raw water lubricated drip-less stern gland, etc.
In a system without an anti-siphon valve, when the engine is shut down, the raw water continues to siphon into the exhaust system until it reaches the same level as the outside, i.e., the waterline. If the engine is installed below the waterline the water will flow back up the exhaust pipe and into the engine itself.
The result is expensive and disheartening, more so if you're not tied to the dock!
The siphon breaker is a small valve that closes under raw water pump pressure when the engine runs, and opens when the engine stops, allowing air into the pipe, thus preventing the water from siphoning. Therefore it must be installed somewhere in the raw water line between the raw water pump and the mixing elbow. It is not necessary on vessels where the lower side of the mixing elbow is above the waterline at all times, e.g. planning launches.

Flapper valve: A flap is sometimes put over the exhaust outlet to prevent water flowing into the exhaust pipe from a following sea. Used mainly on launches as the shape of the stern delays its lift, allowing a transom exit exhaust to go under the water and scoop up a litre or two. Dont rely on this being your only method of defence as it is not always successful.

Mixing elbow: The point where the exhaust gas and the cooling raw water meet. The exhaust then transports the water through the exhaust hose and out of the boat.
There are 2 types of elbow, a 'low' rise (top picture) and a 'high' rise. (below) The later has advantages, mainly in sailboats, to prevent water in the exhaust flowing back into the engine if the height from the water in the waterlock to the lowest point of the exhaust elbow is inadequate.

high rise exhaust

Muffler: A noise reducer installed in-line in the exhaust pipe. Note that it is not a lock or water container.

Raw water: water taken from outside the boat to the inside for cooling the engine or flushing the toilet, etc. It can be either salt or fresh and sometimes other stuff, (sewage) depending on the liquid you're operating in.

Waterlock muffler: While the engine is stopped this 'box' collects the water in the exhaust and forms an air lock . The water is contained so that it doesn't shoot back into the engine while sailing or at anchor. The 'lock' prevents salt laden air from flowing backward and forward through the engine thereby preventing valves, valve seats, etc. from corrosion. It also reduces exhaust noise.
Have a look at the picture above and dont be tempted to install it backwards, it acts like a slingshot, producing the opposite effect to the one you intended!
Dont install it athwartship, in a sailboat, or a launch rocking and rolling at anchor, it acts like a teapot (on one angle of heel) and gently pours all the seawater back into the engine!

Installation tips:

The minimum inside diameter at any point of the exhaust system must be the same as the outside diameter of the mixing elbow.
Yanmar recommends increasing the size of the hose by 25mm / 1 inch where it exits the waterlock or muffler. This size increase isn't necessary on low hp engines but is recommended on 88hp and over or any vessel with a long exhaust run, e.g. a mid mounted engine.
The exhaust gas backpressure measurement is more important on turbocharged engines and must be measured immediately after the turbocharger. Yanmar tolerances for exhaust gas backpressure are higher than other manufacturers so they should be religiously adhered to. Exceeding the backpressure results in a loss of horsepower and an increase in exhaust manifold and cylinder head temperature which can lead to premature engine failure.
The exhaust run must have as few bends and curves as possible and they need to be as large a radii as possible. Sharp bends are a no-no!
For larger engines, e.g. 6CX series and 720-hp 6SY-STP models I recommend the designer submit exhaust system drawings to the engine supplier for an exhaust gas flow calculation to be carried out, before the exhaust is installed.

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