Air resources board staff report public hearing to consider adoption of emission standards and test procedures fo
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- Figure 6
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These lists are not all-inclusive. The engine exhaust in boats is treated differently than land-based engines. For the majority of inboards, the engine exhaust is ducted horizontally to the rear of the boat and passes through the transom, exiting just above the water line. In sterndrive and outboard engines, the engine exhaust is ducted through the lower propeller shaft and exits below water through the propeller hub. In all these drive-systems “used” cooling water is added to the exhaust gases inside the exhaust pipes, and exits with the exhaust gases as a spray. This is done primarily for safety reasons, to minimize heat generation from otherwise hot exhaust pipes within a confined engine compartment. Figure 6 shows a cut-away view of a typical sterndrive exhaust system configuration. After exiting the exhaust manifold the exhaust gases are ducted up for a short distance through the exhaust riser before reversing direction and being ducted downward. The static water level in the boat is approximately even with the bottom of the exhaust manifold in the photo. This means that lake or sea water will fill the exhaust pipe when the engine is off up to approximately the middle of the rubber coupling on the right lower corner of the photo. Thus, the riser provides a labyrinth or seal which protects against the outside water traveling back up the exhaust pipe into the engine cylinders. The riser is typically water-jacketed. It is in the down leg or elbow that the water is directly mixed with the exhaust gases. After this point the exhaust gases are cool enough so that rubber pipes and joints can be used for the exhaust pipes. Figure 6C
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