Designing the Exhaust System

After puzzling over how to fasten the engine in place, the next design problem centered on the exhaust system.  The problem in brief was finding a way to position a water lift muffler sufficiently lower than the exhaust outlet on the engine so that water from the exhaust system would not flow back into the exhaust manifold when the engine was not running.  On our old engine, we dealt with this problem by fabricating an exhaust riser, or gooseneck, that routed the hot gases up a six-inch vertical rise and then back down before injecting the raw water outflow from the cooling system.  This is a common solution, and it keeps water out if the engine, but it is prone to failure from vibration and corrosion, and creates a foot or more of pipe in the engine compartment that runs at temperatures of several hundred degrees.  This must be insulated, and it ultimately radiates its heat into the engine compartment and thus into our living space.  In a cold climate that might be of little consequence, but in the tropics, where we spend most of our time, it is uncomfortable.

The problem was exacerbated by Yanmar's specification of a three inch diameter exhaust hose, which necessitates a large water lift muffler to provide sufficient volume to hold the water that accumulates in the vertical rise of the hose as it wends its way out through the transom.  Most of the mufflers designed for three-inch hose sat up too high in the space behind the engine to provide the down slope and drop of several inches required.  After extensive searching, we found a Vetus muffler that would fit down into the valley above the stuffing box, providing the required drop.  The locations of the inlet and outlet fittings were also adjustable through a broad range, as well.  This will allow us to locate the muffler in otherwise unused space, where it won't interfere with our storage bins below the cockpit, and it allows routing the exhaust hose in an out of the way manner as well.  Of course, this being a boat, there was a trade-off.  The muffler was roughly twice as expensive as more typical, competing products.  The savings from not having to fabricate a custom riser more than offset the additional cost.

The location for the new muffler, looking down from above.

  Stuffing box is below left end of tape.

The new muffler in position.  Note storage bins to right of muffler.