An underwater exhaust is known as a clamshell, an eductor, and a cowl, among other names. If you visit any yard with larger vessels hauled out, you'll see them. They can be round, square, short, long, or barely noticeable. They vary in design as much as the vessels that have them, but what's their purpose?
An underwater exhaust is just that — the exhaust exits the bottom of the boat when the vessel reaches cruise speed. At idle or at low-speed operation, the exhaust gases exit via a bypass above the waterline. This way, you won't have gurgling and a need to push a column of water with exhaust before the gases can escape.
Some of the advantages of an underwater exhaust include:
The system is quiet as the exhaust exits below the waterline and will be far from the vessel as it surfaces and the boat moves at cruise speed.
No main muffler is needed. In comparison, many European builders use a large, dry muffler mounted over the engine. While this will be a quiet system, it poses some major issues - that steel muffler will expand as exhaust temperatures increase and will then reduce in size at ambient temperature, which can cause cracks. This also generates heat in the engine room, even with insulation, and makes it look crowded.
A properly designed underwater system will have a bypass about one-third the size of the main underwater outlet. For example, if you have a 14-inch (35.6 cm) main exhaust outlet, the bypass should be five inches (12.7 cm). For attenuation, a muffler is used in line on the bypass.
Issues that would affect underwater exhaust performance mostly involve proper design. The clamshell needs to be sized properly and should cover about 50 percent of the outlet, which will produce the Venturi effect and help the exhaust or column of water out from under the boat. The cowl's shape will vary also, depending on the vessel's cruise speed. Another important design consideration is the outlet's depth - too shallow and the exhaust will be heard if the vessel is rolling in rough seas as the outlet moves above the waterline, and too deep and the engine may not be able to "blow" the column of water out the stalk to enable the gases to be released.
Some of the stalk connections have hoses and clamps; some have flanges with or without butterfly valves, depending if the vessel is classed and by whom. While most vessels are candidates for an underwater exhaust system, it's mostly for vessels larger than 24 meters.
This article was originally published in the March 2023 issue of Dockwalk.