Helena Frye
Boats can have underwater exhaust systems, which means that exhaust exits through the bottom of the boat, or at the transom or the side of the boat below the waterline. In a Mercruiser stern drive setup, exhaust gases are expected to exit through the prop hub, creating bubbles at the aft end of the prop during idle. However, one boater noticed bubbling near the transom, which may indicate a problem.
| Characteristics | Values |
|---|---|
| Exhaust gases | Leave the combustion chamber and go into the manifolds |
| Mix with water from the manifolds | |
| Travel down to the Y pipe at the transom plate | |
| Go through the exhaust bellows | |
| Exit through the prop hub | |
| Exit through the transom at idle | |
| Exit through the bottom of the boat, the transom, or the side of the boat below the waterline |
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What You'll Learn
Underwater exhaust systems
Exhaust gases leave the combustion chamber and go into the manifolds, after the riser it mixes with water from the manifolds and travels down to the Y pipe at the transom plate, through the exhaust bellows and out the prop hub. This is known as an underwater exhaust system, where exhaust exits through the bottom of the boat, or at the transom or the side of the boat that is below the waterline.
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Exhaust gases and bubbles
The exhaust gases from a boat engine leave the combustion chamber and go into the manifolds, where they mix with water from the manifolds. They then travel down to the Y pipe at the transom plate, through the exhaust bellows and out the prop hub. In some cases, the exhaust gases may exit at the transom or the side of the boat that is below the waterline. This is known as an underwater exhaust system.
Underwater exhaust systems can offer many advantages to the overall design of a boat. In some cases, going underwater may be the only choice due to the difficulty of routing exhaust from the engine room to the stern, or poor initial design planning. However, it is important to note that going underwater adds another dimension to the exhaust system, which may not be initially apparent.
When a boat is idling, the exhaust is designed to exit from the transom. If the exhaust were allowed to exit the prop at idle, the motor would overheat every time the shifter is put into reverse as the exhaust gases would be pushed past the water pick-ups. This can be observed by the presence of bubbles at the aft end of the prop during idle.
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Exhaust exiting through the bottom of the boat
In terms of the exhaust gases themselves, these leave the combustion chamber and go into the manifolds, after the riser it mixes with water from the manifolds and travels down to the Y pipe at the transom plate, through the exhaust bellows and out the prop hub.
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Exhaust exiting at the transom
Underwater exhaust systems offer many advantages to the overall design of a boat. In some cases, going underwater may be the only choice because exhaust routing from the engine room to the stern is close to impossible, or good planning was not done during the initial design.
However, going underwater adds another dimension to the exhaust system, which many builders soon find out at first seatrials. For example, if the exhaust is allowed to exit the prop at idle, you would overheat the motor every time you put the shifter into reverse as the exhaust gases would then be getting pushed right past the water pick-ups.
In a Mercruiser stern drive setup, you would expect to see lots of bubbles at the aft end of the prop during idle.
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Exhaust exiting at the side of the boat
Underwater exhaust systems can offer many advantages to the overall design of a boat. In some cases, going underwater may be the only choice because exhaust routing from the engine room to the stern is close to impossible, or good planning was not done during the initial design. However, going underwater adds another dimension to the exhaust system that many builders soon find out at first seatrials.
In a Mercruiser stern drive setup, exhaust gases are expected to exit through the prop hub exhaust. This means that during idle, you should see lots of bubbles at the aft end of the prop. If you see bubbling near the transom, this could be a problem.
Exhaust gases leave the combustion chamber and go into the manifolds, after the riser it mixes with water from the manifolds and travels down to the Y pipe at the transom plate, through the exhaust bellows and out the prop hub.
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Frequently asked questions
Boat exhaust can exit through the bottom of the boat, the transom, or the side of the boat below the waterline.
This is when exhaust exits through the bottom of the boat, the transom, or the side of the boat below the waterline.
It may be the only choice in many applications because exhaust routing from the engine room to the stern is close to impossible, or good planning was not done during the initial design.
It adds another dimension to the exhaust system that many builders soon find out at first seatrials.
It looks like lots of bubbles.
