Engine Exhaust Discharge Location

[AJ]

Hi all,

I was considering the location of the old Atomic 4 engine exhaust on my boat and wondering how high above the waterline such exhausts should be. This photo of my boat moored in calm waters on Lake Champlain indicates that the blue boottop exactly follows its current waterline:



There is no sign of the outboard motor on the stern, so I'm assuming it was stowed in a locker at the time this photo was taken. I intend to repower with an inboard diesel this summer and have been thinking if there will be any further squatting. The new diesel will weigh less than an A4 and the diesel fuel tank of approximately 18 gallons will be of smaller capacity than the original 25 gallon gas tank (even though diesel weighs more it should still be about 20 pounds lighter). So, on paper, I shouldn't think there would be a problem with reusing the original exhaust discharge.

However, the discharge does seem rather close to the waterline; just above the boottop:



Is there a recommended distance from the waterline for exhaust discharges? And do you think the combination of a waterlock in the sump below the engine and a gooseneck on the inside of the discharge would eliminate any potential problems with the current location?

AJ

[Tim]

I'd rather see it up higher in the counter, but either way the outlet will still be submerged much of the time during sailing. With a high loop (or manufactured gooseneck), this rarely presents a problem in terms of backflow. If you use the existing location, you should still run your high loop and gooseneck as high beneath the deck as physically possible. Under all normal circumstances, this basic setup works effectively to prevent backflow through the exhaust, even when the outlet is submerged during sailing.

Also, from the looks of your existing outlet, there's a good chance you may need to increase its size for your diesel. Most diesels use a 2" ID outlet. If you need to increase the size, that's another good reason to relocate it to a more conventional location higher in the counter. I think it'd be nice to have it high enough that people in the cockpit won't submerge it when you're relaxing at anchor or dockside.

There's no set distance that an outlet should be above the waterline. The criteria for installation found in ABYC P-1 include the basic admonition that "the exhaust system should be designed and installed to prevent cooling water, rain water, or raw water from entering the engine through the exhaust system under all normal operating conditions". How to go about accomplishing this is something that varies from installation to installation, so there is no means of providing specific guidance in terms of height above waterline, etc.

[AJ]

I'd rather see it up higher in the counter, but either way the outlet will still be submerged much of the time during sailing. With a high loop (or manufactured gooseneck), this rarely presents a problem in terms of backflow. If you use the existing location, you should still run your high loop and gooseneck as high beneath the deck as physically possible. Under all normal circumstances, this basic setup works effectively to prevent backflow through the exhaust, even when the outlet is submerged during sailing.

Thanks for the information, Tim. I'll be sure to do that.

Also, from the looks of your existing outlet, there's a good chance you may need to increase its size for your diesel. Most diesels use a 2" ID outlet.

That's a good point. Would you happen to know offhand if that is true for the Beta Marine BD722?

AJ

[Mark.Wilme]

There is no sign of the outboard motor on the stern, so I'm assuming it was stowed in a locker at the time this photo was taken. I intend to repower with an inboard diesel this summer and have been thinking if there will be any further squatting. The new diesel will weigh less than an A4 and the diesel fuel tank of approximately 18 gallons will be of smaller capacity than the original 25 gallon gas tank (even though diesel weighs more it should still be about 20 pounds lighter). So, on paper, I shouldn't think there would be a problem with reusing the original exhaust discharge.

If you are worried about the weight, the distribution and the waterline then load the boat up with those same weights (bricks, water, lead ingots - whatever and then check from a dinghy as to where your water line is.

Just my 2c ....

[Tim]

Also, from the looks of your existing outlet, there's a good chance you may need to increase its size for your diesel. Most diesels use a 2" ID outlet.

That's a good point. Would you happen to know offhand if that is true for the Beta Marine BD722?

As a matter of fact, I do. The BD 722 uses a 2" ID hose (technically, it's 50mm).

[AJ]

Also, from the looks of your existing outlet, there's a good chance you may need to increase its size for your diesel. Most diesels use a 2" ID outlet.

That's a good point. Would you happen to know offhand if that is true for the Beta Marine BD722?

As a matter of fact, I do. The BD 722 uses a 2" ID hose (technically, it's 50mm).

Thanks again, Tim. It appears a new thru-hull for the exhaust will be in order then. Now I just have to figure out the least ugly way to place the unattractive new thru-hulls for the exhaust and bilge pumps in this area:

1. 2" hose for exhaust
2. 3/4" hose for Jabsco diaphragm pump
3. 1.5" hose for Whale Gusher manual pump
4. 1.5" hose for Rule 3700 backup pump

Probably put 2" and 1.5" thru-hulls on the counter Glissando-style; preferably a set that has similar diameters when measuring the width of their flanges in order to keep things looking as tidy and symmetrical as possible. That would still leave me with finding a place to discharge the 3/4" hose of the Jabsco pump. Or I could simply ditch the idea of having a Rule pump as an emergency backup and route the Jabsco through the 1.5" thru-hull with the Whale pump. Hmm.

AJ

[Tim]

Link: Bronze Exhaust Flange



These are pretty nice, and look appropriate. Plus, they prevent the drips from cascading down your counter, instead encouraging the dirty water from your exhaust or bilge pump to drop straight down without that capillary action.

They come in 1.5" and 2" (and larger) sizes. It's too bad you can't get at 3/4" version.

[AJ]

These are pretty nice, and look appropriate. Plus, they prevent the drips from cascading down your counter, instead encouraging the dirty water from your exhaust or bilge pump to drop straight down without that capillary action.

Thank you for the suggestion, Tim. That does look like it would prevent any dirty streaks down the hull from the discharges. I guess I'll have to make an aesthetic decision as to whether I think potential streaks or an extruding proboscis sticking out of the hull is less desirable.

AJ

[Noah]

AJ, in the last couple of years, especially with the big outboard on the back Mwingo always squatted a bit in the stern. I'm not sure what the inboard will do, but be careful.

[AJ]

Thanks for the info, Noah. I've come to the conclusion that the existing through hull so close to the waterline is not long for this world!

AJ