Underwater exhaust pros and cons

[andy65]

Looking at a Northern Bay 36 with an underwater exhaust. Exits on the bottom of the hull. Uncommon? Anything to be concerned about? It is probably quieter and less diesel smell. What are the downsides?

 

[c1steve]

Most likely the noise would be much more audible to fish, so for bluefin tuna it would be a poor choice. I am suspicious that cylinder corrosion would be higher with an U/W exhaust. However as water is always sitting in waterlift mufflers, which work okay, perhaps it is less of an issue than one would think.

 

[GLA]

every henriques I have seen has under water exhausts, know a bunch of owners who do well tuna fishing
my only concern would be introducing air and a bubble stream ahead of the prop.

 

[leaky]

Don't know anything about it, other than it's done. The boats I have seen with it are production cruisers, not downeasts.

Underwater Marine Exhaust Systems - Seaboard Marine

You'd think it would potentially increase back pressure but maybe a total non issue. There is such a thing as a bypass that can be used in these systems too.

For fishing I don't know, some people say engine sound attracts fish, some thinks it spooks them, maybe sometimes it does and other times it doesn't, certain fish certain times etc.. etc.. I tend to generally want the engine cut, especially when trying to pick up macks, but if things seem too quiet sitting on the hook we might startup the engine and try to give the fish something to investigate - at times I swear the generator brings them up.. On a trolling boat, or of course a stick boat, I think I would prefer the exhaust not be run this way since it would leave you no choice.

One big benefit has gotta be the exhaust layout. By running it under the water you have maximum pitch. I wonder if you may also be able to have it be much shorter, ie you can probably exit to the sides of the engine versus having to run it all the way to the transom.

Jon

 

[andy65]

[QUOTE="leaky, post: 279343, member:
One big benefit has gotta be the exhaust layout. By running it under the water you have maximum pitch. I wonder if you may also be able to have it be much shorter, ie you can probably exit to the sides of the engine versus having to run it all the way to the transom.
Jon[/QUOTE]
On the other hand, does being below the surface result in an upward force of water into the exhaust due to water pressure?

 

[leaky]

[QUOTE="leaky, post: 279343, member:
One big benefit has gotta be the exhaust layout. By running it under the water you have maximum pitch. I wonder if you may also be able to have it be much shorter, ie you can probably exit to the sides of the engine versus having to run it all the way to the transom.
Jon

On the other hand, does being below the surface result in an upward force of water into the exhaust due to water pressure?[/QUOTE]

Pressure yes. Upward force of water no, not really, since water only rises to sea level.

 

[Toolate]

At idle/trolling speeds the bypass lets out all of the exhaust so no bubbling or anything. Once underway it seems like almost nothing comes out the bypass pipes. I don’t tuna fish so no idea on the noise levels affecting the bite but at low rpm the exhaust is effectively the same as above water exit setups.

 

[ArchHibb]

You'd think it would potentially increase back pressure but maybe a total non issue.

Interestingly, a properly designed outlet in an underwater system will scavenge exhaust gases. A wedge immediately forward of the outlet creates the vacuum - same concept as the prop vent discussed here on the forum. I've seen this on most Sabre powerboats like Toolate's.

 

[leaky]

Interestingly, a properly designed outlet in an underwater system will scavenge exhaust gases. A wedge immediately forward of the outlet creates the vacuum - same concept as the prop vent discussed here on the forum. I've seen this on most Sabre powerboats like Toolate's.

Ya I don't doubt it can be a good working system (ie link I posted). Hell its not that uncommon to see boats with conventional exhaust half under water, or to back up for miles fighting fish with a wave covering the exhaust, and neither seems to cause problems.

 

[Downrigga]

Most likely the noise would be much more audible to fish, so for bluefin tuna it would be a poor choice. I am suspicious that cylinder corrosion would be higher with an U/W exhaust. However as water is always sitting in waterlift mufflers, which work okay, perhaps it is less of an issue than one would think.

In my opinion it's excellent for bluefin tuna fishing. They are trained to follow the drone of the draggers waiting for the haulback.

 

[Kailua Kid]

Looking at a Northern Bay 36 with an underwater exhaust. Exits on the bottom of the hull. Uncommon? Anything to be concerned about? It is probably quieter and less diesel smell. What are the downsides?

One downside would be not being able to see or maybe hear whether raw cooling water is pumping through the exchanger into the exhaust. My GM 26 has a diverter at the transom that directs the exhaust to a point a few inches below the waterline. I miss not being able to hear and see the raw water exiting the exhaust just so I know immediately if there is a problem with the raw water cooling loop. (Are those diverter units really useful? Should I take it off?)

 

[cb34]

One downside would be not being able to see or maybe hear whether raw cooling water is pumping through the exchanger into the exhaust. My GM 26 has a diverter at the transom that directs the exhaust to a point a few inches below the waterline. I miss not being able to hear and see the raw water exiting the exhaust just so I know immediately if there is a problem with the raw water cooling loop. (Are those diverter units really useful? Should I take it off?)

Unless you are trolling or sticking, who runs their motor?

 

[Kailua Kid]

Unless you are trolling or sticking, who runs their motor?

At start-up. Very good time to discover the exhaust is dry. Checking for flow is part of my light-off routine on the other boat, and that routine has served me well a few times. Others probably have a better way to check for that, like I suppose opening up a hatch and checking for flow in the strainer.

 

[Ditchdigger]

I'm with you Kailua. I guess I am old school, or taught right, but as soon as I start the boat, my next move is to walk to the stern, and see if it is pumping good water. Lots of water coming out the exhaust, makes for a happy boat !

 

[andy65]

Isn't there some sort of alarm available to indicate whether adequate water is pumping?

 

[WoundUpMarine]

Isn't there some sort of alarm available to indicate whether adequate water is pumping?

Yes you can get paddle type flow alarms that go in the flow of water, or temp type alarms that are a band around your exhaust. A lot of people just like seeing the water flow for themselves, I totally get it.

 

[Genius]

you could run a small line (1/8") from the raw water before the mixer. Route it up high above the water line on the sides or the transom to act as a tale tell. Even with a normal wet exhaust, this is a good idea. I've seen twin engine installs like this, they would actually color code the small through hulls red/green.

 

[Blitzen]

you could run a small line (1/8") from the raw water before the mixer. Route it up high above the water line on the sides or the transom to act as a tale tell. Even with a normal wet exhaust, this is a good idea. I've seen twin engine installs like this, they would actually color code the small through hulls red/green.

Yes the tell tail is the way to do it properly and how it is done with underwater exhaust. Most likely a way better system even without underwater exhaust. A well placed tell tail so it is easy to see all the time like at the hauling station on a lobster boat, there is no need to depend on your alarm to warn you if you are pumping water. It can also act as a visual indication as to how well the pump is working. Just like an outboard.

 

[WoundUpMarine]

Bringing this one back up as it's a good idea for my Duffy, I want to try and keep the stern clean, with less soot and this seems like a great solution. @Blitzen do you have any pictures of your 36 or the new boat's underwater outlets? I'm curious how much of a lip is used to create a good vacuum while cruising ahead.

 

[Diesel Jerry]

In my opinion you should have the engine box up (or access) at start up. I will often tie a ribbon onto the sea strainer and it will “dance” with water flow. I have also done the tell tale trick. You can also put your hand on the exhaust elbow if it’s not pumping you will know soon enough.