Real cause for air bubbles in exhaust out of prop

[Hans Grouper]

I read tons of information on the web about air bubbles coming out of the prop when in idle and engine cold or warm, and when running. There are tons of assumptions out there but I have not found one which says this is the problem of the air bubbles and this is the remediation and when done it is cured. People specifically with yamaha F2XX series claim it is exhaust corrosion.There are Suzuki and Mercury and Honda which have the symptoms too. Clogged auxiliary exhaust ports are a supposed culprit too. Bad fuel or other mechanical issues are named as well. So does anyone had big air bubbles out of an engine prop through the exhaust and fixed that for good? If so what caused them? There are two topics on this site here as well but I did not see a conclusion. Thank you for any insights.

 

[seasick]

I read tons of information on the web about air bubbles coming out of the prop when in idle and engine cold or warm, and when running. There are tons of assumptions out there but I have not found one which says this is the problem of the air bubbles and this is the remediation and when done it is cured. People specifically with yamaha F2XX series claim it is exhaust corrosion.There are Suzuki and Mercury and Honda which have the symptoms too. Clogged auxiliary exhaust ports are a supposed culprit too. Bad fuel or other mechanical issues are named as well. So does anyone had big air bubbles out of an engine prop through the exhaust and fixed that for good? If so what caused them? There are two topics on this site here as well but I did not see a conclusion. Thank you for any insights.

How do you know they are air bubbles and not exhaust gas bubbles?

 

[SkunkBoat]

There is supposed to be exhaust coming out of the prop.

 

[seasick]

There is supposed to be exhaust coming out of the prop.

 

[journeyman]

Kinda on topic... I just read that the new Yam 425 redirects the exhaust while in reverse at low rpm to lessen cavitation on the props. Interesting monster...

 

[enfish]

Kinda on topic... I just read that the new Yam 425 redirects the exhaust while in reverse at low rpm to lessen cavitation on the props. Interesting monster...

Kinda not on topic... the boat I grew up with had an old OMC electric shift outdrive. The exhaust on that drive was routed out through the trim tab on the anti-cavitation plate, so the prop always had clean water to bite into. You want to talk about power in reverse... that boat had it. The "braking" distance of that boat was a lot shorter than my current Grady.

 

[journeyman]

Kinda on topic... I just read that the new Yam 425 redirects the exhaust while in reverse at low rpm to lessen cavitation on the props. Interesting monster...

Kinda not on topic... the boat I grew up with had an old OMC electric shift outdrive. The exhaust on that drive was routed out through the trim tab on the anti-cavitation plate, so the prop always had clean water to bite into. You want to talk about power in reverse... that boat had it. The "braking" distance of that boat was a lot shorter than my current Grady.

I thought the topic was how outboards exhaust....... :roll:

 

[enfish]

Kinda on topic... I just read that the new Yam 425 redirects the exhaust while in reverse at low rpm to lessen cavitation on the props. Interesting monster...

Kinda not on topic... the boat I grew up with had an old OMC electric shift outdrive. The exhaust on that drive was routed out through the trim tab on the anti-cavitation plate, so the prop always had clean water to bite into. You want to talk about power in reverse... that boat had it. The "braking" distance of that boat was a lot shorter than my current Grady.

I thought the topic was how outboards exhaust....... :roll:

We were, which is why I started with the caveat that my post was kind of off topic. But, my point was, I guess, what's new is old... The new Yamaha 425 re-routes the exhaust to lessen cavitation on the prop in reverse. That type of design existed in an OMC drive that came out in 1964, and worked quite well for giving better reverse control. We've come full circle in exhaust routing designs

 

[journeyman]

We were, which is why I started with the caveat that my post was kind of off topic. But, my point was, I guess, what's new is old... The new Yamaha 425 re-routes the exhaust to lessen cavitation on the prop in reverse. That type of design existed in an OMC drive that came out in 1964, and worked quite well for giving better reverse control. We've come full circle in exhaust routing designs

Ah, now I get it! Still foggy as I just got up from the night shift a while ago and coffee has not kicked in yet... I tell my wife to never ask me complicated questions at this time. I'm feeling"old" and wish I was "new"!!!

 

[Hans Grouper]

To come back to the first on topic responses when I wrote air bubbles, I meant whatever gas from the combustion in the motor. Key is that there are bubbles of gas coming out of the exhaust from the engine lower unit when in neutral, idle or running. So any comment on my question would be great with my air/gas correction.
To the second not on topic response. This is not cavitation what I am referring to since I have bubbles from the lower unit without the prop spinning.
Thank you

 

[SkunkBoat]

There is nothing wrong. It is supposed to do that.

 

[Halfhitch]

I'm confused

 

[DennisG01]

Air goes into the engine and combines with fuel. Spark ignites the mixture. Boom. The leftover air/mixture goes out of engine so new air can come in. Leftover air travels down the exhaust tube and out the prop. The prop is below the waterline. Exhaust air is lighter than water so it rises. Neutral, forward, reverse... does not matter... this is how it is. In fact, it's pretty much exactly the same principle as when you fart in the bathtub. What do you have going on that is different from this?

 

[wspitler]

Here's my experience, but don't understand why. I have two 2007 F250s and the port one had the exhaust corrosion repair in 2011 at about 250 hours, under warranty. Before the repair they both "bubbled" about the same amount. After the port one was repaired, new oil pan, augmenter, etc. etc. it has a significant amount more exhaust bubbles coming out of the prop hub than the starboard one. My guess is the seals are better on the port. They both run fine and have over 800 hours on them now. I have personally bore scoped both and no major corrosion was seen. The starboard one takes longer to get to operating temp, but my guess is that is a thermostat issue. Don't worry about it (like I did at first)!

 

[Hans Grouper]

Bill,

Your experience is very helpful information since you have the twin engines config, thank you.
My takeaway from Bill's experience is:
Even a F250 year 2007 can have dry exhaust corrosion issues
Dry corrosion issues can happen to F250 year 2007 with lower hours (lt 300)
A 2007 F250 can run in excess of 800h without having any major exhaust dry corrosion
Bubbles of gas through the lower unit exhausts form a 2007 F250 is NOT an indication of exhaust dry corrosion
Bubbles of gas from the lower unit appear in 2007 F250 with or without exhaust dry corrosion issues[/list][/list]

There is one last thing to understand:
And I read the feedback from others in this post about the exhaust bubbles and that it is normal.
I have seen videos were twin outboards are on idle and one was bubbling gas the other not at all.
https://www.youtube.com/watch?v=Q82rBK6mbOE
I also have seen single F2xx at the doc not bubbling at all. How could that be?

My current 2007 f250 bubbles, 470h, and my previous boat 2007 f250 bubbles as well, now about 580h. Both engines have not been checked for dry exhaust corrosion, so I do not know if mine has issues or not,but I will scope at end of season and provide an update. Hopefully we can answer this topic comprehensively at one point provide all the insight needed for the community.

Any more experiences, insights, and knowledge on this topic are highly appreciated.
Thanks

 

[DennisG01]

Gas/bubbles comes out of EVERY engine. I'm not sure what you aren't understanding about that. Based on what you have written, you seem reluctant to believe that exhaust gases coming out are normal. As I eluded to above, though, you need to further explain what is going on and whether it is excessive/out of the norm. Posting a video would help.

Edit. I just saw that other video you posted. A thought... your engine has an upper exhaust port just below the top cowling, I think? If that is clogged, it would force much more exhaust gasses down to the prop.

 

[Hineser Out]

I had twin 2006 f250s on my old boat, 28 Southport, and noticed an excessive amount of bubbles coming from the props at idle. This was when I just put it back in the water in the spring. Turns out the dumbass mechanic, marina owners son, broke off the oil pressure sensors when trying to get the oil filters off. This caused the engines to idle about 100 rpms higher causing a little more exhaust to go through the props. Had to take it to the Yamaha dealer to figure that out. Of course I change my own oil now. So that’s one cause why bubbles come through the prop exhaust at idle.