Exhaust question - Side outlet

[Latestarter1]

Is that correct? The exhaust gases may go into the water under the hull but as the boat travels forward at 20kts or so and the props throw water out from under the hull as well, a fair amount of exhaust gases must also exit from under the transom too and hence get sucked back onto the boat. If I'm wrong why do I get exhaust residue across the whole of my transom, not just local to the by pass outlets which on my boat are on the sides of the hull?

Mike,

One the very reasons behind use of underwater exhaust was to suppress start up, low load smoke smoke issues, fairing of exhaust outlets also does a good job burying exhaust was astern. If relief pipe is correctly sized very little exhaust gas passes out at WOT due to the vacuum effect of main underwater exhaust.

IF a vessel is potentially overloaded will suffer sooting no exhaust design is 100% fix for that, and many are particularly in the Mediterranean where vessels may have been propped for Northern European ambient temperatures.

 

[RogerRat]

John,

Looking at the pictures conventional underwater exhaust arrangement, relief pipe looks a little large.

#1 CAT police their installations very well and have a distaste of underwater exhausts. The issue is that maximum back pressure allowed by CAT is 3 inches of Hg however this number is at W.O.T, no idle back-pressure limit is given or required for installation review. Sizing the relief pipe is a hit and miss affair, and yours seem a tad large.

#2 CAT normally have 1/8th NPT pug in the exhaust riser. If is was my call I would obtain 0 to 5 inches Hg Magnaheilic gauge or similar and fit into the riser.

#3 Fit a valve in the bypass line and run the engine at idle, close the valve until you see tiny sign of flutter from the gauge then stop.

#4 Listen out for signs of underwater exhaust 'farting' sure sign that too much exhaust gas being pushed into underwater exit at rest.

#5 Take a look at results of restricting bypass, if you have reduced the volume significantly then result, if you want to improve further add a flap.

As to comparing 3126 with 3208,apples oranges and a few grapes thrown in for good measure, and discussion re 'station wagon' effect causing sooting from transom mounted exhausts, what has this got to do with OP's discussion, if underwater exhausts properly faired 99.9% of gas and soot go into the water.

Good luck.

Thanks Latestarter, (Dr. Moteur)

You never fail to surprise me with depth of experience...

No doubt you're right on the money as per norm but I didn't quite understand points #1-#5. :ambivalence: 'Hg Magnaheilic' looks like a spelling mistake.

Can you tell me why Azimut would state not to run the engines while stationary for more than 5 minutes? The idea of running both up to temperature appeals at this time of year if not going out but I haven't done this at all due to this recommendation / instruction.

There is also a strange affair when coming down off the plane, I can actually feel the 'farting' of the underwater exhausts briefly as the speed reduces to displacement speed. As much as I would like to reroute or remove tell tales, I'd be worried about messing up a designed balance.

A bit like changing props, it always seems to be a bit of a black art and best left to hydrodynamic experts.

RR

 

[Deleted User YDKXO]

One the very reasons behind use of underwater exhaust was to suppress start up, low load smoke smoke issues, fairing of exhaust outlets also does a good job burying exhaust was astern. If relief pipe is correctly sized very little exhaust gas passes out at WOT due to the vacuum effect of main underwater exhaust.

IF a vessel is potentially overloaded will suffer sooting no exhaust design is 100% fix for that, and many are particularly in the Mediterranean where vessels may have been propped for Northern European ambient temperatures.

OK so please explain to a simpleton like me. What happens to the exhaust gases once they exit the underwater exhausts?

 

[rafiki_]

NB

Thanks Latestarter, (Dr. Moteur)

You never fail to surprise me with depth of experience...

No doubt you're right on the money as per norm but I didn't quite understand points #1-#5. :ambivalence: 'Hg Magnaheilic' looks like a spelling mistake.

Can you tell me why Azimut would state not to run the engines while stationary for more than 5 minutes? The idea of running both up to temperature appeals at this time of year if not going out but I haven't done this at all due to this recommendation / instruction.

There is also a strange affair when coming down off the plane, I can actually feel the 'farting' of the underwater exhausts briefly as the speed reduces to displacement speed. As much as I would like to reroute or remove tell tales, I'd be worried about messing up a designed balance.

A bit like changing props, it always seems to be a bit of a black art and best left to hydrodynamic experts.

RR

Roger, I have the 3126's in my Az 39. The handbook also says don't run at stationary for more than 5 mins, the reason in my book is for pollution, ie the impact of the exhaust gases on the immediate environment. I don't think there is any significant impact on your engine from running stationary, unless there is a cooling concern. I have run mine for half and hour to warm the oil before changing, with no obvious detrimental impact.

 

[RogerRat]

NB

Roger, I have the 3126's in my Az 39. The handbook also says don't run at stationary for more than 5 mins, the reason in my book is for pollution, ie the impact of the exhaust gases on the immediate environment. I don't think there is any significant impact on your engine from running stationary, unless there is a cooling concern. I have run mine for half and hour to warm the oil before changing, with no obvious detrimental impact.

Hah,
So greeeen, better not run them at all then.

thanks,

RR

 

[johng39]

John,

Looking at the pictures conventional underwater exhaust arrangement, relief pipe looks a little large.

#1 CAT police their installations very well and have a distaste of underwater exhausts. The issue is that maximum back pressure allowed by CAT is 3 inches of Hg however this number is at W.O.T, no idle back-pressure limit is given or required for installation review. Sizing the relief pipe is a hit and miss affair, and yours seem a tad large.

#2 CAT normally have 1/8th NPT pug in the exhaust riser. If is was my call I would obtain 0 to 5 inches Hg Magnaheilic gauge or similar and fit into the riser.

#3 Fit a valve in the bypass line and run the engine at idle, close the valve until you see tiny sign of flutter from the gauge then stop.

#4 Listen out for signs of underwater exhaust 'farting' sure sign that too much exhaust gas being pushed into underwater exit at rest.

#5 Take a look at results of restricting bypass, if you have reduced the volume significantly then result, if you want to improve further add a flap.

As to comparing 3126 with 3208,apples oranges and a few grapes thrown in for good measure, and discussion re 'station wagon' effect causing sooting from transom mounted exhausts, what has this got to do with OP's discussion, if underwater exhausts properly faired 99.9% of gas and soot go into the water.

Good luck.

Wow, that has to be about the most comprehensive answer ever, do I need to send money

I have just lifted the boat out, so can take a picture of the fairing also under the boat and will look at the solution you state above before drilling holes and re routing pipes.

So if I understand correctly, the relief pipe is for idle and should not be exiting any fumes at high planing speed? If I use the gauge as you suggest, I guess putting it on the riser and testing the backpressure at various engine loads with various states of blockage of the relief pipe will give me all of the data I need.

Thanks

John

 

[rafiki_]

D

Hah,
So greeeen, better not run them at all then.

thanks,

RR

I guess that Azi have ISO 14001, hence have to write this sort of garbage in their handbooks. If you ran your Snazi at idle for a couple of hours in an ornamental pond, you just might have some sort of negative impact on the environment.

 

[Portofino]

Re - 5 min engine idle .it's not a green reason it's a cooling reason -by the book
My MAN manual says the same " do not let engines idle for more than 5 min turn them off " -it goes on to qualify that with some blurb about NOT allowing the engies to cool down -too cold -.at tick over
In the real World @ fuel pontoon Waiting -que or marina manoeuvre / waiting etc nobody I know turns off engines .
Cos you would look a right plonked if there was a restart problem ! Drifting about in full glare
Insurance claim (s) would make interesting reading .
Suspect summit to do with cylinder bore wash with excess fuel -just a guess LS 1 ?

 

[Latestarter1]

Wow, that has to be about the most comprehensive answer ever, do I need to send money

I have just lifted the boat out, so can take a picture of the fairing also under the boat and will look at the solution you state above before drilling holes and re routing pipes.

So if I understand correctly, the relief pipe is for idle and should not be exiting any fumes at high planing speed? If I use the gauge as you suggest, I guess putting it on the riser and testing the backpressure at various engine loads with various states of blockage of the relief pipe will give me all of the data I need.

Thanks

John

John I suspect in practice you will will see little activity on the gauge at varying engine loads. Normally back pressure only starts to rise as you approach W.O.T unless exhaust design is dire. Typically you will only see anything on the gauge, above say 2,300 or 2,500 rpm on 2,800 rated engine.

If you can obtain a Fluke meter real easy to graph results, either way your reasoning is on track.

If builder has done the job correctly vacuum created by underwater exhaust will always encourage gases to take the line of least resistance.

Good luck.

 

[Robg71]

You have the experts here for exhaust issues, but i can recommend muc-off for cleaning up the transom.... Im pretty certain my old sabres will out soot yours
(Having said that, after i did the turbos, didnt need swabbing down last time out (guernsey and back)
i have tried a few cleaners too....
Ignore all packaging that says marine..... And charge more for it.... Its exactly the same stuff.....
Last lot i bought was buy one get one free from demon tweaks at a good saving....

 

[Deleted User YDKXO]

OK so please explain to a simpleton like me. What happens to the exhaust gases once they exit the underwater exhausts?

LS1 how about a response to this?