Exhaust question - Side outlet

[johng39]

After my first year with my new boat I have annoying black soot marks up the side of my boat that I have cleaned numerous times. Now there is not much I can do about the engines producing the smoke, but wondered if re-configuring the tell tale on the side may solve the issue?

The boat has underwater exhausts and before the bend that takes the exhaust out of the hull underwater, there is a take-off for the tell tales at the side. I have not measured this as yet, but my guess is around a 60mm pipe that exits just above the waterline. The annoying part is that there is a moulding on the exterior that points the gases down into the water, so any helpful soul passing me a mooring line gets soaked by the gases being blown into the water.

So underway the gases (due to the fact that the side exhaust is taken of the top of the elbow) blow down the rear third of the boat and the water / soot gets stuck to the boat. Regardless of polishing /waxing it is a sod of a job to remove it........ So, can I:

a) Block the things off
b) reconfigure to take the outlet from the bottom of the elbow (where water will be) and hopefully the mixture of gas and water will leave less soot down the side.

I call these tell tales, but they are not really as you cannot see water from them, my feeling is that these are necessary to stop pressure build up due to the underwater exhaust configuration perhaps? I am no expert, hence the question and possible reasons.

Also if I am correct and the pressure needs to come from the exhaust side exit, would this be true at planing speeds where the soot is generated? If so I may be able to rig up a flap that comes down to block this off at planing speeds.

If I cannot, then I have the option to extend the pipe to the rear of the boat and cut new side exhausts I guess.

Help greatly appreciated.

 

[Bandit]

You should be able to see water droplets coming out to tell you that some water is being injected to cool the exhausts but the main reason is to stop a boat rolling at anchor and siphoning water back through the exhausts.

 

[johng39]

You should be able to see water droplets coming out to tell you that some water is being injected to cool the exhausts but the main reason is to stop a boat rolling at anchor and siphoning water back through the exhausts.

Thanks for the reply, but you cannot see water as all it does is blow gasses directly into the sea water soaking everyone withing 5 feet. You could not possibly tell if it were water from the exhausts or just water being whipped up. Hence they are not really tell tales. Are you saying that the only reason for this side exhaust is to stop siphoning and not to stop pressurisation of the exhaust system?

 

[Portofino]

I would not block it off your aption a
They do not like backpressure -page 3
http://www.dtic.mil/dtic/tr/fulltext/u2/a542327.pdf

 

[mlines]

I would also hazard a guess that it might not make much difference?

Our previous sterndrive equipped boat had the exhaust venting under water and we still got greasy black stains on the transom

 

[Deleted User YDKXO]

Agree with mlines. If you've got significant soot deposits on the side of your hull or transom, they're likely to have come from the underwater exhausts anyway. After all, the exhaust fumes have got to escape from under the hull somewhere. I've had a number of boats with underwater exhausts and all suffered to a greater or lesser degree from sooting of the lower aft sides of of the hull. Is this a new(ish) boat with modern electronically controlled engines? If so, it might be worth having the engines checked out because modern engines shouldn't be producing a lot of exhaust deposits

 

[Paul1962]

May I suggest you speak to Halyard marine. They have a system that mixes the exhaust water with the exhaust gas. The soot etc from the exhaust gas transfers into the exhaust water which is then ejected through an underwater exit. The clean and now cool remains of the exhaust gas exits above the water line where ever you want. There is still some sootiness but nowhere like being without the system.
Sorry for the poor explanation but speak to Halyard - a system such as this was fitted to a Thames cruise boat that had a Gardner diesel in it and this system was fitted to overcome the problem you describe - and it did!

This may help

Regards

 

[Latestarter1]

After my first year with my new boat I have annoying black soot marks up the side of my boat that I have cleaned numerous times. Now there is not much I can do about the engines producing the smoke, but wondered if re-configuring the tell tale on the side may solve the issue?

The boat has underwater exhausts and before the bend that takes the exhaust out of the hull underwater, there is a take-off for the tell tales at the side. I have not measured this as yet, but my guess is around a 60mm pipe that exits just above the waterline. The annoying part is that there is a moulding on the exterior that points the gases down into the water, so any helpful soul passing me a mooring line gets soaked by the gases being blown into the water.

So underway the gases (due to the fact that the side exhaust is taken of the top of the elbow) blow down the rear third of the boat and the water / soot gets stuck to the boat. Regardless of polishing /waxing it is a sod of a job to remove it........ So, can I:

a) Block the things off
b) reconfigure to take the outlet from the bottom of the elbow (where water will be) and hopefully the mixture of gas and water will leave less soot down the side.

I call these tell tales, but they are not really as you cannot see water from them, my feeling is that these are necessary to stop pressure build up due to the underwater exhaust configuration perhaps? I am no expert, hence the question and possible reasons.

Also if I am correct and the pressure needs to come from the exhaust side exit, would this be true at planing speeds where the soot is generated? If so I may be able to rig up a flap that comes down to block this off at planing speeds.

If I cannot, then I have the option to extend the pipe to the rear of the boat and cut new side exhausts I guess.

Help greatly appreciated.

I was waiting for clues as to engines or installation as nothing further posted..........

If you have true underwater exhausts they rely on forward motion of the hull to eliminate any back pressure, however when hull not moving through the water underwater exhaust requires a relief pipe/vent pipe to prevent excessive back pressure with engine idling.

Without detailed layout of your system very difficult to surmise what is happening, a well designed underwater exhaust will emit little gas/water from the relief tube.

Both Centek and Hallyard manufacture mixer chambers for use with underwater exhausts none of this stuff is rocket science, Germans invented the concept for their E Boats in WWII and got it right, since then couch engineers at various builders have seemed intent on ballsing the simple concept up.

 

[benjenbav]

Would it be possible to fit a cowl over each outlet so that the smoke is carried away from the topsides?

 

[johng39]

Wow, I do 1 day of DIY and all these answers, thanks.

The boat is an azimut 46 with v8 cats 3208 with underwater exhausts that exit around 3 metres from rear of boat. The tell tales have a cowling that directs the fumes down, but that doesn't work.

If this is only really relieving pressure at idle / low speed then a flap could block it at high speed. I will look at the Halyard solutions, thanks for the info.

Thanks for all the replies and sorry for the slow response.

Deleted User I believe you have first hand experience of this type of boat

 

[RogerRat]

Wow, I do 1 day of DIY and all these answers, thanks.

The boat is an azimut 46 with v8 cats 3208 with underwater exhausts that exit around 3 metres from rear of boat. The tell tales have a cowling that directs the fumes down, but that doesn't work.

Hi John,
I had similar sooting problems on my previous Phantom 38, It would soot up the whole transom and gate access in no time. Always better / cleaner if cruising faster.
As you know I have the CAT 3126B' straight 6 engines in mine and the smut is minimal compared to the Phantom's TamD 63s'. I wouldn't have thought that the exhaust gases would be any different between these engines and the layout and exhaust tell tales are the same. I cruise about 23-4 knots, if this helps. When pootling, there is less reverse air flow behind the boat anyway so I think you're less likely to get sooting.

I know what you mean about the splashing of someone on the pontoon but actually this is nothing in comparison to some sunseekers. I've found the annoying bit is noise level, at idle on departure and arrival, I have to give thumbs' up signals as anyone on the pontoon can't hear me unless I shout, not very cool, makes us sound like a 'right pair'. IfyouknowwhatImean.

I spoke to Azimut about this at the show and I haven't heard anything more but I also forgot to chase the technical department, but at least I have received the Azi specific parts now.

Hope to see you at Libs,
Roger.

 

[johng39]

Morning Roger

I thought you may have said that about the later engines. I would like to not have the problem and if any of the solutions offered here works for me, then problem solved. After 9 months and 7000 litres of fuel, this is the only issue with the boat. One very happy chappy. I have some pictures of the exhaust for latestarter so will post them in a mo. Thanks everyone, helpful advice as ever.

@ RR Have you done enough hours to know what the fuel burn is like? Mine does 115lph at 21 knots.

 

[johng39]

Morning Roger

I thought you may have said that about the later engines. I would like to not have the problem and if any of the solutions offered here works for me, then problem solved. After 9 months and 7000 litres of fuel, this is the only issue with the boat. One very happy chappy. I have some pictures of the exhaust for latestarter so will post them in a mo. Thanks everyone, helpful advice as ever.

@ RR Have you done enough hours to know what the fuel burn is like? Mine does 115lph at 21 knots.

 

[johng39]

Some images

https://www.dropbox.com/s/g5tm3jvxbhwchce/2015-01-04 10.33.23.jpg?dl=0

https://www.dropbox.com/s/jxeathj2hqlm55a/2015-01-04 10.32.40.jpg?dl=0

https://www.dropbox.com/s/a3vj9d9c2itigyl/2015-01-04 10.32.55.jpg?dl=0

No silencers, just straight out the bottom, plus the tell tale/pressure relief pipe.

 

[jfm]

Cat usually specify above waterline relief exhausts to deal with back pressure of underwater exhausts on a stationary boat. My boat, which has Cat engines and underwater main exhausts, has them. You can re route them eg to the transom under the swim platform (which is where mine are). Just use ordinary 60mm exhaust hose. You could also fit a deflector over the outlet to divert the spray somewhere else.

The 60mm sizing seems generous in yours. Mine are also 60mm, on 32 litre engines. So I wouldn't worry about any slight back pressure increase on your boat, from lengthening the hose or fitting a deflector

 

[Deleted User YDKXO]

The boat is an azimut 46 with v8 cats 3208 with underwater exhausts that exit around 3 metres from rear of boat. The tell tales have a cowling that directs the fumes down, but that doesn't work.
Deleted User I believe you have first hand experience of this type of boat

Yes johng39, I should have recognised your avatar! I have had an AZ46 with the Cat 3208 engines and indeed 2 other boats with these engines. Whilst the 3208 is known for being a robust engine (particularly in its lower powered versions), it does not have electronically controlled fuel injection and as such it is known for chucking out a fair amount of soot and unburnt fuel in its exhaust. With flybridge boats like the AZ46 punching such a large hole in the air, they are bound to create some back pressure which causes exhaust soot to be deposited around the aft end of the boat. All of my Cat 3208 powered boats used to get quite dirty around the transom and rear quarters and yes it was a never ending job to keep them clean. IMHO the best you can do is ensure that the hull of your AZ46 is well polished every season and then regularly clean off the exhaust soot with a good marine cleaner and brush

 

[johng39]

Cat usually specify above waterline relief exhausts to deal with back pressure of underwater exhausts on a stationary boat. My boat, which has Cat engines and underwater main exhausts, has them. You can re route them eg to the transom under the swim platform (which is where mine are). Just use ordinary 60mm exhaust hose. You could also fit a deflector over the outlet to divert the spray somewhere else.

The 60mm sizing seems generous in yours. Mine are also 60mm, on 32 litre engines. So I wouldn't worry about any slight back pressure increase on your boat, from lengthening the hose or fitting a deflector

The re-route is a definite option and yes my eyesite is not what it was, they are 50mm. Just climbed in for a proper look.

Thanks all and yes Mike, if I could find a cleaner that easily wiped it off, I would take this option rather than the others mentioned. I will investigate while she is out of the water for the next few weeks and update my findings.

Thanks again.

 

[RogerRat]

@ RR Have you done enough hours to know what the fuel burn is like? Mine does 115lph at 21 knots.

To be honest, I haven't completed any long passages and I also had problems with the port engine limited to 70% while complete diagnostics revealed three separate faults, filters, throttle position sensor and a faulty starter relay giving variable/weird voltage spikes back to the micro commanders. Still sorted now and running very nicely but I was limited to 18/19 knots from Aug-Nov. I am happy to report tho, that I am pleasantly surprised by the fuel economy so far.

With 450s' under there, I was expecting about 600 litres per 100 Nm at a reasonable 22-24 knots. I tend to run in this European manner as St Peter Port is almost bang on 100Nm. Filling up on arrival gives you a pretty good feel for consumption and also it's relevant to the sea conditions experienced during the passage.

Your figures, 115 litres per hour @ 21 knots tells me we're about right. You're burning 575 litres per 105Nm. I think you could hope to get 1 gallon per mile below 20 knots, say 18ish, not bad eh? :encouragement:

Perhaps Deleted User, can recall what sort of fuel burn he had on his Azi?

I also quite like the idea of relocating the teltale breathers to the transom, if only to reduce sound and pontoon splash.

RR

 

[Latestarter1]

Some images

https://www.dropbox.com/s/g5tm3jvxbhwchce/2015-01-04 10.33.23.jpg?dl=0

https://www.dropbox.com/s/jxeathj2hqlm55a/2015-01-04 10.32.40.jpg?dl=0

https://www.dropbox.com/s/a3vj9d9c2itigyl/2015-01-04 10.32.55.jpg?dl=0

No silencers, just straight out the bottom, plus the tell tale/pressure relief pipe.

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.

 

[Deleted User YDKXO]

Jand 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.

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?