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Exhaust/ silencer


Crow

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Lads  I've a dry exhaust,can I ,or is it acceptable to run a stainless pipe straight down the back of the transom below the water line,

as the engine  / manifold  is 20 inch above waterline there's no chance of water entering engine,nick

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When the boat is low in the water for whatever reason you will increase the exhaust back pressure, though a few "inches of water" is probably no big deal.

You need to be sure that turbulent lock filling etc really cant send a gulp of water into the engine.  Might be other issues too, but can't think of them just now.

............Dave

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On 26/03/2017 at 15:58, Crow said:

Lads  I've a dry exhaust,can I ,or is it acceptable to run a stainless pipe straight down the back of the transom below the water line,

as the engine  / manifold  is 20 inch above waterline there's no chance of water entering engine,nick

I'd agree there is very little chance but NO chance? Really?

I'm puzzled about why you would want to do such a thing in the first place.

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Well if the water gets as high up as the exhaust port I'm well and truly sinking,or have already,   z drive to Mitsubshi L3e on a narrowboat is a bit of a one off ,unconventional doesn't mean it's not going to be any good,L3e are normally heat exchanger cooled,with wet exhaust,keeping the exhaust quiet,   now a  1000 cc L3e revs to 3000 plus and makes a bloody racket,not like your 2 pot lister plodding at 1200 rpm so it either needs a silencer or  a exhaust pipe in the water and let the river do the silencing I've tryed it with a car type silencer and all was well , but wind was blowing fumes back on to rear deck, does this unpuzzle any thing

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Oh heck, I can see why you'd want to shut that up! ;) However, holes beneath the waterline are something to avoid whenever possible imho, particularly as it strikes me that most of that noise is mechanical and not originating due to poor exhaust silencing.

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When the engine gets switched off, the exhaust will be filled with hot gas. This will cool, and contract. Will it's volume decrease enough to empty the 20" of pipe, allowing said pipe to fill with water?

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Boat builders are often trying to find ways to make a boat a bit quieter, none of them have ever put the exhaust below the waterline.  There must be a good reason for this.  My concern is that very hot exhaust gas will make the steel exhaust pipe hot, and fairly empty of water when the engine is running with a lot of very cold water flowing up the pipe when the engine slows to an idle or stops.  This will give a big thermal shock to the pipe and set up a lot of stress with the temperature differentials.  The pipe will crack and split after a number of cycles and then the boat will sink.

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Its worth a try, just stick it under the water and see if it helps. If it does quieten it down then re do it properly, you need a loop / swans neck to stop  water coming back up the pipe or syphoning and a syphon break as well, a little hole at the highest point with a stub welded on it then a pipe from that to a high point, top of transom or something. Others will probably suggest better ways but I doubt it will make it much quieter anyway.

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1 hour ago, Chewbacka said:

Boat builders are often trying to find ways to make a boat a bit quieter, none of them have ever put the exhaust below the waterline.  There must be a good reason for this.  My concern is that very hot exhaust gas will make the steel exhaust pipe hot, and fairly empty of water when the engine is running with a lot of very cold water flowing up the pipe when the engine slows to an idle or stops.  This will give a big thermal shock to the pipe and set up a lot of stress with the temperature differentials.  The pipe will crack and split after a number of cycles and then the boat will sink.

I'm sure there are such things as underwater exhausts on boats, an outboard motor is one example, but the crucial difference is they are wet exhaust systems and therefore presumably you wouldn't get the above scenario.  Having said that the OP seems to be suggesting just sticking a few inches of his exhaust in the canal, it's hard to see that creating problems.  I suppose it wouldn't do much for the noise though, you might have to shove it down a foot or more before it made a significant difference.

For some reason I'm reminded of that joke about the hippo farting.  

 

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 I agree but in addition all the exhaust smoke will rise up right by the transom/stern with no force to blow it clear of the boat. This is likely to cover the back of the boat in soot and allow the slightest breeze from behind to blow the exhaust over the steerer.

Do a temporary lash up and see

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15 minutes ago, Tony Brooks said:

 I agree but in addition all the exhaust smoke will rise up right by the transom/stern with no force to blow it clear of the boat. This is likely to cover the back of the boat in soot and allow the slightest breeze from behind to blow the exhaust over the steerer...............................................

In which case our OP has a very appropriate name.

Come on Crow, we all want to see what happens...

 

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I think the sheer volume of exhaust gas coming out of an exhaust pipe will spray large volumes of water EVERYWHERE.

A two litre engine doing 1,500rpm is chucking out 9 cubic metres of gas per minute out of the exhaust, without even accounting for the rise in temperature (and therefore volume) from the combustion. 

This is, I suspect, why it isn't normally done.

Edited by Mike the Boilerman
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The trip boat I skipper has two exhaust outlets on the transom, one 3" diameter for the main engine exhaust, and one 2" diameter for the generator (a 20kW 3000 rpm Vetus unit).

 

Both can become submerged when the 8000 litre pump out tanks are full.

 

Both have substantial swans necks, raised nearly 3' above the outlets, to remove any possibility of water entering the exhausts.

 

If submerged, they happily burble away and chuck spray and smoke backwards and upwards- there isn't really any difference between the loudness with them out of the water, or under water.

 

Having a pipe facing vertically downwards into the water might be different, but I'd be more inclined, based on the loudness of the tripboat exhausts, to use a silencer and box in the engine a bit.

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Well I'll let you know,  most my other mods have worked,and like that old hippo it doesn't take any more pressure to fart in water than it does on land,so i don't think that engine will know the difference,it'll be Easter when I finally get it back in water ,when I've done a few snagging jobs and sorted a prop that matches the engine,I recon ive made the swim and keel to good ,and the prop calcs are no where in the Parrish  13x8 is to fast even at 1000 rpm I've no controll over mooring and manovering ,trouble is 8 inch pitch is as low as I can get  for a Enfield ,need to try to make up,or get a 13x6. That'll be a challenge on its own

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On 28/03/2017 at 00:13, Crow said:

Well I'll let you know,  most my other mods have worked,and like that old hippo it doesn't take any more pressure to fart in water than it does on land,so i don't think that engine will know the difference,it'll be Easter when I finally get it back in water ,when I've done a few snagging jobs and sorted a prop that matches the engine,I recon ive made the swim and keel to good ,and the prop calcs are no where in the Parrish  13x8 is to fast even at 1000 rpm I've no controll over mooring and manovering ,trouble is 8 inch pitch is as low as I can get  for a Enfield ,need to try to make up,or get a 13x6. That'll be a challenge on its own

 

I'm reasonably sure when you rev the engine, the exhaust gas will just create/blow itself a hole in the water and sound just the same because the end is once again in fresh air.

Regarding the prop, I suggest an easy way of reducing it would be to grind an inch off the tips of each blade with a grinder. Change it into a 11 x 8 in ten minutes flat!

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20 hours ago, Tony Brooks said:

 I agree but in addition all the exhaust smoke will rise up right by the transom/stern with no force to blow it clear of the boat. This is likely to cover the back of the boat in soot and allow the slightest breeze from behind to blow the exhaust over the steerer.

Do a temporary lash up and see

 I went to visit a standby generator installation which utilised an "exhaust gas scrubber" once. Basically it was a tank full of water which the exhaust gases were forced to bubble through,  to remove any black smoke.

It worked well, but polluted the water in the tank so required daily water changes. The only downside was it created clouds of steam, but I suspect this was due to the relatively small volume of water in the tank.

Apparently exhaust gas scrubbers were first fitted to diesel engined trains used in the mining industry.

So I'm sure an exhaust extended below the water line would work provided measures were taken to prevent  back siphoning. 

Not sure if it would attenuate the sound much though.

Edited to add the last sentance.

Edited by cuthound
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10 hours ago, Crow said:

Yes it's only a little 24 ft springer water bug.  2900 kg   Is dia of prop in relation to pitch a workable solution

Good Grief Christina, how on earth have you managed to fit a diesel engine and an Enfield outdrive to a Waterbug - and doesn't it trim ever so slightly down at the stern..?

Have you got pictures of this set up I am intrigued.

I don't know anything about outdrives - presumably there's no reduction ratio to play with in which case you need to reduce the prop diameter, if you can get it down to 10x8 that should be ok, surely a prop specialist could do that.  

But I'd love to have a go on a Waterbug that has a 13x8...

Edited by Neil2
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Neil when you open the throttle the bow lifts and she pulls like a car from the lights ,I've over a period of time posted photos on this site,look for (peppermint pig).  I'll be posting a few more soon

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11 hours ago, Crow said:

Neil when you open the throttle the bow lifts and she pulls like a car from the lights ,I've over a period of time posted photos on this site,look for (peppermint pig).  I'll be posting a few more soon

Oh I think I would leave it...  The canal equivalent of a Q car..

Or you could sell it to a water skier.

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Right, I've managed to find that bit of vid you posted on the live aboard section.  

Can't you just rig up a bigger silencer on the transom - or is there some reason why you can't have an external silencer on a boat?  

I do think the answer to the excess power is a few inches off the prop dia.   I had a WB with a Honda 15 and it was too much with the standard prop even at low revs (though great for rivers) I think I changed it to 10x7 and that was ideal.

If you get this sorted it's going to be a very versatile boat.  I love Waterbugs.    

 

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On 29/03/2017 at 09:59, Neil2 said:

Right, I've managed to find that bit of vid you posted on the live aboard section.  

Can't you just rig up a bigger silencer on the transom - or is there some reason why you can't have an external silencer on a boat?  

I do think the answer to the excess power is a few inches off the prop dia.   I had a WB with a Honda 15 and it was too much with the standard prop even at low revs (though great for rivers) I think I changed it to 10x7 and that was ideal.

If you get this sorted it's going to be a very versatile boat.  I love Waterbugs.    

 

 

I agree. The main problem with waterbugs is the petrol outboard. A compact inboard diesel running via a Z drive seems a great idea! 

A higher proportion of the work done by a blade is done at the tips, so small reductions in diameter will cut down the thrust especially at low speeds which is exactly what the OP needs. 

I still haven't seen the video. Got a link?

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