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Replacing my exhaust and how to choose a hospital silencer?


jetzi

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4 hours ago, Cheshire cat said:

In the end I bought the largest silencer that would fit in the space.

Yes... that's what I've been thinking. Having said that, I dont know what the difference is between an ordinary silencer and a hospital silencer. I think I've got an ordinary one, but only because it doesnt look very big.

 

Buy something that fits the existing holes and space, get it fitted, and see how it goes :) 

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On 14/08/2019 at 20:39, WotEver said:

As an aside, didn’t someone

mention recently that for private boats the exhaust no longer has to be lagged? I can’t be arsed to go look through the BSS checklist right now. Obviously a better job with it lagged though, and safer. 

I've just had a hospital silencer fitted, and it is not lagged (though the connections to and from it are). It doesn't get very hot. Is the argument against lagging that it traps moisture in the winter months and so aids corrosion? 

I now get absolutely no noise from the exhaust, even standing on the towpath. The downside is that I have a number of "new" rattles and squeaks that were obscured before. And I have to recalibrate my (aural) rev counter as the one on the boat has not worked for over 20 years ....

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6 hours ago, Richard10002 said:

Buy something that fits the existing holes and space, get it fitted, and see how it goes :) 

As above really. This could be done for less than £100. Going with your other plan I would suggest the cost will be around £700 ish. Cost everything carefully and don't assume/guess low prices without checking. 

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7 hours ago, Richard10002 said:

Yes... that's what I've been thinking. Having said that, I dont know what the difference is between an ordinary silencer and a hospital silencer. I think I've got an ordinary one, but only because it doesnt look very big.

 

Buy something that fits the existing holes and space, get it fitted, and see how it goes :) 

 

Noise attenuation and thus size for the same attenuation.

 

"Hospital silencer" is a term devised by a marketing man. Silencer manufacturers (as opposed to resellers) do not recognise the term.

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18 hours ago, Machpoint005 said:

Probably worth talking to Beta Marine -- that spec might be very easy to meet.

But how do I calculate the back pressure? Surely that is not something Beta can help with since they specify the requirement (80mm Hg) rather than the effect of the exhaust system?

 

12 hours ago, Cheshire cat said:

Hmmm. I'd say that's beyond repair. What does it look like the other side of that bulkhead? At least you won't have too much trouble removing the old one.

There is another 6 inches of silencer beyond the bulkhead, then a 90 degree elbow on the outlet sending the exhaust vertically down 2' in a 2" pipe, another 90 degree elbow and another 2' of pipe to the engine. I haven't taken the lagging off any of this but I assume the pipework also needs replacing.

 

12 hours ago, Cheshire cat said:

I went to TW Marine with the idea of having a hospital silencer fitted but they persuaded me it was a bad idea. There wasn't enough space

I don't have the space problem a a cruiser stern. I measured that the silencer could easily be 12 inches in diameter and up to 4 feet long and all it would mean is cutting a larger hole in the bulkhead. I don't think I'd want an extremely massive one as then it might make other engine maintenance a bit awkward. But other than that space is no reason for me to forgo getting a larger silencer.

 

1 hour ago, mad dog n' Englishman said:

As above really. This could be done for less than £100. Going with your other plan I would suggest the cost will be around £700 ish. Cost everything carefully and don't assume/guess low prices without checking. 

I went to P&S Marine yesterday and got a quote for the replacement. Everyone is so busy that no one actually wants the job, so I'm acutely aware that the cost is inflated. They said at least a day's work plus all the bits and I'm looking at around 600 gbp. That's with a 5 inch silencer like the one I have. They said if I want to fit a larger silencer the answer is no to that job,

 

Good advice, I'll definitely put together the parts list and cost it all before I take the plunge.

 

1 hour ago, cuthound said:

"Hospital silencer" is a term devised by a marketing man. Silencer manufacturers (as opposed to resellers) do not recognise the term.

Yes, this is what I have found in my travels too. Thanks for pointing this out. "Hospital silencer" seems to just be a non-technical term for "awkwardly big but very quiet silencer".

Edited by ivan&alice
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34 minutes ago, ivan&alice said:

But how do I calculate the back pressure? Surely that is not something Beta can help with since they specify the requirement (80mm Hg) rather than the effect of the exhaust system?

 

Ask the silencer maniufacturer? They will know the figures for their products.

The maximum 80mm of mercury is quite high value though. Less than that isn't a problem!

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1 minute ago, Machpoint005 said:

Ask the silencer maniufacturer? They will know the figures for their products.

The maximum 80mm of mercury is quite high value though. Less than that isn't a problem!

Odd that you have to ask them, and they don't specify the figure in the description of their products, as it is so important. Makes it quite hard to compare.

 

Also, I presume that lengths of pipe, pipe diameterand elbows contribute to back pressure too? One would think there would be a rule-of-thumb formula available.

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36 minutes ago, ivan&alice said:

"Hospital silencer" seems to just be a non-technical term for "awkwardly big but very quiet silencer".

 

Yes, it is. The term "silencer" is also a misnomer -- better ones are "attenuator" (but nobody will understand it) or "muffler" (which is American English, but at least it is accurate).

 

A "hospital silencer" may well have dissipative materials (absorption) as well as the standard reactive section (like the silencer muffler on a car).

 

 

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56 minutes ago, Machpoint005 said:

 

Yes, it is. The term "silencer" is also a misnomer -- better ones are "attenuator" (but nobody will understand it) or "muffler" (which is American English, but at least it is accurate).

 

A "hospital silencer" may well have dissipative materials (absorption) as well as the standard reactive section (like the silencer muffler on a car).

 

 

lovely word : muffle.  just something about it, but I can't put my finger on it.

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1 hour ago, ivan&alice said:

Odd that you have to ask them, and they don't specify the figure in the description of their products, as it is so important. Makes it quite hard to compare.

 

Also, I presume that lengths of pipe, pipe diameterand elbows contribute to back pressure too? One would think there would be a rule-of-thumb formula available.

You  are correct about length & diameter of pipes etc. However I am going to stick my neck out here and some will chop it off.

 

For any reputable silencer make the exhaust back pressure will not be an issue on a canal boat when the silencer is new. It may be when its old and falling apart internally or badly carboned up. If it is too higher then it will reduce the volumetric efficiency of the engine  and thus reduce the power produced but I don't see that as being a major issue for canal boats. Most have engines that are far to powerful for everyday needs anyway.  In really bad cases of excess back pressure it may cause the engine to smoke because the burned gasses remaining in the cylinder will stop sufficient air being drawn in.

 

A boat is unlikely to have a tuned exhaust where the column of moving exhaust gasses is used to help draw gasses from the cylinder and that is where back pressure becomes really important, I suspect more a high performance two stoke problem than a boat type diesel engine.

 

My feeling s that as long as you use the largest bore pipework you can without too many porblems and sue bends rather than elbows all will be fine.

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1 hour ago, Tony Brooks said:

You  are correct about length & diameter of pipes etc. However I am going to stick my neck out here and some will chop it off.

 

For any reputable silencer make the exhaust back pressure will not be an issue on a canal boat when the silencer is new. It may be when its old and falling apart internally or badly carboned up. If it is too higher then it will reduce the volumetric efficiency of the engine  and thus reduce the power produced but I don't see that as being a major issue for canal boats. Most have engines that are far to powerful for everyday needs anyway.  In really bad cases of excess back pressure it may cause the engine to smoke because the burned gasses remaining in the cylinder will stop sufficient air being drawn in.

 

A boat is unlikely to have a tuned exhaust where the column of moving exhaust gasses is used to help draw gasses from the cylinder and that is where back pressure becomes really important, I suspect more a high performance two stoke problem than a boat type diesel engine.

 

My feeling s that as long as you use the largest bore pipework you can without too many porblems and sue bends rather than elbows all will be fine.

Agreed,back pressure on a boat exhaust is largely irrelevant.

Lagging, not required on a silencer or even the pipes and fittings unless it is in an area where excess heat could cause damage or accidental burns to operative.

I lag silencers with old fire blankets, much easier than rolls of glassfibre.

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Thanks, I shan't worry too much about the back pressure then - I will just endeavour to use gentle bends and perhaps upgrade to 2" pipework and that should cover my larger silencer.

 

I took the plunge and removed the lagging from the rest of the exhaust system to see the damage. My suspicion that the lagging was structural proved correct.

 

The pipes are flexible sections with BSP fittings connecting into sharp 90 degree elbows. The pipe itself seems OK but the nuts and fittings and crumble to the touch so I am going to replace the whole lot.

 

My next question is: how the heck am I going to remove the pipe from the exhaust flange on my engine? I have a strong suspicion that they will be completely bonded together. The exhaust flange does seem a bit stronger than the pipework but until I attempt to dismantle it I can't be sure if that too will need replacing.

 

It looks to me like the easiest way for me to dismantle everything would be to remove the exhaust flange and gasket, which is held in place to the engine by four nuts and provides a piece of threaded pipe (presumambly BSP) for the exhaust pipe to connect onto. Then I can simply replace the flange (and gasket) and everything thereafter.

 

I haven't been able to source an exhaust flange specific to my Beta 38 / BV1505 engine. I found these at ASAP: https://www.asap-supplies.com/exhaust/exhaust-engine-outlets/multi-choice-exhaust-flanges

 

Apart from being eyewateringly expensive, they mention " a spray head exhaust outlet as part of a water injection bend for your boat exhaust outlet" which leads me to believe they might be for wet systems?

 

Do you think that replacing the flange and gasket is a good solution? Any idea where I could get an exhaust flange specific to my engine?

 

 

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15 minutes ago, ivan&alice said:

 

 

It looks to me like the easiest way for me to dismantle everything would be to remove the exhaust flange and gasket, which is held in place to the engine by four nuts and provides a piece of threaded pipe (presumambly BSP) for the exhaust pipe to connect onto. Then I can simply replace the flange (and gasket) and everything thereafter.

 

 

 

 

I removed the flange and took it to my local garage who got it cherry red to remove the pipe sub. Used a new gasket when I put it back using the old flange. You may be able to get a new one from Beta if you need it.

 

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41 minutes ago, ivan&alice said:

Do you think that replacing the flange and gasket is a good solution? Any idea where I could get an exhaust flange specific to my engine?

 

 

If the flange is still in good order can you not cut the old fitting off and have a new threaded pipe welded back on?

Ebay does sell a suprising good selection of exhaust flanges-you may get lucky and find one that suits.

I also used ebay for my threaded pipe sections between fittings -search for 'threaded 1 1/2 pipe' - saved me  alot of hassle. (Guessing if yours is a 2" pipe its 1 1/2 bsp)

 

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2 hours ago, PaulJ said:

If the flange is still in good order can you not cut the old fitting off and have a new threaded pipe welded back on?

Ebay does sell a suprising good selection of exhaust flanges-you may get lucky and find one that suits.

I also used ebay for my threaded pipe sections between fittings -search for 'threaded 1 1/2 pipe' - saved me  alot of hassle. (Guessing if yours is a 2" pipe its 1 1/2 bsp)

I did have a look at eBay but I couldn't see any suitable flanges. Thanks for the tip on the pipe though - that will be helpful.

 

2 hours ago, ditchcrawler said:

I removed the flange and took it to my local garage who got it cherry red to remove the pipe sub. Used a new gasket when I put it back using the old flange. You may be able to get a new one from Beta if you need it.

Do you mean they heated it up?

 

I think I might be able to cut the pipe off lengthways with an angle grinder, if I'm careful not to cut into the thread, as the pipe is brittle and I think I could break it off in pieces. There would even be a fair bit of screw thread left on the flange if I just cut the pipe off cross-sectionally, might be enough to re-use the flange ..

 

How are they normally fixed? Are they welded, gunked or just screwed on? My worry is that once I start the job I need to finish it quickly! My engine is my electricity and my hot water so I really can't be without it for long. So I think I'd rather buy the flange and have it available, I can always return it if I end up not needing it.

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1 minute ago, ivan&alice said:

I did have a look at eBay but I couldn't see any suitable flanges. Thanks for the tip on the pipe though - that will be helpful.

 

Do you mean they heated it up?

 

I think I might be able to cut the pipe off lengthways with an angle grinder, if I'm careful not to cut into the thread, as the pipe is brittle and I think I could break it off in pieces. There would even be a fair bit of screw thread left on the flange if I just cut the pipe off cross-sectionally, might be enough to re-use the flange ..

 

How are they normally fixed? Are they welded, gunked or just screwed on? My worry is that once I start the job I need to finish it quickly! My engine is my electricity and my hot water so I really can't be without it for long. So I think I'd rather buy the flange and have it available, I can always return it if I end up not needing it.

Yes red hot then unscrewed the pipe, I think mine had a female coupler welded to the flange.

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I also sent the following email to TW Marine:

 

I need to replace my narrowboat's exhaust system and I would like to upgrade to a "hospital" silencer while I'm at it.
 
The maximum size silencer my boat could possibly accommodate is 12" diameter and 4' length. The exhaust skin fitting is 2' above the engine exhaust outlet. My current engine (Beta 38 / BV1505) has an exhaust flange with a 1&1/2" BSP thread, which appears to be bonded to the old exhaust system.
 
I have found a few options online
 
 
which all look quite similar (12" x 36", with side entry), and range from 255 GBP to 315 GBP.
 
These seem a bit overkill and I would like a slightly smaller and cheaper silencer. You were recommended to me and I thought I would ask what you have available?
 
Note I would also need around 2 feet of flexible hose for the riser, a gentle 90 degree bend and exhaust lagging. I would also need to free the exhaust flange or replace it.
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25 minutes ago, ivan&alice said:

I also sent the following email to TW Marine:

 

I need to replace my narrowboat's exhaust system and I would like to upgrade to a "hospital" silencer while I'm at it.
 
The maximum size silencer my boat could possibly accommodate is 12" diameter and 4' length. The exhaust skin fitting is 2' above the engine exhaust outlet. My current engine (Beta 38 / BV1505) has an exhaust flange with a 1&1/2" BSP thread, which appears to be bonded to the old exhaust system.
 
I have found a few options online
 
 
which all look quite similar (12" x 36", with side entry), and range from 255 GBP to 315 GBP.
 
These seem a bit overkill and I would like a slightly smaller and cheaper silencer. You were recommended to me and I thought I would ask what you have available?
 
Note I would also need around 2 feet of flexible hose for the riser, a gentle 90 degree bend and exhaust lagging. I would also need to free the exhaust flange or replace it.

https://www.midlandchandlers.co.uk/products/bellows-1-1-2-bsp-m-m-18-o-a-length-e324

https://www.midlandchandlers.co.uk/products/exhaust-flange-bowman-1-1-2-bsp-ea-045

 

My exhaust system has two of these in it https://www.midlandchandlers.co.uk/products/pipe-union-black-2-11340-09-ea-123

Edited by ditchcrawler
added a bit
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On 16/08/2019 at 07:12, mad dog n' Englishman said:

As above really. This could be done for less than £100. Going with your other plan I would suggest the cost will be around £700 ish. Cost everything carefully and don't assume/guess low prices without checking. 

Here's my parts list. Most of this stuff from Midland Chandlers, who had the largest hospital silencers and also the only place I could find the flexible exhaust pipe (with the sort of braided exterior).

 

6.5       GBP    Exhaust Flange Gasket
24.9     GBP    Exhaust Flange Bowman 1.5" BSP (female)
14.9     GBP    Malleable Iron 90 degree long bend 1.5" BSP (male to female)
63.9     GBP    24" Flexible Exhaust Pipe 1.5" BSP (male to female)
255      GBP    Hospital Silencer 12" diameter X 32" length 1.5" BSP (male side entry to male exit)
4.3       GBP    Malleable Iron Socket 1.5" BSP (female to female)
14.2     GBP    600mm Black Medium Steel Tube 1.5" BPS (male threaded one end) (tailpipe)
46.9     GBP    30m Exhaust Lagging (titanium not fibreglass)
6.37     GBP    Copaslip 100g (to ensure the system will come apart in future)
434.06 GBP    TOTAL

 

I'll hopefully be able to return some of this if I manage to save some of my parts like my existing flange.

 

I would have thought that going from male to female in the direction of gas flow would be ideal, but the parts don't seem to work out that way.

 

I also have decided to site the silencer entirely within my engine bay, which will allow me to cut just a 50mm hole in the bulkhead and hull to let my tailpipe out. I'm thinking that this will also keep it even quieter while freeing up locker space.

 

I'm going to call T W Marine tomorrow and see if they can't do a better deal, but if not I'm going to go with this list from Midland.

 

Please comment if you see any issues with this plan. Thanks all for the advice.

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On 17/08/2019 at 18:23, ditchcrawler said:

I removed the flange and took it to my local garage who got it cherry red to remove the pipe sub. Used a new gasket when I put it back using the old flange. You may be able to get a new one from Beta if you need it.

 

Just checked my flange and I was wrong, it is a male thread, so with care you could slice the coupling off

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34 minutes ago, ditchcrawler said:

Just checked my flange and I was wrong, it is a male thread, so with care you could slice the coupling off

Mine is also male. So I will try to do the angle grinder most of the way + hammer and chisel. Who knows it might even unscrew ?

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