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Old 27-08-2016, 02:20   #1
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Exhaust modifications

Hi Cruisers,

I have an exhaust dilemma.

A year or more ago I changed our engine from hard mounted to soft mounted. Long story, but in summary, it was HORRIBLE with the hard mounts. It is a 4 cylinder, 3.5 litre engine, so it was just too rough.

As part of the modification I introduced a section of flexible house between the cast iron exhaust pipe and the water muffler, immediately after the water injection point and before the muffler. This has worked well. It has allowed the iron exhaust to move around with the engine, while leaving the muffler hard mounted to the hull.

But, I woke up yesterday with a horrible thought. It suddenly occurred to me that that big iron exhaust pipe must be putting incredible strain on the mounting points to the exhaust manifold, due to its length. It is nearly 1 meter tall, going up well above the waterline before coming down again for the water injection point. All great design in preventing syphoning and inadvertently flooding the exhaust valves, but not great when the engine is jumping all over the place as it does when cold and idling.

I have looked at the possibility of redesigning the exhaust to come straight down from the manifold to the muffler, but there simply is no space, the current muffler placement (about level with the engine head) is the only option available to me.

So now I am thinking I have two options:

1. Weld up a serious support arm to brace the exhaust pipe so that it is not putting strain on the manifold.

2. Put a flexible section between the manifold and the raw water mixing point.

I am hoping for option 2 for a couple of reasons, not least of which is the bracket would be difficult to weld and would add clutter to an already busy engine bay. But I am worried about introducing a flexible coupling BEFORE the water mixing point. I cannot work out if this is possible. Instinct says it will get too hot for a rubber bellows such as I have fitted between the mixing point and the muffler, but maybe I am wrong?

Any suggestions welcome.

Matt

(Pictures attached showing current arrangement and proposed arrangement with extra flexible section.)
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Old 27-08-2016, 02:29   #2
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Re: Exhaust modifications

Matt, that's a lot of exhaust heat sink exhaust before the water cooling. The exhaust should be cooled soon after the manifold.

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Old 27-08-2016, 03:13   #3
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Re: Exhaust modifications

Quote:
Originally Posted by SimonV View Post
Matt, that's a lot of exhaust heat sink exhaust before the water cooling. The exhaust should be cooled soon after the manifold.

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It might be a bit long, but it has done over 5000 hours, including a trip where the engine ran continuously for 5 days and nights (the original owner motored from Brisbane to Vanuatu), so it works ok. With the heat wrap on it, it is kept to a temperature where it is safe to work on the engine even when the engine has been running for a while. I wouldn't want to hold it firmly, but it does not burn you if you brush it with your hand.
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Old 27-08-2016, 04:34   #4
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Re: Exhaust modifications

Yeah, its a bit of a worry, my folks have a similar system but the bolted on riser doesn't seem to have ever given any problems except for rusting out every 15 years or so.

You can get stainless steel flexible billows but they are prone to getting holes and needing replacement every couple of years or they start leaking exhaust gasses. Id avoid using them again unless I really had no other option.

I would favour the brace option and making sure the bolts are all regularly checked and torqued up.

The whole riser can be built quite lightly out of duplex stainless steel to reduce the weight that needs supporting.

Heat is an issue as I am sure you know. Lots of fibreglass insulation tape around it works for me.

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Old 27-08-2016, 04:53   #5
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Re: Exhaust modifications

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... my folks have a similar system but the bolted on riser doesn't seem to have ever given any problems except for rusting out every 15 years or so.
Anything good enough for the NZ Maid is good enough for me.

Is their engine soft mounted or hard mounted?

Matt
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Old 27-08-2016, 15:34   #6
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Re: Exhaust modifications

It used to be hard mounted, but now it's soft mounted. No problems with it so far, but only a 1000 or so miles on it since the change over.

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Old 27-08-2016, 16:08   #7
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Re: Exhaust modifications

The stainless bellows style used on most new cars will probably outlive you before they get any "pin holes".
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Old 27-08-2016, 16:40   #8
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Re: Exhaust modifications

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The stainless bellows style used on most new cars will probably outlive you before they get any "pin holes".


This is the sort I used on my dry exhaust. Welded in by a car exhaust place. They worked, but I found after a few years they leaked a small amount of exhaust gas, inspecting them inside showed corrosion and some small "pinholes".

They were not horribly expensive to replace, but it was a nuisance. Probably on a car you wouldn't notice the slight leak due to it being outside the vehicle. The alignment of them is critical, as is the correct allowance for expansion. There is a type with a solid tube inside it for high temperature and high flow environments that reduces the heating of the bellow parts.



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Old 27-08-2016, 16:54   #9
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Re: Exhaust modifications

[QUOTE=Snowpetrele. There is a type with a solid tube inside it for high temperature and high flow environments that reduces the heating of the bellow parts.


I'm familiar with that style and would never trust it inside a boat. There is a metal graphite seal inside the coupling that wears out.
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Old 27-08-2016, 18:54   #10
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Re: Exhaust modifications

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I'm familiar with that style and would never trust it inside a boat. There is a metal graphite seal inside the coupling that wears out.
No, Thats not the type I am talking about.
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Old 27-08-2016, 20:04   #11
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Re: Exhaust modifications

Gilow, We had the same exact set-up on Panope's previous engine for about 20 years. The iron pipe "riser" was about 2 feet tall and it had the "serious support arm" to keep it rigid to the engine block. The brace was quite crude being bolted together (my Dad was not a welder) but it worked fine, happily shaking away for all those years.

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Old 27-08-2016, 21:09   #12
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Re: Exhaust modifications

These have flow liners inside them. As shown they are quite tight. For a marine engine you would probably need bigger tolerances. They need to be mounted the correct way around. No metal bellow likes being insulated (heat is the enemy) so they are best left bare, or given only a loose shroud around them.

If I was doing one from scratch for a dry exhaust or a big riser I would try to put it in its own trunk with a separate electric exhaust fan to keep it cool and vent any stray exhaust gasses out of the boat. It's not a question of if it will leak, but when. Small leaks, expansion and heat are a big part of why water cooled exhausts are more popular than dry exhausts.
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Old 28-08-2016, 03:32   #13
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Re: Exhaust modifications

Hey guys. Thank you all for the terrific replies.

On balance, after reading what you have written, I think I will go ahead and start welding up the bracket. The flexible coupling looks like it will be very difficult. Particularly problematic if it needs to be welded to the exhaust pipe as the current riser is (I think) cast iron. Then between the need for careful alignment and the possibly limited life span it all just feels like trouble.

Ben, I'd love to know what prompted your folks to convert to a soft mount after all those years.

Steve, you've made me feel a little better, not least for reminding me how much more difficult these sorts of jobs were before I could weld.

Fair winds etc, Spring in a few days here and I for one am looking forward to some more pleasant sailing conditions.

Matt


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Old 28-08-2016, 05:10   #14
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Re: Exhaust modifications

Anyone have a go to reference manual or guide for engine exhausts, & how to set them up? As well as inspect them?
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Old 28-08-2016, 16:09   #15
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Re: Exhaust modifications

http://www.gerrmarine.com/BOAT_MECHA...S_HANDBOO.html this is a pretty good starting point.

Matt, my folks switched over partly to make alignment easier. The old system needed shimming to the exact heights and everything was getting old so when they rebuilt the beds they added flexible mounts. It's now much smoother, as you would expect.

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