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Old 28-07-2021, 16:15   #1
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Exhaust expertise please

Is there anyone on this forum who is a bof/fundi/guru on exhaust gas principles, and who would deign to explain to this ignoramus in a way that he can grasp?

Looking at the Vetus installation recommendations: after the water-injection elbow, a large diameter rubber pipe (75mm) is connected to the large 75mm, 10.5 liter Vetus water-lock (all good thus far, and no issues with my understanding as yet), BUT then this whacking great pipe continues (75mm dia) and it is connected to PVC piping to make a large loop to close to the deck, a lift of nearly 1.3m, before dropping down again to just above water-level. This means that the exhaust gasses have to hold a LOT of water in suspension going up that large 1.3m lift. Also, when the engine is shut down, all that water in suspension at the time falls back into the water-lock.
My intuition tells me that a smaller diameter pipe will use less engine power as less water will be held in suspension. My gut feel is that a 40mm dia pipe would be ample. This would also give greater reserve capacity in the water-lock, which is a good thing. Is there a gas/fluids engineer out there that can correct me with a reasoned explanation?

As a 2nd issue - It costs energy to get the water up the 1.3m incline: is there another good way of preventing seawater backflowing into the outlet pipe? Is so I could exit the outlet at about 0.8m height, which is still maybe 0.6m above sea-level.

Thanks in advance.
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Old 28-07-2021, 17:12   #2
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Re: Exhaust expertise please

No, no, no!

OK, I'm not a gas/fluids engineer. but bear with me.

Leaving aside the water, clearly a large diameter pipe offers less resistance to fluid flow. I'm sure you know this. So bigger is better exhaust wise.

Now for the water, regardless of pipe diameter, the amount of water ejected equals the amount of water injected by the raw water pump. It can never be more. So if we have to get the water out, it is better to use the lowest resistance to the flow as possible. Again this means a larger diameter pipe rather than a smaller one.

Diesels prefer low back pressure, big pipes lower the back pressure.
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Old 28-07-2021, 17:18   #3
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Re: Exhaust expertise please

Quote:
Originally Posted by JeremyFowler View Post
......
As a 2nd issue - It costs energy to get the water up the 1.3m incline: is there another good way of preventing seawater backflowing into the outlet pipe? Is so I could exit the outlet at about 0.8m height, which is still maybe 0.6m above sea-level.

Thanks in advance.
Consider the maximum height of a wave smacking up against the exhaust outlet. It is possible for it to reach gunnel height. Reduce the max height of the exhaust loop with caution.
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Old 28-07-2021, 18:02   #4
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Re: Exhaust expertise please

From practical observation and some intense research when I re-did my exhaust system I would add the following comments..


You need to use a diameter recommended by the marine engine manufacturer. Even though the engine will run with a smaller diameter exhaust, this increases back pressure which can effect engine performance.


Water isn't pushed along the pipe in large volume, as the design of the water lock only allows small volumes of water to be "pulsed" (for want of a better word) through the exhaust system post waterlock.


Maximum lift should be no more than about 1.2m from the base of the waterlock (IIRC).



Minimizing the length of exhaust system is the most beneficial way to reduce the volume of water within the exhaust and which will flow back to the water lock. Routing is the most obvious factor, but the design of the swan neck can also influence this. For example, just having flexible hose in a loop versus a "proper" swan neck increases the length of the exhaust system.



If you need to lower the head height of the exhaust, a method that helps prevent back flow is to use a swan neck with a larger diameter tube post "hump" compared to the exhaust system and outlet through hull in general. This tube has greater volume which helps prevent the water overflowing back into the exhaust in heavier seas as it slows the rise of water inside the tube. A rubber flapper valve over the exhaust outlet will also help here.


One final note about PVC is that it can have a low melting point and might not be suitable for exhaust use unless specifically selected for the task. Some types of PVC have maximum service temperatures as low as 60 deg C.
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Old 28-07-2021, 19:07   #5
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Re: Exhaust expertise please

Thanks for the replies, but...
Wotname, I get your argument that the amount of water IN has to be the same as the water OUT, but the issue is the water that is carried in the system at any one moment that has to be held in suspension by the exhaust gasses. Extending the argument ad absurdum to an enormous diameter and length exhaust pipe means that the water that needs to be suspended is enormous too, although the back pressure will be approaching zero for the exhaust gas alone. It is a combination of water weight and gas back-pressure. This is what I'm trying to get my head around.
Reefmagnet, thanks for your points. The flap valves are readily available I see.
The pvc has worked fine to date, so the operating temp must be ok.
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Old 28-07-2021, 19:34   #6
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Re: Exhaust expertise please

Yanmar 4JH4TE, 72hp with turbo. Center cockpit.

I have had a LOT of issues with exhaust runs. Engine sits below water line and is a Looong way from the transom. With unlimited budget I would reroute exhaust out side and/or use a water separator. On Earth I have to cope otherwise.

The Vetus water lock muffler did overheat enough to deform the inlet pipe. The exhaust elbow was modified to lift it above water level, good. Cheap **** Yanmar exhaust elbow keeps leaking at the factory welds and broke off the flange due to engine vibration and real thin tubing. Custom made exhaust snapped stud off of turbo due to extra unsupported weight. Now back to an aftermarket exhaust elbow but it is below waterline. Which is an issue if the anti-siphon should fail.

More to your point. The real long exhaust has a Vetus check valve that allows one way flow. It keeps water from flowing back into thr water lock. Then there is a goose neck before exiting through a flapper. The water lock is now a custom ss job in a deep bilge, sized to hold the water in the hose. Now that won’t help if there is a following sea for a prolonged period. Or, as I had one night, a swell against tide situation that caused nasty pitching with the sea slamming into the transom. Ug!

I think I understand your question well enough. It is easier to push a small volume than a big volume. So why not use a smaller exhaust hose?

My GUESS is that the resistance due to water weight is only for a small period of time. It is only every 1 second or so that a belch of water is disgorged. At 2,000 rpm that is once every 33 engine revolutions. That is something the engine can tolerate, especially since the exhaust gasses are compressible and there is no big shock to the engine. There is a trade off and the engines desire for low back pressure wins.

I keep thinking there has to be a better way, my wallet disagrees.
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Old 28-07-2021, 19:40   #7
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Re: Exhaust expertise please

To get your head around how the water gets expelled, it is probably easier to imagine the water being vacuumed out of the exhaust, as the principle is pretty much the same in relation to the pressure differential.
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Old 28-07-2021, 20:25   #8
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Re: Exhaust expertise please

Hi Jeremy, your concerns are well founded and one of the “into service “ checks an engineer makes with a new installation is to measure the amount of raw water remaining in the aqualock at shutdown....... and often the results are startling.... as far as half way up the pipe to the mixer elbow. So the remedies are ....1 multiple aqualocks.... 2.Larger single Aqualock....3.lower the bulkhead loop ( tricky but do able) and finally, drop the water out of the exhaust via a water separator and run the bulk of the exhaust practically dry.
An additional problem can arise from the siphon break loop. if it’s high up with long plumbing this drop down amount needs to be taken into consideration because one side of that loop drops directly into the mixer elbow at shutdown with some speed and no exhaust back pressure. One of the hardest things to get totally correct with a new engine installation is the wet exhaust and if you get it wrong the results are anywhere between damaging and catastrophic....... often the original old exhaust system was badly designed from the manufacturer and owners frequently say “ it’s worked for years like it is, why change it” and it’s hard to argue against that logic.
I have instances of the 4 different solutions (though I’m sure there are many more solutions) , if you’re interested in the fine detail I can post or PM me.
Pete.
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Old 29-07-2021, 07:21   #9
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Re: Exhaust expertise please

Quote:
Originally Posted by JeremyFowler View Post
... BUT then this whacking great pipe continues (75mm dia) and it is connected to PVC piping to make a large loop to close to the deck, a lift of nearly 1.3m, before dropping down again to just above water-level...
Dave Gerr recommends that the anti-siphon exit loop rise no more than 42"

See ➥ https://www.cruisersforum.com/galler...r&imageuser=79

And ➥ https://www.cruisersforum.com/galler...r&imageuser=79


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Old 29-07-2021, 07:30   #10
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Re: Exhaust expertise please

Gord,

That is all well and good, except some boats don’t fit that profile.

Then you need to do something different.

In this image there is no goose neck, but the functionality is there because he shows a rise in the exhaust pipe.
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Old 29-07-2021, 07:59   #11
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Re: Exhaust expertise please

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Originally Posted by hpeer View Post
That is all well and good, except some boats don’t fit that profile.
Then you need to do something different.

In this image there is no goose neck, but the functionality is there because he shows a rise in the exhaust pipe.
And a downward slope [min. 1/4" per Ft.] in the exit.


Here's how Yanmar illustrates it:
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Old 29-07-2021, 08:06   #12
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Re: Exhaust expertise please

Northern Lights have an excellent technical paper, with exhaust System Installation Information:
“Please Don't Drown Me”https://www.northern-lights.com/medi...t_drown_me.pdf
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Old 29-07-2021, 08:46   #13
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Re: Exhaust expertise please

Reefmagnet: “One final note about PVC is that it can have a low melting point and might not be suitable for exhaust use unless specifically selected for the task. Some types of PVC have maximum service temperatures as low as 60 deg C.”

I installed an exhaust temp meter on my Yanmar 3JH4E just after the exhaust elbow. In the Med in summer I get a temp of 37/38 °C at normal RPM of 2500. At WOT 3200 rpm the temp is 48°C. I expected a much higher temperature, but these are the values observed.
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Old 29-07-2021, 09:07   #14
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Re: Exhaust expertise please

Great info above.
Does anyone include an exhaust flapper?
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Old 29-07-2021, 09:09   #15
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Re: Exhaust expertise please

Quote:
Originally Posted by Wotname View Post
No, no, no!

OK, I'm not a gas/fluids engineer. but bear with me.

Leaving aside the water, clearly a large diameter pipe offers less resistance to fluid flow. I'm sure you know this. So bigger is better exhaust wise.

Now for the water, regardless of pipe diameter, the amount of water ejected equals the amount of water injected by the raw water pump. It can never be more. So if we have to get the water out, it is better to use the lowest resistance to the flow as possible. Again this means a larger diameter pipe rather than a smaller one.

Diesels prefer low back pressure, big pipes lower the back pressure.
Not sure all assumptions here are correct....?
-A larger pipe offers less resistance to the exhaust as a whole, but higher velocity, (smaller pipe) ejects better. Think of it this way; if the pipe was 3 feet diameter and rising up, how much water would go out? Probably none.
-Further: the amount of water ejected does NOT equal the amount of water injected... that's why there is water left in the system to flow back into the muffler (or engine!)
Just a thought........
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