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Old 13-06-2010, 17:52   #1
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25' Yanmar 3GM30 Exhaust Run (!)... Max Length / Height ?

Greetings Folks,

My current situation: Yanmar 3GM30, raw water cooled. Engine is mounted amidships, so being a 40' boat, there's about 20' to the transom (direct). Old exhaust run was roughly 8m (24-25'?) long including turns etc and didn't have a gooseneck - just went uphill, with only the exhaust fitting in the transom itself facing down. Total lift from bottom of waterlock was just over 1m (3').

According to what I've read. Everything about this install was Evil(tm) - no gooseneck, too long, too much backpressure etc etc etc.. However, it did work. I have no baseline data like HP lost/temps etc etc.. but it start and ran fine. Only proviso here is that the engine was new (38 hours) to the boat at the time of purchase, and I haven't run it for more than about 10 hours total after that when I started the refit - so since I'm planning on long-term cruising - if this exhaust system is destined to kill the engine in the long-term, but can survive short-term, then that's Really Bad.

As I'm replacing everything in the boat, I'm trying to fix the exhaust run Evil(tm) as well... however, the geometry of the boat does not allow any flexibility (short of running exhaust hose over mattresses/settees etc...

I can't use a gas/water separator. I can't exhaust closer to the engine above the WL. I can't use a standpipe/drystack. Etc etc.. Not good.

After much thinking - there are two things I can do:

One is to follow the exact old layout and add the gooseneck (say 40cm/just over a foot). (But I'm adding more backpressure/length etc - so I don't want this to be the straw that breaks the camel's back)...

Or Two is to cut the exhaust length almost in half, put the gooseneck amidships to loop WELL above the HWL, but exhaust about 10cm (3") below the WL (but I can also add another 3" below WL to the opposite side as well so it looks a bit like a North Sea exhaust... but wet and underwater.

I have no idea what the 3GM30's backpressure limit is - however have found this document for the 3YM30:

http://psyberspace.com.au/yanmar/installationmanual.pdf

Which states on p.5 that the max allowable backpressure is 1500mmAq, which using an online converter has yielded to be 2.1psi...

Now I have no idea how this relates to water and gas in an exhaust pipe and am now even more confused. Who knows, maybe the original install was well within spec and the Yanmar 3GM30 can push a 1000' column of water 40' straight up?

If somebody could enlighten me and let me know whether the original was OK and I should just copy and add the gooseneck and be done with it... or offer some other advice, I would GREATLY appreciate it.

Thanks muchly in advance!
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Old 14-06-2010, 08:43   #2
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.445 psi per foot of water column. do the math and then round off conservatively.
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Old 14-06-2010, 12:36   #3
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I would stick with the original geometry of the exhaust install....

Unless you can get in touch with the builder.
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Old 14-06-2010, 13:15   #4
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Its difficult from a verbal description of the system to see what you really need. You may want to get in touch with a really good engine installer, have him look at the system and see what he recommends.
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Old 14-06-2010, 14:57   #5
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If I recall correctly, back pressure can be lessened by using larger diameter exhaust. 25' is indeed a long exhaust pipe, but I would think exiting to the air would still be less backpressure than exiting below the water line.

It's also hard to tell if you have good water flow in the exhaust if you can't see it.
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Old 14-06-2010, 16:49   #6
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.445 psi per foot of water column. do the math and then round off conservatively.
Thank you. Is this for height only (lift) or length as well? Also - is this for solid water or a mix of gas/water?
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Old 14-06-2010, 16:50   #7
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I would stick with the original geometry of the exhaust install....

Unless you can get in touch with the builder.
Original builder is long gone unfortunately... :\
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Old 14-06-2010, 17:01   #8
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Its difficult from a verbal description of the system to see what you really need. You may want to get in touch with a really good engine installer, have him look at the system and see what he recommends.
Unfortunately I'm in Australia and good services of any industry are very rare. Especially marine... everybody claims to be an expert, charges HEAPS and doesn't know anything. :\

Just as an example - before I bought a torque multiplier, I thought I would have a shipyard tighten my keelbolts and replace the plates/washers. (This is the "best" yard in Sydney harbour).. cost me a PILE and the as*holes took my stainless plates out and replaced them with mild steel... this is in the bilge! ... and you can't argue either as the reply is "Who are you to question us anyway? We are licensed shipwrights and this is how we do it..." (I then bought a torque multiplier and redid the lot and replaced with nice thick 316 plates..)

Anyway, I digress... but this is my problem, I have no access to decent paid professional advice. (Maybe that was the original builder's problem too!

I will call the Yanmar distributor here today and see what advice I get, but I'm not holding my breath on that one either...

Will post back with info though...
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Old 14-06-2010, 17:17   #9
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Originally Posted by Jetexas View Post
If I recall correctly, back pressure can be lessened by using larger diameter exhaust. 25' is indeed a long exhaust pipe, but I would think exiting to the air would still be less backpressure than exiting below the water line.

It's also hard to tell if you have good water flow in the exhaust if you can't see it.
Good point. On this note, I've read that you can tee a small diameter hose before the raw water injection and take that to somewhere visible, which has the effect of reducing water volume in the exhaust (which again, I've read that is generally too much anyway and causes unnecessary backpressure...)

I think I agree with you on the above water exit being less pressure though despite being double the hose length.
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Old 14-06-2010, 17:42   #10
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or run dry exh out the stern

If you've got anyway to do a water lift immediatly behind the engine then raise the wet exahust to a level always above the heeled waterline you can use a water/gas seperator to dump the water overboard at any convenient location. This leave you with a mostly dry exh run out the stern of the boat.......meaning you can now have dip and rises (ie not 100% nessarry to have a downhill run all the way) as you meander your way to the stern.

here's just one of the many available water/gas seperators Centek Industries - Products , or you can build your own. FWIW i've installed a generator in the hanging in the fwd third of an irwin 37 and run the exh out the stern using this system, it does eliminate most of the back pressure concerns since the biggest contributor to the pressure is the water in the hose.
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Old 14-06-2010, 17:48   #11
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Originally Posted by Jetexas View Post
If I recall correctly, back pressure can be lessened by using larger diameter exhaust. 25' is indeed a long exhaust pipe, but I would think exiting to the air would still be less backpressure than exiting below the water line.

It's also hard to tell if you have good water flow in the exhaust if you can't see it.
a Catalina 30 has about 20' of exhaust hose curled up in it.


OP,
What your describing is not out of the ordinary.
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Old 14-06-2010, 17:49   #12
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Originally Posted by Sailmonkey View Post
If you've got anyway to do a water lift immediatly behind the engine then raise the wet exahust to a level always above the heeled waterline you can use a water/gas seperator to dump the water overboard at any convenient location. This leave you with a mostly dry exh run out the stern of the boat.......meaning you can now have dip and rises (ie not 100% nessarry to have a downhill run all the way) as you meander your way to the stern.

here's just one of the many available water/gas seperators Centek Industries - Products , or you can build your own. FWIW i've installed a generator in the hanging in the fwd third of an irwin 37 and run the exh out the stern using this system, it does eliminate most of the back pressure concerns since the biggest contributor to the pressure is the water in the hose.

I would love to use a gas/water separator, but don't have the space. This would indeed eliminate all of my problems, but I've looked at all the manufacturers and can't find anything that will fit into a space only 10cm (3") deep (which is all I have)...

You mention building one - do you have any info on that? If I can make a real small/slim one, then that could definitely be an option!
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Old 14-06-2010, 17:56   #13
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Quote:
Originally Posted by akio.kanemoto View Post

You mention building one - do you have any info on that? If I can make a real small/slim one, then that could definitely be an option!
If you can't get it above the waterline it won't work. I have built one, but it was larger in overall area and volume than the available product, but did fit nicely into the convoluted area that it needed to fit.

I have no info to offer, but they're really simple, just two fiberglass pipes inside a box with a baffle between and a drain at the bottom.
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Old 14-06-2010, 18:00   #14
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a Catalina 30 has about 20' of exhaust hose curled up in it.


OP,
What your describing is not out of the ordinary.
Howdy,

I've read your other various diesel related posts and thank you for your input on my thread.

If this is not out of the ordinary, then just adding the gooseneck should not overly tax the system?

Also - something else that has been bothering me is that the exhaust run is all uphill until the gooseneck which will be just short of the outlet. I have a vetus waterlock which the specs say holds 5L.. exactly how much of the hose is filled with water? Definitely not 100%, but how little? The reason I ask is that when the engine is stopped, all the water in the hose will run back and fill the waterlock... looking at the exhaust run - I would have guessed that the hose would contain more than 5L and thus will fill the waterlock and fill the exhaust manifold etc... Thoughts?
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Old 14-06-2010, 18:11   #15
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If you can't get it above the waterline it won't work. I have built one, but it was larger in overall area and volume than the available product, but did fit nicely into the convoluted area that it needed to fit.

I have no info to offer, but they're really simple, just two fiberglass pipes inside a box with a baffle between and a drain at the bottom.
If it's just gas and there are no restrictions on sags in the exhaust and convoluted runs, I can definitely get it above the WL... I just can't get it above the WL if it has to be "always uphill to the gooseneck with no dips" etc..

Two pipes in a box I can definitely do. Is marine ply soaked in penetrating epoxy with more epoxy on it, then glassed on the outside enough or does the box need to be all made of glass?
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