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Old 12-06-2015, 05:59 PM   #1
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help with exhaust question

I feel like I have seen this discussed somewhere but my search of the archives was not fruitful.
I have already stipulated that I am not a motor head so go easy on me.

I looked at a boat this weekend that had this 1" steel riser on the exhaust that looked like an add-on. Looking for information on what this might be about.
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Old 12-06-2015, 06:08 PM   #2
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That loop with the two 90 deg els is a half hearted attempt to put in a vacuum breaker loop to prevent water from being syphoned up when the engine is shut off. But to work successfully you need an antisyphon valve at the top of the loop. Go to Defender and search for antisyphon va!be and you will see what I mean.

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Old 12-06-2015, 06:37 PM   #3
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Take it off and re-attach the hose to the tranny cooler. I've never seen that before and really don't know what it would do other than add 2' to your cooling system.
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Old 12-06-2015, 07:37 PM   #4
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I think David might be on the right track. It looks like that rig might supply water to the propeller shaft seal. The riser might be an attempt to prevent water from back flowing to the engine? Very strange.
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Old 12-06-2015, 08:44 PM   #5
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Thanks, comments so far support my head scratching. Parks, this is a poor picture to get the whole view but that hose that you see headed for the shaft seal does not come from this added loop. The loop just leaves near the manifold and re-enters the exhaust further downstream. Baffled me. I also did not see how it could provide a vacuum break without a one way valve in the top horizontal section.
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Old 12-06-2015, 09:04 PM   #6
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Quote:
Originally Posted by klee wyck View Post
Thanks, comments so far support my head scratching. Parks, this is a poor picture to get the whole view but that hose that you see headed for the shaft seal does not come from this added loop. The loop just leaves near the manifold and re-enters the exhaust further downstream. Baffled me. I also did not see how it could provide a vacuum break without a one way valve in the top horizontal section.
This is from our FL SP135. The vent line at the top exists about 12" above the water line though about 1/8" hole. The line on the right dumps water on the inboard side of the cutlass bearing just after the stuffing box. I'm not sure if it's needed but after 8k plus hours on the engine it's worked so far.

Edit: At about 1000 to 1200 rpms water will dump out of the 1/8" hole. At higher rpms it's a steady stream. While under way it's one of the boat checks.
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Old 12-06-2015, 09:39 PM   #7
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Gentlemen:

We already have one thread that refuses to die, discussing unproven additions to our fuel systems. Lets not get another one going on our exhaust systems.

Or maybe we could route the raw water through an Algae-X on its way to the mixing elbow?
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Old 12-06-2015, 10:12 PM   #8
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Quote:
Originally Posted by Larry M View Post
This is from our FL SP135. The vent line at the top exists about 12" above the water line though about 1/8" hole. The line on the right dumps water on the inboard side of the cutlass bearing just after the stuffing box. I'm not sure if it's needed but after 8k plus hours on the engine it's worked so far.

Edit: At about 1000 to 1200 rpms water will dump out of the 1/8" hole. At higher rpms it's a steady stream. While under way it's one of the boat checks.

Thanks Larry, that makes sense. The one I am looking at is a 120 from about 70 hulls earlier in the series than yours and maybe a year before they made the switch to the 135. In a quick look,I cannot find pictures of this apparatus on other 120s so maybe this guy thought it was a good idea to add it and was not complete.
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Old 12-06-2015, 11:26 PM   #9
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My 2 Lehman 90's have these well made units, complete with a screw-on siphon breaks, feeding water to the exhaust elbows. Since they are bolted to the engines they eliminate the problem of attaching the loop to a bulkhead which may not be close.
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Old 12-07-2015, 12:26 AM   #10
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That through-hull plumbing just behind the engine is goofy too.
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Old 12-07-2015, 08:15 AM   #11
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I would also be concerned that the outlet of the muffler is above the engine. There appears nothing to stop exhaust water from flowing back into the engine innards. If it ever fails to start close the seacock as with generators before cranking a lot.
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Old 12-07-2015, 08:38 AM   #12
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I will leave discussion re siphon breaks to others.
What I see, and the picture may be misleading, is the exhaust above the lift muffler going up to a 90 and then it appears is if it runs from there slightly up hill. It also looks like there some more space above that 90 and that as the highest point in the exhaust system it should be as high as possible, and everything after that going down hill until the exit at the boot.

An inexpensive safety precaution as well as some additional noise reduction could be from a larger lift muffler. When the engine turns off the water in the lift hose drains back into the muffler. Larger mufflers have more reserve capacity to hold that water safely.

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Old 12-07-2015, 03:07 PM   #13
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Further to my earlier post about an anti siphon valve, let me elaborate:


Virtually all sailboats have a vented loop with an anti siphon valve. On sailboats the water injection point is often below the water line. In this case when the engine stops, water can continue to siphon. So boat builders put a loop in the hose just before the mixer and an anti siphon valve which is nothing more than a check valve with a very weak spring that will break the vacuum.


In Larry's photo the small diameter hose at the top of the SS loop that exits well above the water line does the same thing but without a valve.


In your case, Bill the builder didn't really know what he was doing, he had seen other installations and he thought he was doing the same thing. But without a vent of some sort, it will not necessarily break the vacuum.


So first check the height above or below the water line where the water squirts out of the mixer. If it is a few inches above the water line, then forget it, you don't need anything. If it is below, then consider adding a vent- either out the side of the boat like Larry's or much easier with an anti siphon valve like this one: Forespar MF-841 Replacement Duck Bill / Cap Assembly


You will probably have to drill and tap the top of the loop for a 1/4" pipe thread. Then install the anti siphon valve with a 1/4" SS nipple.


I know that the foregoing was more than most of you wanted to know about anti syphon loops and I am probably showing off a bit by posting all of this drivel, but the OP wanted to know, so now he has it.


David
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