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Old 11-13-2018, 02:19 PM   #1
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Dry Stack Anti-Vibration

I have a dry stack for my little 3 cylinder Yanmar. The engine for its size is a vibrating beast. The engine is connected to the 1 1/4" diameter exhaust pipe via a 1' long steel flexible coupling. The stack is hard bolted with a couple brackets to the aft plywood bulkhead between the wheelhouse and the aft deck. The stack is in the wheelhouse and surrounded with an insulated sheet metal shroud. I have done various things to minimize noise and I am thinking about replacing the hard mounts attaching the stack to the bulkhead. I am not completely sure that this would reduce noise but the stack does vibrate quite a bit and inside the house there is a kind of drumming noise. I can get some anti-vibration mounts that would fit easily and they are cheap. Has anyone else done this?
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Old 11-13-2018, 03:00 PM   #2
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Can you post a pic of the flexible connection between engine and stack?

A 3cyl propulsion engine is going to shake a lot, especially with proper flexible motor mounts. Worse at dead idle. That flexible connection needs to be very flexible!!
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Old 11-13-2018, 05:09 PM   #3
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[QUOTE=Ski in NC;714537]Can you post a pic of the flexible connection between engine and stack?


This is the flange between the engine and stack. Actually it is the old one, I replaced it with one exactly the same.

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Old 11-13-2018, 05:53 PM   #4
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If I had it in my hands I bet it would be quite stiff. I have used those only when engine motion was minimal, like a 6-71.

Does it flex at all by hand?

Most of the low rpm vibes on a 3cyl are in the form of the block rotating around the crank side to side (roughly) and at high rpm the front end wiggles in a rotating fashion opposite of the same vibe on the back end (waddle).

If flex tube goes straight up from the manifold, it would need to flex in compression which I doubt it likes. Better for the flex tube to run parallel to crankshaft as then it would flex in a bend.

Might consider automotive style flex tube, but those tend to be a little leaky and not last as long.
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Old 11-13-2018, 07:29 PM   #5
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There are 2 things that need to happen in a resiliently supported dry stack exhaust system.
You need to support the weight, and you do that in compression with isolators
And you need to stabilize and support the piping in the stack

You want to attach all these isolators to the stiffest structure you can, like a structural bulkhead or a cabin side. This allows the isolators to perform better.

You can use the isolators you pictured, horizontally in compression to support the weight, and with threaded rod to stabilize. The threaded rod allows the rubber to get away from the hot pipe, and 2 is usually adequate per location, through an 8 ft trunk maybe 2 locations

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Old 11-13-2018, 08:42 PM   #6
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You need to isolate and support the weight of the exhaust stack and then connect the engine to the stack. Then you need a pipe that is flexible but will contain the exhaust so that no vibration is transferred. That bellows you are using is intended to allow for expansion/contraction due to heating and is not really a good vibration damper. This one would be better (but get a longer one).
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Old 11-16-2018, 12:35 AM   #7
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Good one,
any mfgr. name suggestions?
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Old 11-16-2018, 12:43 AM   #8
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Old 11-16-2018, 07:42 AM   #9
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Flexible Exhaust
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