Wet exhaust through aluminum hull

Discussion in 'Metal Boat Building' started by Deering, Apr 30, 2015.

  1. Deering
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    Deering Senior Member

    What would be the appropriate material to use for the wet exhaust hull penetration (above WL) on an aluminum boat?

    I'm thinking that thick-walled 5086 aluminum tubing welded to the hull penetration would be the simplest and most durable, but I'm wondering if the exhaust gas/water slurry would be too corrosive? How is it done on other aluminum hulls?
     
  2. gonzo
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    gonzo Senior Member

    I think a bolted aluminum fitting with a flange would be better. If you do get corrosion, it will be easier to change.
     
  3. yofish

    yofish Previous Member

    I have built aluminum versions of Elastomufflers that lasted for years out of 6061 schd 40 pipe. The AL will get eaten but will take a looong time. Cheap and easy to do. Like gonzo says, bolting is more intelligent.
     
  4. Kevin Morin
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    Kevin Morin Junior Member

    Wet Exhaust Thru Hull in Aluminum

    Deering, I've mostly welded the through hulls in all boats and that includes the few wet exhausts I've done independent of I/O drives.

    My reason to suggest the welded pipe is the corrosion will mostly likely be limited to the inside of the pipe. IF you're talking a 2,000 hour/yr. running boat... that's one thing and the pipe may pit through in a decade at that rate of engine hours? But if you're talking sports use where only a hundred hours is typical, then welded in Sched. 40 is going to give you a decade's service with wall thickness left to spare. And if you turn up to Sched 80; well how old are you? It will probably last lifetime.

    My concern with installing hull mount flanged and bolted through hulls is the increased area for corrosion of the hull. The flange is a crevice corrosion site- waiting to happen. So what does that require? Well a gasket or some super goop and bolt holes that will be individual crevice corrosion sites, and or sleeves and isolators or more goop or .... on and on. Why not weld, yes it will have to cut out to repair but the hull will remain intact, the pipe is all you'd replace- whenever that came along?

    With a flange you may be replacing the hull in a big doughnut around the old flange too?

    I see it as less work to make a nice pipe fitting to weld to the hull and expect the grand kids to find a welder to replace if time comes? I've see most corrosion from lapped surfaces that aren't water tight, and would say they were more of a headache than the welded on pipe?

    My 2 cents

    Cheers,
    Kevin Morin
    Kenai, AK
     
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  5. Deering
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    Deering Senior Member

    Thanks all. Kevin, my thinking has been in line with yours. If I'm going to use aluminum, then I'll weld it. If I'm going to bolt, might as well go with composite or stainless. Grinding out a weld isn't that much work if it only needs to be done every decade or two.

    One thought I had today was that I could epoxy-coat the inside of the aluminum pipe after it's welded in. The paint would resist any acid and salt in the water, and it should be cool enough for the epoxy to hold up.

    Kevin, I hope boating season is looking up in Kenai. Wettest April on record by a long shot here in Juneau, and that's saying a lot in Southeast!
     
  6. yofish

    yofish Previous Member

    Can't quibble what Kevin says. It isn't worth debating one method over the other as the difference is small. He is correct, if made of schd 80, it would outlive you. If you can't stand the idea of 6061, AK Copper & Brass has extrusions in 5086 at a substantial premium.

    I doubt that coating the inside of the tube would provide any benefit; it would most likely flake off from the expand/contract. Even if it were to fail your son after you were gone, the repair would be too easy!

    I mentioned those fake elastomuffles, they were cans made from 8" pipe that always had some water in them, were only 2' from the manifolds and outlasted two sets (4) different 350 Chevy IO's.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    We always use S/Steel for engine exhausts. But for gensets we do use ally. And as you also note we weld them in. We also are required to have a thicker doubler/insert plate around the pipe. The plate is a minimum thickness of 1.5 times the local plate.

    Also Class do not allow the use of 6000 series for such exhausts, only 5000 series. We nominally use 5083-O, but the purchasing dept also complain it is hard to get hold of.

    We also coat the insides with Belzona.
     
  8. waikikin
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    waikikin Senior Member

    Belzona is incredible material, have patched some carroty old intercoolers with it, still going ok after a fair while, it's exxy but worth it to save.
    Interested in the S/Steel use in the exhausts, I know that it's in common use but thought that exhaust temps & chemicals could be adverse to the S/Steel or is there more really special grades appropriate for this purpose?
    Jeff.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Nope. Just standrad 316L
     
  10. baeckmo
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    baeckmo Hydrodynamics

    Weld al with inner dia equal to wet hose outer dia. Let the hose stick through the al stub. Use additional short hose connecting outside al-stub and outside wet hose. Works perfect; no exhaust in touch with alu-component!
     
  11. Deering
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    Deering Senior Member

    baeckmo, I really like your approach. I can think of several variations on that, and I think they all would work. The task now will be to see if I can find suitably sized aluminum tubing to accommodate the hose diameter without too large of a gap.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    That's kind of how we do it too :D

    Genny Exhst.jpg
     
  13. yofish

    yofish Previous Member

    I may be wrong but I don't think that that drawing is what baeckamo is describing unless there is rubber INSIDE the 5083 -0 tube.
     
  14. Deering
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    Deering Senior Member

    Ad Hoc, it looks like you're exhausting into the tunnel of the catamaran. Am I interpreting that correctly? If so, does the noise bounce around inside the tunnel and create more noise in the cabin than if you went outside the tunnel or through the transom?
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Correct.

    No issues with noise, the exhaust is far aft. And its far more preferable to exhaust fumes hitting you in the face along side when berthed.
     
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