Aluminum Drainplug?

Discussion in 'Materials' started by CaptHarry, Mar 20, 2024.

  1. CaptHarry
    Joined: Mar 2024
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    Location: Tallahassee

    CaptHarry New Member

    Ahoy

    I'd like to add drainplugs to an aluminum hull houseboat. I've tried to google search but no luck.

    Anyone out there know where I might find the traditional threaded ones to accommodate a 1" hole? Or what is used in this application?

    Thanks in advance! Capt Harry

    What I'm doing is adding 6 thru-hull drain plugs in a 60' tri-hull houseboat hull where none existed. Reason is 6 mos annual storage to let water flow out. These will be welded in, aluminum drain plug to the aluminum hull. My concern is these are below the watertline and not sure how brass bungs will react with the aluminum in salt water.
    Aluminum Bung Drain Plug w/Brass Plug Pontoon Boat Vessel Gas Tank Oil Tank | eBay
     
    Last edited: Mar 21, 2024
  2. comfisherman
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    comfisherman Senior Member

    Do you need a specific alloy, and are we talking a aluminum coupling? Aluminum close couplings come in all manner of sizes, I'll make a couple calls in the am to see if my local spot still stocks 5086 versions. When a lot of alloy Boats were being built they had a few runs done.
     
  3. kapnD
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    kapnD Senior Member

    Threaded PVC or ABS pipe plugs should work without causing corrosion problems.
     
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  4. Barry
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    Barry Senior Member

    A close coupling can be cut in half and welded in. We did this in any aluminum fuel tanks that we built.
    The problem is that screwing an aluminum plug or fitting into an aluminum pipe fitting can often gall the threads. Causing leakage issues after a few tightings. Kapn D below suggested plastic plugs
    and they are certainly a better choice that aluminum to aluminum

    Alternatively, you can insert a stainless (316 highly recommended) reducing bushing into the aluminum welded in coupling. And into the reducing bushing another stainless plug. When removing the smaller plug just use two wrenches, one to hold the reducing bushing from loosening and the other on the plug.
    Stainless also has the propensity to gall, so do not overtighten.

    Over the last 20 odd years, on my own aluminum salt water kept boats, I would use a stainless bushing and a brass plug and never had any issues. BUT, my salt water boats would
    only sit in the ocean for 6 months or so at a time. I never noticed any corrosion of the brass plug but as it was softer than the stainless, I never had issues with galling.
    The purpose of the stainless bushing between the aluminum hull and the brass plug was to reduce the galvanic corrosion potential between the brass and aluminum. Brass plugs should never be screwed into aluminum.

    The plastic plugs suggested above seems to be the best alternative and will mitigate any corrosion possibilities.

    (just an aside: people often use teflon tape on pipe fittings. The purpose is not to provide a smoother or less friction joint make up. The teflon disappears on the thread faces upon
    tightening. Some of the tape residual will fill any inconsistencies in the thread mating interfaces. Mainly at the roots and tips of each adjacent thread. )
     
    Last edited: Mar 21, 2024
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  5. BlueBell
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    BlueBell . . . _ _ _ . . . _ _ _

    Why?
     
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  6. comfisherman
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    comfisherman Senior Member

  7. CaptHarry
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    CaptHarry New Member

    To drain the hull while out of the water.
     
  8. CaptHarry
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    CaptHarry New Member

    Forgive my ignorance, I've only worked with wood and fiberglass boats over the years.
    What I'm doing is adding 6 thru-hull drain plugs in a 60' tri-hull houseboat hull where none existed. Reason is 6 mos annual storage to let water flow out. These will be welded in, aluminum drain plug to the aluminum hull. My concern is these are below the watertline and not sure how brass bungs will react with the aluminum in salt water.
    Aluminum Bung Drain Plug w/Brass Plug Pontoon Boat Vessel Gas Tank Oil Tank | eBay
     
  9. comfisherman
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    comfisherman Senior Member

    I wouldn't do it with salt water.
     
  10. Barry
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    Barry Senior Member

    I did not recommend a brass plug in contact with aluminum but a brass plug put into a stainless reducing bushing. Do not use the scully flange with the brass plus as shown on ebay.
    Aluminum + brass + salt water = corrosion

    Comfisherman said not even to use stainless due to corrosion. I have not seen stainless corrode in 3 months underwater but he spends more time on the water than most.

    So go with plastic. Easy to buy. I am not sure if you can find a plug made from Marelon. Lots of marine products made from this plastic
     
    Last edited: Mar 23, 2024
  11. comfisherman
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    comfisherman Senior Member

    It won't corrode in a single year, but you will get some white powder with stainless over time. Marelon is a good suggestion as well.

    My main work skiff has 4x plastic plugs in npt coupling like linked above. My through hull for the drain used more frequently is a threaded chunk of 5086 with a 316 stainless plug. Anytime there is dissimilar metals there will be some level of electrolysis, but if the combo has a life measured in decades it's good enough.
     
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  12. C. Dog
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    C. Dog Senior Member

    Re the galling propensity of certain metals, I have found nickel anti-seize compounds safe on aluminium, plus inert waterproof greases such as lithium based, some pipe sealants, PTFE thread tape, all help to lubricate the thread and avoid seizing. No lubricants/sealants with copper or molybdenum near aluminium however, or expect a sieve.
     
  13. gonzo
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    gonzo Senior Member

  14. CaptHarry
    Joined: Mar 2024
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    CaptHarry New Member

    I ordered an aluminum bung the will accommodate a 1/2' NPT plug. Also found 1/2" NPT aluminum plugs online. These should work.
    I've got a mobile welder that specializes in boats coming to do some mods on the hull. He will weld the bungs in place. 20220429_112137.jpg cheers resized.PNG I removed the I/Os and will be replacing them with outboards as to be able to lift them completely out of the water. This particular boat will be more house than boat, for when I get REALLY old. We have a small (34' Marine Trader) trawler that we use for traveling.
    Thanks all for the info! Capt Harry
     

  15. Barry
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    Barry Senior Member

    I would not run aluminum NPT plugs into an aluminum bung.
    Just fine tuning here.
    There are couplings that you can cut in half to make up a "bung" and we have done this when we could not get scully flanges. But you can expect that with welding that the coupling can
    warp slightly, enough to change the profile of the female thread opening. So when you screw in you plug it may 1) leak or 2) gall the thread face on removal

    A scully flange has much more material around it, reducing warpage and when we could get them, we used them to mitigate leaks.

    Along with building many aluminum tanks, with draw lines, return lines etc, we found that even running in relatively low torque hose barbs, that when you put them in, it was one time only
    ie we figured out the direction of the 90 hose barb, tightened them up and they stayed. When we erred and could not get another turn on the fitting and had to take it out, I would say that 50%
    of them would gall.
    Gonzo, Comfisherman and myself, and perhaps others suggested plastic pipe plugs.

    You might get lucky with aluminum on aluminum.
     
    Last edited: Mar 23, 2024
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