Aluminium boat with no welds?

Discussion in 'Boat Design' started by stonedpirate, Jun 19, 2011.

  1. stonedpirate
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    stonedpirate Senior Member

    Hi,

    I know you can rivet the sheet metal to the frame, but can the frame itself be put together with rivets or does it need to be welded?

    Anyone ever seen an ally frame made with rivets?

    Thanks
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Go look at the early hydrofil boats by Rodriquez:
    http://www.rodriquez.it/fastferry/hydrofoils.php

    All riveted.

    You can join any plate to any frame by any method you wish. All you have to do is satisfy yourself that the method of jointing can trasmit the loads from plate to frame/stiffener.
     
  3. nukisen
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    nukisen Senior Member

    Of course you can! why shouldn´t ?
    I have only seen my dads small fishing boat made in this way but is als welded.
    Actually he´s boat have several damage and holes in the bottom and still is floating. A few times when the winter have come we have removed the ice with an axe. Then a few time there have been accidents and holes in the hull have ben made.
    This holes have been repaired with a pitch hammer and a sledge on the other side.

    This works and I think he´s boat will long for my generation also and it was my grand father bought it when he returned to Sweden after a few years emigrated to US.
    So I think this boat is over 80 years old by now.

    Aluminium is a very good good formable material. And if you let it oxide without polish it. It will last for ages.
     
  4. daiquiri
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    daiquiri Engineering and Design

    Yes, you can do it. The joints have to be watertight in order to avoid water penetration and crevice corrosion between parts. A layer of epoxy or other insulating watertight material between the plates to join can be a good idea. If possible, use at least two rows of rivets with spacing equal to around 4 times the rivet diameter (this will vary depending on the construction detail and loads). The rivets have to be made of aluminum alloy.
    You can download rules for construction of aluminum small craft from one of classification societies (Lloyd's, DNV, GL) and get the correct dimensions and spacing of rivets from there.
    Cheers!
     
  5. CDK
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    CDK retired engineer

    In a TV documentary I once saw how aircraft skin was attached to the frame using cyanoacrylate only.

    My own experiences with superglue are all negative and I have always distrusted commercial airliners, but they do not all fall apart in turbulent air, so with proper surface treatment it might also work for aluminum boats.
     
  6. Jeremy Harris
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    Jeremy Harris Senior Member

    I've done a fair bit of alloy bonding on aircraft structures and it can work very well, but it is extremely intolerant of poor working conditions. Getting a good bond means careful control of temperature, humidity, surface cleanliness etc and involves some fairly involved surface preparation techniques. It's a well-proven technique, first used back in the 1940s with Redux, but since then various anerobic adhesives, epoxies and polyurethanes have been used to good effect. Most of my experience has been with epoxy adhesives (particularly the Hysol range), but these do have problems with high temperatures (they start to soften and have much reduced bond strength at the sort of temperatures that might be experienced on a dark coloured alloy skin in the tropics).

    Surface treatment prior to bonding is usually by using etching followed by a chromic acid anodise (something like Alodine) treatment.

    If it were me, then I think I'd opt for using an epoxy adhesive for an alloy boat below decks, making sure that the bond areas were adequate and that the joints were designed to primarily take loads in shear, with limited exposure to tension and definitely no peel loads. Above decks, where the temperature might possibly approach the softening temperature of epoxy in the tropics (we're talking of temperatures close to boiling point here though, so this may not be a problem with careful design) then either look to use mechanical fasteners or perhaps the Redux process (essentially phenyl formaldehyde and polyvinyl, as used on aircraft skins).

    Despite its age, Redux has proven to be a very capable aluminium alloy adhesive. There are still many aircraft flying today with skins that were bonded on using this process more than 60 years ago, and the bond failure rate is very low. My only reservation on using it for a whole boat hull bonding system is the fact that it's much harder to work with than something like Hysol 9462. Hysol comes in a neat two pack dispenser gun with a mixing nozzle, which makes it very easy to use.

    This is probably a bit off-topic, but this alloy boat frame that I've built is riveted together in the main, with epoxy bonds at the stem and transom:
    [​IMG]

    This frame was for a fabric skin-on-frame boat, but it could equally easily have been clad in thin sheet alloy, riveted or bonded to the frame.

    Jeremy
     
  7. FAST FRED
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    FAST FRED Senior Member

    Look at Feathercraft boats or Grumman dinks.

    Riveted construction is fine at modest speeds , 40K .

    Riveting is cheap , repairable but noisy.

    Use "marine" aluminum rivets , not aircraft , and they must be driven (hammered) rivets , not pop rivets.

    FF
     
  8. Jeremy Harris
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    Jeremy Harris Senior Member

    Why the insistence on driven rivets over self-setting (often referred to by the "Pop" trademark) rivets? I've a lot of experience with using Avdel and Monobolt self-setting rivets on aircraft structures. They are strong, easy to use, very reliable and have a lot of advantages over traditional bucked rivets, not least being their ability to be placed from one side only.

    I'd agree that alloy "Pop" rivets from the local hardware store are not a good choice, they have a very poor shear strength and tend loosen over time when exposed to varying loads, but a modern monel, or maybe passive stainless, Avdel type rivet will have a shear strength only limited by the bearing stress of the hole in the sheet, not the strength of the rivet.
     
  9. CDK
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    CDK retired engineer

    Thank you for the extensive explanation Jeremy.
    It does nothing for my distrust of airliners though.... I just hate situations where I'm not in control.
     
  10. wardd
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    wardd Senior Member

    they wear faster, not as strong and more expensive

    and they usually are not that much faster to install than shot rivets
     
  11. Steve W
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    Steve W Senior Member

    If i were doing this i would use both adhesive and rivets for a couple of reasons,one being the belt and suspenders approach and another being the need to have something in the joint to prevent moisture, even just salt laden sea air from getting into the joint and causing problems down the road. As an interesting side note, i work at a marina and we have quite a few large aluminum boat trailers stored from customers boats and they,unlike steel trailers are all bolted together,no welding at all, my understanding is that it is because aluminum welds are not as strong as the parent metal, perhaps someone with more knowledge than i can confirm or dispute this?
    Steve.
     
  12. Jeremy Harris
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    Jeremy Harris Senior Member

    I shared your distrust of bonding metals in aircraft, but gradually this faded as I started to see how reliable bonding can be if done properly. The one thing I dislike about this process is the inability to properly check whether or not a bonded metal joint is up to spec, the emphasis has to be on getting the process right and trusting that the joint is OK, unlike rivets and welds, that can be inspected and X rayed.

    BTW, better take care if you ever drive one of the Lotus cars made in the last ten years or so, as they have an aluminium alloy chassis glued together with epoxy...............
     
  13. Jeremy Harris
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    Jeremy Harris Senior Member

    No rivet should wear out, as the strength comes in large part from the friction between the faying faces of the metal it's holding, with the ultimate joint strength coming from the bearing stress limit of the material being joined. If rivets are moving ("working") then they either weren't set properly or they are inadequately specced for the job.

    Something like the Avdel Monobolt rivets I referred to earlier are the equal of bucked rivets in terms of their ability to hold sheet material together, are easier to set (no riveting skill needed, can be fixed by a single operator from one side) and are easier to inspect for correct setting (they have a visible indicator). See here for details: http://www.avdel-global.com/en/products/breakstem-fasteners/monoboltr.html

    Cost is an issue, as bucked rivets are dirt cheap. However, the speed with which self-setting rivets can be dipped, inserted and set, plus the fact that they will form a very small proportion of the overall build cost, makes cost less of an problem than it could be.
     
  14. wardd
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    wardd Senior Member

    i can teach you to rivet in a couple hours

    and rivets and rivet holes do wear and loosen and the metal under the heads can wear from movement

    when i was in the service mechanical or friction locking rivets were only permitted when solid rivets couldn't be used
     

  15. Jeremy Harris
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    Jeremy Harris Senior Member

    That's a very good approach. Wet riveting is one of my preferred techniques for just this reason. Using rivets to take potential peel loads from an otherwise bonded joint is another good strategy, as it allows the number of fasteners to be reduced over a purely riveted structure and also provides some additional protection from early bond failure in a joint.

    You're right about heat treated aluminium alloys needing to be re-heat treated post weld, which is one reason that larger alloy structures are often bolted, bonded or riveted rather than welded. Even one of the ubiquitous, 6000 series alloys that we see every day in bike frames, window frames etc loses around 30% or more of its strength in the heat affected zone around a weld unless its re-heat treated after welding. This heat treatment involves careful temperature control over a fairly long period of time, so a pretty big oven would be required to take a boat trailer.
     
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