Complex bends in aluminium

Discussion in 'Boat Design' started by Storm_Eagle, Aug 19, 2020.

  1. Storm_Eagle
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    Storm_Eagle Junior Member

    Hi.
    How are complex bends in aluminium hulls made?
    For example like the bottom of the pontoons of a sailing catamaran hull:
    [​IMG]

    It bends in 3 directions at the same time on the bottom middle. It is curved longitudinally, sideways, and vertically.

    The simple "one curve" bends like on the side panels of the pontoons is easy to understand. It is just to let the panels follow a frame, but for "multi curve" bends you will have to stretch the metal somehow? Are they hydroformed against a mold or some other technique like that? If so it is pretty much out of reach for hobbyists to do right?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Where there's a will, there's a way. One method not recommended for home use, was to place an explosive on a sheet of metal, over a hole in the ground excavated to the desired shape. Then fire it, I am sure there would be 343 laws nowadays prohibiting such an idea ! More realistically, you are talking about relatively sophisticated metal working techniques, like stretch forming, or plate rolling. Not really basic skills, or methods not requiring expensive plant. This little boat built in alloy, features a hollow flare in the topsides forward, from stretch forming.
    Q.jpg
     
  3. Storm_Eagle
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    Storm_Eagle Junior Member

    Maybe there is a way to hack DIY hydroforming? Using a pressure washer, and making a mold out of plywood and then backing it up with reinforced concrete for strength to stand up against the hydroforming pressure? Is it possible? Or will you just crack the concrete from the forces?
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    It is just a lot easier to do, with other building materials, rather than metal. The ease of working the metal would vary with the alloy and temper, it ain't a backyard project, really.
     
  5. Rumars
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    Rumars Senior Member

    Nobody hydroforms (or explosive forms) plating, that would mean investment in forms. Plating is streched/shrunk cold the old fashioned way by powerformers or wheeling. It can also be be done entirely by hand with hammers against dolly/sandbag, just takes a lot longer. The achievable finish is a function of the workers skill, good workers can produce plating that needs minimal or even no fairing compound.
    If you want to see the machines at work google kraftformer and english wheel, for the hand process "panel beating".
     
  6. Storm_Eagle
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    Storm_Eagle Junior Member

    After some google image searching i found this:
    [​IMG]

    Looks like they just have enough stringers in the frame for support and thin enough aluminium sheets and then just use ratchet straps to force the aluminium sheet to conform to the frame shape....

    Beating it with hammers, and english wheel rolling takes to much skills and time for my liking.
     
  7. Rumars
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    Rumars Senior Member

    Cheapest and quickest way for a hobbyist would be a radius chine hull, where the plates are only rolled.
    The plates in your picture where wheeled, judging from the marks. Wheeling is not that complicated if you use a powered wheel, and the machine can be homemade. There is an element of skill but that is also true for the welding part, and both can be learned.
    Hand beating entire panels is something nobody does, that's what powerformers are for.
    Here, look at this project, nice big boat buildt by a single man. http://nordkyndesign.com/nordkyn/construction/shell-construction/
     
  8. KeithO
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    KeithO Senior Member

    https://www.amazon.com/gp/product/1...at-20&linkId=1c559cb7d0e4b19c8f37d2ab33768a9e

    There is another method. But it results in a lot of narrow plates and a lot of welding. However it eliminates the wheeling altogether. The small full round bilge boat shown on the cover of the book has some extreme curvature and was built using the proposed method. Instead of wheeling, each individual plate is developed with lines that are 3 degrees apart and by marking the bend lines on the flat plate and performing identical bends on a press brake using radiused tooling, each of the plates is completely prefabricated without any voodoo.

    The work is split into 2 main parts, firstly designing the plates themselves, on which the author does not reveal too much of the workflow. Then once the plans are done, the process of defining how much bend stroke is needed for an exact 3 degree of bend for different lengths of bend. Once that is known for each plate thickness involved, the flat plate profiles are cut out by water jet and deburred, the bend lines transferred to the plates and then plates bent up 1 bend at a time in the press and hull plating can proceed as fast as the rigger and welder can work.

    Based on the example in the book, the final product needed fairing. Now it is possible that a larger hull may need less fairing to allow for bare aluminum hulls. I think that wheeling produces a superior product but it does require skilled operators with a lot of experience because there is virtually no automation. So more expensive. Avoiding the need to paint for the life of a boat is possibly worth more than the difference in the plating cost. It may be hard to even get on the schedule of a shop with the needed skills because they may not even be interested in merely building your hull, there is a lot more money to be made for them on fitting out the interior and installing systems.
     
  9. Squidly-Diddly
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    Squidly-Diddly Senior Member

    same way they make aluminum cans. I'm really amazed at how well that works.

    I once looked at an old Grumman Sportboat that had HUGE deep dents all over the bottom, but no splits. It looked like it was used as a toboggan over wet round river rocks more that a boat.

    Pretty sure they used big stamping machines similar to how the bang out auto-body parts for aircraft in WW2 to get rough curves then placed and marked overlap and trim with power-shears, then re-place into position for rivets.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Given that panel beating is an indentured trade, I think we can discount that a good job of forming compound curves can readily be done by an amateur, though I did know a fella who made some very passable panels for a 100-year old vintage car, as a very determined amateur, but he was unusually dedicated to the task. I recall telling him that it would be easier to make them from GRP, but of course that was sacrilege, being not faithful to the original.
     
  11. KeithO
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    KeithO Senior Member

    Stretching aluminum sheet to make a canoe:

    Actual aluminum forming starts at 45 sec.
     
  12. Storm_Eagle
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    Storm_Eagle Junior Member

    Yes. Stretch forming is probably how it is done. As i can not imagine it will pay off to invest in stamping presses as huge as a sailboat hull will need. Or if that big presses even exist.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    You don't introduce complex curves - simple!

    Only if, as noted above by Rumars, very very thin plate hydroforming... but that is for small dinghy's etc.

    In other words, no one introduces curves in 3 directions....it is a poor design and lacking in shipbuilding practice knowledge, if a hull design using a metal, has such complex curves.
     
    Ilan Voyager likes this.
  14. Storm_Eagle
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    Storm_Eagle Junior Member

    It is not that simple. As you can see from the clip above even a simple aluminium canoe has compound curves trough the whole hull.

    With the exception of flat bottom sump boats / fan boats almost all hull shapes have compound curves. especially towards the front of the hull where you want it to become narrower to a point and at the same time curve both sideways and upwards to make a bow.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    Only boats that will be made of composite.

    Metal hulled vessel do not have complex curves because the shapes are not developable. You may find small regions of some bows of some vessels, but these have endless small plates to attempt to create such a complex curve - but it is extremely labour intensive and not good for fatigue properties either.

    Only someone with no shipyard experience or knowledge of plate forming would ever try to design a hull with complex curves.....

    Except for small little dinghy's. There are plenty about that are hydroformed in this way. It doesn't scale!!
     
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