20 Aluminum Boat Build

Discussion in 'Metal Boat Building' started by Gnohk_Tad, Feb 25, 2018.

  1. Gnohk_Tad
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    Gnohk_Tad Junior Member

    Hello All - I am new to this forum and would like some advise or feed back on this build. I pick up the plans from Glen-l. Frames and keel has been assembled and I am creating the template to plate the hull right now. I am using 1/4 inch aluminum for the hull and a bit concerned about being able to bend it into shape. Should I be worried or should I downsize to 3/16 or 1/8.
     

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  2. gonzo
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    gonzo Senior Member

    What do the plans call for?
     
  3. Gnohk_Tad
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    Gnohk_Tad Junior Member

    Plans call for 1/4 or 3/16 , this is my first build so I am a bit nervious about bending the 20 foot aluminum plate into place. What types of tools do i need to bend the two halves together?
     
  4. rwatson
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    rwatson Senior Member

    This clip shows the temporary forms to support the plate while assembling. For large plate sections, you can even do more substantial formwork with 2 x 4 's

     
  5. Gnohk_Tad
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    Gnohk_Tad Junior Member

    do you have any recommendations on how to build the form?

    Also how can I calculate the curvature of the boat to build this form?
     
  6. rwatson
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    rwatson Senior Member

    The best way is to cut up MDF ( say 12mm - 16mm)

    You should have drawings with the frame dimensions, so a few of the molds can be formed using these dimensions.

    The other technique is to make supports that are approximately the right shape, but have 2 x 4s attached to uprights that are fastened with tech screws. Once you have laid the sheet over the supports, you can jack up the "approximated" frames to push the sheet into a fair shape.

    Its not so hard to give the frame shape and placement dimensions to a person with a Cad program, and they can draw up the hull shape, and produce a precise set of mold shapes to use. HalfSBBottom.JPG
     
  7. Gnohk_Tad
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    Gnohk_Tad Junior Member

    Great advice, but it seems like a lot to go through given I have the frames set up and the template for the hull plating ready. In any case the plate I am using is 1/4 inch how would the using this mold help me bend the plate into shape?

    The boat is 20 feet long by 8 feet wide i re uploaded the picture of the boat as it sits today.

    This boat is a one off as most boats built by amateurs. Personally building a CAD model of the boat and then creating a mold for it seem a bit excessive.

    I appreciate your help, anyone else have some ideas ?
     

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  8. rwatson
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    rwatson Senior Member

    Edit:
    Just had a relook through the pictures, so I think I have a better idea what you are asking..

    You are planning to pull down the 1/4" bottom plate while sitting on a metal frame I guess, so you want to bend it to shape before offering it up to the aluminium frame for tacking.

    The normal way is to simply apply pressure with timber lengths and compression straps until the plate conforms to the metal jig. You can hold it in place with clamps at each edge with clamps alone, or you can use timber strips clamped over the aluminium , with the timber clamped to each steel frame.

    PS - if you get a really difficult piece to bend, this has some info

     
  9. rwatson
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    rwatson Senior Member

    More details of pulling plates to frames.


    Al01.png Al02.png

    Note how they drill a hole through the bottom of the sheet, and insert an eye hook, so they can pull a curve into the bottom sheet at the bow.

     
  10. rwatson
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    rwatson Senior Member

    Found a good video on the welding process for aluminium

     
  11. Gnohk_Tad
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    Gnohk_Tad Junior Member

    Thanks for the info!!!

    I appreciate the feedback.

    What type of boats have you guys built ?
     
  12. Barry
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    Barry Senior Member

    Some comments
    Our niche was building 18 to 24 foot by 8 foot beam aluminum jet boats but have built up to 36 feet.

    1) If you are not an extremely experienced ALUMINUM fabricator, don't try to heat the pieces to bend them. Just a slight hesitation when
    applying the torch/heat and you will find the aluminum will just drop out of shape. This is considerably different than heating steel to a red hot phase and having it hold its shape. Aluminum oxide has a higher melting temperature than the aluminum underneath so the shape can be retained
    until the outer temp is reached and entire section can just fall away. As an experiment, take a length of aluminum and heat it and you will discover what I am explaining. As we fit 1/4 inch stringers onto 1/4 inch hull bottoms, the welding heat from the welds would always introduce a bit of camber and we would heat the stringers to straighten them out. We had temp sticks, a crayon, that as we heated the stringer, we would just take the crayon and make a mark with it on the heated aluminum, if the crayon melted and got shiny, then that is the temp of the material. I do not want to give you inaccurate information here but, memory is questionable, I think we used 800 degree temp

    2) The sweep of the bow appears quite tight. Generally if the bow has a fast or tight curve then the chine cannot also have the same tight turn or
    the plate will not fit flat. As you have a plan from a known builder, there is a good chance that the two are compatible to make a developable surface
    and that the sheet will in fact lie on both edges tightly.
    As we built on a steel, male jig, we had tabs welded on the male jig and if a piece of aluminum was not snugging up, we would get under the jig and
    drill up through the hull, drop a bolt in it and pull the plate down to the jig. After, just don't forget to weld up the holes.

    3) I see that you have pieces of maybe plywood to make perhaps make a pattern for the sheet. You should make the template as one compete
    piece, joining the ply with a thin batten at the 8 foot intervals, thin so that the shape or curve of your pattern is not impacted from the hard
    spot that a heavy batten can produce at the joint. As this is a problem, on the 36 that we built, on a jig that was not proven to be developable, we took thin plywood, maybe 1/8 inch, cheap wall panelling, and made up a flat sheet about 40 feet by 4 feet wide by offsetting the sheets
    and gluing them one on top of the other, so that the joints offset would act as a single piece.
    If you make a template from small independent pieces there is a chance that you will have fitment issues.

    4)I expect that your 20 foot sheets are long enough to make the side panels of the hull. If not, make the butt joint on a flat surface before
    trying to weld it onto the framing/jig. Other wise you will have a disjointed joint, had to. You need the sheet to act as a single unit as when you bend it around a form to get a fair curve

    5) We used 1/4 bottoms and 1/8 sides. I cannot see any reason why you would consider 1/4 for the sides. On the 36, we used a 3/16 side but then
    it was 36 feet long.

    6) Welding. Unless absolutely critical, I would not use Tig on a boat hull. The slow speed of tig 7) welding introduces a lot of aluminum expansion
    into the sheet so that you actually can weld in deformation in the hull. Some will say not, and occasionally we used Tig but not often.
    When you are doing a butt weld you must cut out the backside of the weld before welding the backside. Most important!!

    7) When welding in any other structure, to the hull or most anywhere else, try to ensure that you have a weld bead on each side of each other.
    Aluminum work hardens extremely easy. If you weld say a 1/4 inch piece onto another 1/4 inch piece only on one side, it is extremely easy
    to break this weld due to stress concentration. And the stress concentration due to the notch effect can cause failure due to small localized work hardening

    8) Also when you start and stop any long seams, you must cut back the bead where you stopped to ensure that when you stopped the first bead
    there was not a weld crevice due to the bead cooling quickly. You can try this out on some scrap. If you get to the end of the weld, just
    draw back the wirefeed to minimize this effect, but we always cut the end of the weld bead back say 1/2 inch to ensure a fresh start of the next continuous bead
    9) When butt welding sheets, the weld must be cut from the back side to welded material. We used a standard 7 1/4 inch skil saw, and would cut
    the weld from the backside down until the thin black line of the unwelded plate disappeared.

    If I think of anything else I will let you know
     
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  13. Gnohk_Tad
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    Gnohk_Tad Junior Member

    Thank you for the input. I will post some pictures of the progress.
     
  14. Barry
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    Barry Senior Member

    Re cutting the back side of a butt weld
    Normally we use a regular 7 1/4 inch skil saw, 60 - 80 tooth carbide. You have to manually pull the guard back to expose the bottom of the blade and often have to have the
    shoe part of the saw up so 2 1/2 inches of the blade is exposed. To see the exact path of the blade into the back side of the joint, you have to get quite close to it.
    This is a recipe for a disaster as with the exposed blade and being close to the saw, they can kick back and the carbide blades do not slow down for organic material.
    It was a safety policy in our shop that any potential kick back path did not cross over a thigh, leg etc.
    I would recommend that if you are in the shop after hours or whatever, that you have someone around in case of a problem

    On occasion I had seen other aluminum boat builders install a 4 1/2 inch carbide tooth blade in a 4 1/2 inch grinder. They have less mass and if they bind they are harder to control kick back but sometimes due to access, they are used. After trying one ONCE, we never had one in our shop

    Alternatively, a 1/8 inch cut off blade in the same 4 1/2 inch hand grinder will work but you have then a further task to get rid of all of the material off the disc to remove any chance of
    including the fibre, abrasive in the weld. Again, if we only had access for a 4 1/2 inch grinder and there was a risk of kick back

    So if you are on the back side of a butt weld and cutting the material to the sound weld joint, as you start you will see a distinct thin black line, the pieces butting together, then when you hit
    solid material, it will look like the rest of the cut.

    Recommend two levels of eye protection, only as your head is so close to the hot chips, safety glasses and face shield,

    Did not intend to be so wordy, but this is one of the most dangerous hull welding processes of constructing a hull, short of dropping it on someone when you roll it over
     
    Last edited: Feb 28, 2018

  15. Gnohk_Tad
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    Gnohk_Tad Junior Member

    What is the purpose of cutting the backside of the weld ?
     
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